# RobotProcessor: The Art of Processing Robot Data

This tutorial will teach you how to use RobotProcessor, a powerful pipeline system for preprocessing robot observations and postprocessing actions. You'll learn how to create modular, composable data transformations that make your robot learning pipelines more maintainable and reproducible.

**You'll learn:**

1. What RobotProcessor is and why it's better than alternatives
2. How to create and compose processor steps
3. How to save, load, and share your processors
4. Advanced features for debugging and customization

## Real-World Scenarios: When You Need RobotProcessor

Before diving into code, let's understand the real problems RobotProcessor solves. If you've worked with robots and learned policies, you've likely encountered these challenges:

### Observation Key Mismatches

Your environment might output observations with keys like `"rgb_camera_front"` and `"joint_positions"`, but your pre-trained policy expects `"observation.images.wrist"` and `"observation.state"`. You need to rename these keys consistently across your pipeline.

### Image Preprocessing Requirements

Your robot's camera captures 1920x1080 RGB images, but your policy was trained on 224x224 cropped images focusing on the workspace. You need to:

- Crop out the relevant workspace area
- Resize to the expected dimensions
- Convert from uint8 [0,255] to float32 [0,1]
- Ensure the right channel ordering (HWC vs CHW)
- Move everything to GPU efficiently

### Coordinate System Transformations

Your policy outputs desired end-effector positions in Cartesian space, but your robot expects joint angles. You need to:

- Apply inverse kinematics to convert positions to joint angles
- Handle singularities and out-of-reach positions
- Smooth the resulting trajectories to avoid jerky movements
- Clip values to respect joint limits

### State Augmentation

Your environment provides joint positions, but your policy also needs velocities and accelerations. You need to:

- Calculate velocities from position differences
- Maintain a buffer of past states
- Handle episode boundaries correctly

### Multi-Robot Deployment

You want to deploy the same policy on different robots with varying:

- Camera setups (different resolutions, positions)
- Joint configurations (6-DOF vs 7-DOF arms)
- Control frequencies (10Hz vs 30Hz)
- Safety limits and workspace boundaries

### Data Collection and Training

During data collection, you need to:

- Record raw sensor data for dataset creation
- Apply the same preprocessing as during deployment
- Ensure reproducibility across different operators
- Share preprocessing configurations with collaborators

RobotProcessor solves all these problems by providing a modular, composable system where each transformation is a separate, shareable component. Let's see how.

## Why RobotProcessor?

If you've worked with robot learning before, you've likely written preprocessing code like this:

```python
# Traditional procedural approach - hard to maintain and share
def preprocess_observation(obs, device='cuda'):
    processed_obs = {}

    # Process images
    if "pixels" in obs:
        for cam_name, img in obs["pixels"].items():
            # Convert uint8 [0, 255] to float32 [0, 1]
            img = img.astype(np.float32) / 255.0
            # Convert from HWC to CHW format
            img = np.transpose(img, (2, 0, 1))
            # Add batch dimension
            img = np.expand_dims(img, axis=0)
            # Convert to torch and move to device
            img = torch.from_numpy(img).to(device)
            # Pad to standard size
            if img.shape[-2:] != (224, 224):
                img = F.pad(img, (0, 224 - img.shape[-1], 0, 224 - img.shape[-2]))
            processed_obs[f"observation.images.{cam_name}"] = img

    # Process state
    if "agent_pos" in obs:
        state = torch.from_numpy(obs["agent_pos"]).float()
        state = state.unsqueeze(0).to(device)
        processed_obs["observation.state"] = state

    return processed_obs
```

This approach has several problems:

- **Mixed concerns**: Image processing, tensor conversion, device management all in one function
- **Hard to test**: Can't test image processing separately from state processing
- **Device management scattered**: Easy to forget `.to(device)` calls
- **No standardized format**: Each project reinvents the wheel
- **Difficult to share**: Can't easily reuse this preprocessing in other projects

RobotProcessor solves these issues by providing a declarative pipeline approach where each transformation is a separate, testable, shareable component.

## Understanding EnvTransition and Batch Format

RobotProcessor works with two data formats:

### 1. EnvTransition Dictionary Format

An `EnvTransition` is a dictionary that represents a complete transition in the environment:

```python
from lerobot.processor.pipeline import TransitionKey

# EnvTransition structure:
transition = {
    TransitionKey.OBSERVATION: {"observation.image": ..., "observation.state": ...},  # observation at time t
    TransitionKey.ACTION: [0.1, -0.2, 0.3],                                          # action taken at time t
    TransitionKey.REWARD: 1.0,                                                       # reward received
    TransitionKey.DONE: False,                                                       # episode done flag
    TransitionKey.TRUNCATED: False,                                                  # episode truncated flag
    TransitionKey.INFO: {"success": True},                                           # additional info from environment
    TransitionKey.COMPLEMENTARY_DATA: {"step_idx": 42}                              # complementary_data for inter-step communication
}
```

### 2. Batch Dictionary Format

This is the format used by LeRobot datasets and replay buffers:

```python
# Batch dictionary format (used by LeRobotDataset, ReplayBuffer)
batch = {
    "observation.image": torch.tensor(...),      # Image observations
    "observation.state": torch.tensor(...),      # State observations
    "action": torch.tensor(...),                 # Actions
    "next.reward": torch.tensor(...),            # Rewards
    "next.done": torch.tensor(...),              # Done flags
    "next.truncated": torch.tensor(...),         # Truncated flags
    "info": {...},                               # Info dictionary
    # Additional keys are preserved but ignored during conversion
}
```

### Automatic Format Conversion

RobotProcessor automatically handles both formats:

```python
from lerobot.processor.pipeline import RobotProcessor
from lerobot.processor.observation_processor import ImageProcessor

processor = RobotProcessor([ImageProcessor()])

# Works with EnvTransition dictionaries
transition = {
    TransitionKey.OBSERVATION: {"pixels": image_array},
    TransitionKey.ACTION: None,
    TransitionKey.REWARD: 0.0,
    TransitionKey.DONE: False,
    TransitionKey.TRUNCATED: False,
    TransitionKey.INFO: {},
    TransitionKey.COMPLEMENTARY_DATA: {}
}
processed_transition = processor(transition)  # Returns EnvTransition dictionary

# Also works with batch dictionaries
batch = {
    "observation.pixels": image_tensor,
    "action": action_tensor,
    "next.reward": reward_tensor,
    "next.done": done_tensor,
    "next.truncated": truncated_tensor,
    "info": info_dict
}
processed_batch = processor(batch)  # Returns batch dictionary
```

### Using TransitionKey

Use the `TransitionKey` enum to access dictionary elements:

```python
from lerobot.processor.pipeline import TransitionKey

# Good - using TransitionKey
obs = transition[TransitionKey.OBSERVATION]
action = transition[TransitionKey.ACTION]
reward = transition[TransitionKey.REWARD]
done = transition[TransitionKey.DONE]
truncated = transition[TransitionKey.TRUNCATED]
info = transition[TransitionKey.INFO]
comp_data = transition[TransitionKey.COMPLEMENTARY_DATA]

# Alternative - using .get() for optional access
obs = transition.get(TransitionKey.OBSERVATION)
action = transition.get(TransitionKey.ACTION)
```

### Default Conversion Functions

RobotProcessor uses these default conversion functions:

```python
def _default_batch_to_transition(batch):
    """Default conversion from batch dict to EnvTransition dictionary."""
    # Extract observation keys (anything starting with "observation.")
    observation_keys = {k: v for k, v in batch.items() if k.startswith("observation.")}
    observation = observation_keys if observation_keys else None

    # Extract padding and task keys for complementary data
    pad_keys = {k: v for k, v in batch.items() if "_is_pad" in k}
    task_key = {"task": batch["task"]} if "task" in batch else {}
    complementary_data = {**pad_keys, **task_key} if pad_keys or task_key else {}

    transition = {
        TransitionKey.OBSERVATION: observation,
        TransitionKey.ACTION: batch.get("action"),
        TransitionKey.REWARD: batch.get("next.reward", 0.0),
        TransitionKey.DONE: batch.get("next.done", False),
        TransitionKey.TRUNCATED: batch.get("next.truncated", False),
        TransitionKey.INFO: batch.get("info", {}),
        TransitionKey.COMPLEMENTARY_DATA: complementary_data,
    }
    return transition

def _default_transition_to_batch(transition):
    """Default conversion from EnvTransition dictionary to batch dict."""
    batch = {
        "action": transition.get(TransitionKey.ACTION),
        "next.reward": transition.get(TransitionKey.REWARD, 0.0),
        "next.done": transition.get(TransitionKey.DONE, False),
        "next.truncated": transition.get(TransitionKey.TRUNCATED, False),
        "info": transition.get(TransitionKey.INFO, {}),
    }

    # Add padding and task data from complementary_data
    complementary_data = transition.get(TransitionKey.COMPLEMENTARY_DATA)
    if complementary_data:
        pad_data = {k: v for k, v in complementary_data.items() if "_is_pad" in k}
        batch.update(pad_data)
        if "task" in complementary_data:
            batch["task"] = complementary_data["task"]

    # Handle observation - flatten dict to observation.* keys if it's a dict
    observation = transition.get(TransitionKey.OBSERVATION)
    if isinstance(observation, dict):
        batch.update(observation)

    return batch
```

### Custom Conversion Functions

You can customize how RobotProcessor converts between formats:

```python
def custom_batch_to_transition(batch):
    """Custom conversion from batch dict to EnvTransition dictionary."""
    # Extract observation keys (anything starting with "observation.")
    observation = {k: v for k, v in batch.items() if k.startswith("observation.")}

    return {
        TransitionKey.OBSERVATION: observation,
        TransitionKey.ACTION: batch.get("action"),
        TransitionKey.REWARD: batch.get("reward", 0.0),  # Use "reward" instead of "next.reward"
        TransitionKey.DONE: batch.get("done", False),  # Use "done" instead of "next.done"
        TransitionKey.TRUNCATED: batch.get("truncated", False),
        TransitionKey.INFO: batch.get("info", {}),
        TransitionKey.COMPLEMENTARY_DATA: batch.get("complementary_data", {})
    }

def custom_transition_to_batch(transition):
    """Custom conversion from EnvTransition dictionary to batch dict."""
    batch = {
        "action": transition.get(TransitionKey.ACTION),
        "reward": transition.get(TransitionKey.REWARD),        # Use "reward" instead of "next.reward"
        "done": transition.get(TransitionKey.DONE),            # Use "done" instead of "next.done"
        "truncated": transition.get(TransitionKey.TRUNCATED),
        "info": transition.get(TransitionKey.INFO),
    }

    # Flatten observation dict
    obs = transition.get(TransitionKey.OBSERVATION)
    if obs:
        batch.update(obs)

    return batch

# Use custom converters
processor = RobotProcessor(
    steps=[ImageProcessor()],
    to_transition=custom_batch_to_transition,
    to_output=custom_transition_to_batch
)
```

### Advanced: Controlling Output Format with `to_output`

The `to_output` function determines what format is returned when you call the processor with a batch dictionary. Sometimes you want to output `EnvTransition` dictionaries even when you input batch dictionaries:

```python
# Identity function to always return EnvTransition dictionaries
def keep_as_transition(transition):
    """Always return EnvTransition dictionary regardless of input format."""
    return transition

# Processor that always outputs EnvTransition dictionaries
processor = RobotProcessor(
    steps=[ImageProcessor(), StateProcessor()],
    to_output=keep_as_transition  # Always return dictionary format
)

# Even when called with batch dict, returns EnvTransition dictionary
batch = {
    "observation.image": image_tensor,
    "action": action_tensor,
    "next.reward": reward_tensor,
    "next.done": done_tensor,
    "next.truncated": truncated_tensor,
    "info": info_dict
}

result = processor(batch)  # Returns EnvTransition dictionary, not batch dict!
print(type(result))  # <class 'dict'>
```

### Real-World Example: Environment Interaction

This is particularly useful for environment interaction where you want consistent dictionary output:

```python
from lerobot.processor.observation_processor import VanillaObservationProcessor

# Create processor that always outputs EnvTransition for environment interaction
# This avoids format conversion overhead during real-time control
env_processor = RobotProcessor(
    [VanillaObservationProcessor()],
    to_transition=lambda x: x,  # Pass through - no conversion needed
    to_output=lambda x: x,      # Always return EnvTransition dictionary
)

# Environment interaction loop
env = make_env()
obs, info = env.reset()

for step in range(1000):
    # Create transition - input is already in dictionary format
    transition = {
        TransitionKey.OBSERVATION: obs,
        TransitionKey.ACTION: None,
        TransitionKey.REWARD: 0.0,
        TransitionKey.DONE: False,
        TransitionKey.TRUNCATED: False,
        TransitionKey.INFO: info,
        TransitionKey.COMPLEMENTARY_DATA: {"step": step}
    }

    # Process - output is guaranteed to be EnvTransition dictionary
    processed_transition = env_processor(transition)
    processed_obs = processed_transition[TransitionKey.OBSERVATION]

    # Use with policy
    action = policy.select_action(processed_obs)
    obs, reward, done, truncated, info = env.step(action)

    if done or truncated:
        break
```

### When to Use Different Output Formats

**Use EnvTransition dictionary output when:**

- Environment interaction and real-time control
- You need to access individual transition components frequently
- Working with gym environments that expect structured data
- You need the flexibility of dictionary operations

**Use batch dictionary output when:**

- Training with LeRobot datasets
- Working with DataLoaders and batched processing
- Interfacing with existing LeRobot training code
- You need the standardized "next.\*" key format

```python
# For environment interaction - use dictionary output
env_processor = RobotProcessor(
    steps=[ImageProcessor(), StateProcessor()],
    to_output=lambda x: x  # Return EnvTransition dictionary
)

# For training - use batch output (default)
train_processor = RobotProcessor(
    steps=[ImageProcessor(), StateProcessor(), NormalizerProcessor(...)],
    # to_output defaults to _default_transition_to_batch
)

# Training loop
for batch in dataloader:
    processed_batch = train_processor(batch)  # Returns batch dict
    loss = model.compute_loss(processed_batch)

# Environment loop
for step in range(1000):
    transition = {
        TransitionKey.OBSERVATION: obs,
        TransitionKey.ACTION: None,
        TransitionKey.REWARD: 0.0,
        TransitionKey.DONE: False,
        TransitionKey.TRUNCATED: False,
        TransitionKey.INFO: info,
        TransitionKey.COMPLEMENTARY_DATA: {}
    }
    processed_transition = env_processor(transition)  # Returns EnvTransition dictionary
    obs = processed_transition[TransitionKey.OBSERVATION]
    action = policy.select_action(obs)
```

### Why "next.reward", "next.done", "next.truncated"?

The default conversion uses "next.\*" prefixes because this matches the standard format used by LeRobot datasets and follows the convention that rewards, done flags, and truncated flags are the result of taking an action (i.e., they come from the "next" state):

```python
# Standard RL transition format
# (s_t, a_t, r_{t+1}, done_{t+1}, truncated_{t+1})
#  ^     ^     ^        ^           ^
#  |     |     |        |           |
#  |     |     |        |           +-- Result of action a_t
#  |     |     |        +-- Result of action a_t
#  |     |     +-- Result of action a_t
#  |     +-- Action taken in state s_t
#  +-- State at time t

batch = {
    "observation.state": s_t,
    "action": a_t,
    "next.reward": r_{t+1},      # Reward received after taking action
    "next.done": done_{t+1},     # Done flag after taking action
    "next.truncated": truncated_{t+1},  # Truncated flag after taking action
}
```

## Your First RobotProcessor

Let's create a processor that properly handles image and state preprocessing:

```python
from lerobot.processor.pipeline import RobotProcessor, TransitionKey
from lerobot.processor.observation_processor import ImageProcessor, StateProcessor
import numpy as np

# Create sample data
observation = {
    "pixels": {
        "camera_front": np.random.randint(0, 255, (480, 640, 3), dtype=np.uint8),
        "camera_side": np.random.randint(0, 255, (480, 640, 3), dtype=np.uint8)
    },
    "agent_pos": np.array([0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7], dtype=np.float32)
}

# Create a full transition
transition = {
    TransitionKey.OBSERVATION: observation,
    TransitionKey.ACTION: None,
    TransitionKey.REWARD: 0.0,
    TransitionKey.DONE: False,
    TransitionKey.TRUNCATED: False,
    TransitionKey.INFO: {},
    TransitionKey.COMPLEMENTARY_DATA: {}
}

# Create and use the processor
processor = RobotProcessor([
    ImageProcessor(),  # Converts uint8[0,255] to float32[0,1], HWC to CHW, adds batch dim
    StateProcessor(),  # Converts numpy arrays to torch tensors, adds batch dim
])

processed_transition = processor(transition)
processed_obs = processed_transition[TransitionKey.OBSERVATION]

# Check the results
print("Original keys:", observation.keys())
print("Processed keys:", processed_obs.keys())
print("Image shape:", processed_obs["observation.images.camera_front"].shape)  # [1, 3, 480, 640]
print("Image dtype:", processed_obs["observation.images.camera_front"].dtype)   # torch.float32
print("Image range:", processed_obs["observation.images.camera_front"].min().item(),
      "to", processed_obs["observation.images.camera_front"].max().item())      # 0.0 to 1.0
```

## Working with LeRobot Datasets and Replay Buffers

RobotProcessor seamlessly works with LeRobot's batch dictionary format:

```python
from lerobot.datasets.lerobot_dataset import LeRobotDataset
from lerobot.processor.pipeline import RobotProcessor
from lerobot.processor.normalize_processor import NormalizerProcessor
from lerobot.configs.types import FeatureType, NormalizationMode, PolicyFeature

# Load a dataset
dataset = LeRobotDataset("lerobot/pusht")

# Define features and normalization
features = {
    "observation.image": PolicyFeature(FeatureType.VISUAL, (3, 96, 96)),
    "observation.state": PolicyFeature(FeatureType.STATE, (2,)),
    "action": PolicyFeature(FeatureType.ACTION, (2,)),
}

norm_map = {
    FeatureType.VISUAL: NormalizationMode.MEAN_STD,
    FeatureType.STATE: NormalizationMode.MIN_MAX,
    FeatureType.ACTION: NormalizationMode.MEAN_STD,
}

# Create processor with normalization
processor = RobotProcessor([
    NormalizerProcessor.from_lerobot_dataset(dataset, features, norm_map),
])

# Process a batch from the dataset
batch = dataset[0]  # Get first batch
print("Original batch keys:", list(batch.keys()))
print("Original image shape:", batch["observation.image"].shape)

# Process the batch - automatically converts to/from batch format
processed_batch = processor(batch)
print("Processed batch keys:", list(processed_batch.keys()))
print("Processed image range:", processed_batch["observation.image"].min().item(),
      "to", processed_batch["observation.image"].max().item())

# Use with DataLoader
from torch.utils.data import DataLoader

dataloader = DataLoader(dataset, batch_size=32, shuffle=True)

for batch in dataloader:
    # Process entire batch at once
    processed_batch = processor(batch)

    # Use processed batch for training
    # model.train_step(processed_batch)
    break
```

## Integration with Replay Buffers

RobotProcessor works great with replay buffers for online learning:

```python
from lerobot.common.utils.buffer import ReplayBuffer
from lerobot.processor.pipeline import RobotProcessor
from lerobot.processor.device_processor import DeviceProcessor

# Create replay buffer
buffer = ReplayBuffer(capacity=10000)

# Create processor for online data
online_processor = RobotProcessor([
    ImageProcessor(),
    StateProcessor(),
    DeviceProcessor(device="cuda"),
])

# During environment interaction
env = make_env()
obs, info = env.reset()

for step in range(1000):
    # Raw environment observation
    transition = {
        TransitionKey.OBSERVATION: obs,
        TransitionKey.ACTION: None,
        TransitionKey.REWARD: 0.0,
        TransitionKey.DONE: False,
        TransitionKey.TRUNCATED: False,
        TransitionKey.INFO: info,
        TransitionKey.COMPLEMENTARY_DATA: {}
    }

    # Process for policy input
    processed_transition = online_processor(transition)
    processed_obs = processed_transition[TransitionKey.OBSERVATION]

    # Get action from policy
    action = policy.select_action(processed_obs)

    # Execute action
    next_obs, reward, done, truncated, info = env.step(action)

    # Store in replay buffer (can store either format)
    buffer.add(obs, action, reward, next_obs, done, truncated, info)

    obs = next_obs
    if done or truncated:
        obs, info = env.reset()

# Sample and process batches for training
batch = buffer.sample(batch_size=32)
processed_batch = online_processor(batch)  # Processes entire batch
```

## Creating Custom Steps: The ProcessorStep Protocol

A processor step must follow certain conventions. Let's create a complete example that shows all required and optional methods:

```python
from dataclasses import dataclass
from typing import Any, Dict
import torch
import torch.nn.functional as F

@dataclass
class ImagePadder:
    """Pad images to a standard size for batch processing."""

    target_height: int = 224
    target_width: int = 224
    pad_value: float = 0.0

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        """Main processing method - required for all steps."""
        obs = transition.get(TransitionKey.OBSERVATION)

        if obs is None:
            return transition

        # Process all image observations
        processed_obs = dict(obs)  # Create a copy
        for key in list(processed_obs.keys()):
            if key.startswith("observation.images."):
                img = processed_obs[key]
                # Calculate padding
                _, _, h, w = img.shape
                pad_h = max(0, self.target_height - h)
                pad_w = max(0, self.target_width - w)

                if pad_h > 0 or pad_w > 0:
                    # Pad symmetrically
                    pad_left = pad_w // 2
                    pad_right = pad_w - pad_left
                    pad_top = pad_h // 2
                    pad_bottom = pad_h - pad_top

                    img = F.pad(img, (pad_left, pad_right, pad_top, pad_bottom),
                               mode='constant', value=self.pad_value)

                processed_obs[key] = img

        # Return modified transition
        new_transition = transition.copy()
        new_transition[TransitionKey.OBSERVATION] = processed_obs
        return new_transition

    def get_config(self) -> Dict[str, Any]:
        """Return JSON-serializable configuration - required for save/load."""
        return {
            "target_height": self.target_height,
            "target_width": self.target_width,
            "pad_value": self.pad_value
        }

    def state_dict(self) -> Dict[str, torch.Tensor]:
        """Return tensor state - only include torch.Tensor objects!"""
        # This step has no learnable parameters
        return {}

    def load_state_dict(self, state: Dict[str, torch.Tensor]) -> None:
        """Load tensor state - required if state_dict returns non-empty dict."""
        # Nothing to load for this step
        pass

    def reset(self) -> None:
        """Reset internal state at episode boundaries - required for stateful steps."""
        # This step is stateless, so nothing to reset
        pass
```

### Important: get_config vs state_dict

These two methods serve different purposes and it's crucial to use them correctly:

```python
@dataclass
class AdaptiveNormalizer:
    """Example showing proper use of get_config and state_dict."""

    learning_rate: float = 0.01
    epsilon: float = 1e-8

    def __post_init__(self):
        self.running_mean = None
        self.running_var = None
        self.num_samples = 0  # Python int, not tensor

    def get_config(self) -> Dict[str, Any]:
        """ONLY Python objects that can be JSON serialized!"""
        return {
            "learning_rate": self.learning_rate,  # float ✓
            "epsilon": self.epsilon,              # float ✓
            "num_samples": self.num_samples,      # int ✓
            # "running_mean": self.running_mean,  # torch.Tensor ✗ WRONG!
        }

    def state_dict(self) -> Dict[str, torch.Tensor]:
        """ONLY torch.Tensor objects!"""
        if self.running_mean is None:
            return {}
        return {
            "running_mean": self.running_mean,    # torch.Tensor ✓
            "running_var": self.running_var,      # torch.Tensor ✓
            # "num_samples": self.num_samples,    # int ✗ WRONG!
            # Instead, convert to tensor if needed:
            "num_samples_tensor": torch.tensor(self.num_samples)
        }

    def load_state_dict(self, state: Dict[str, torch.Tensor]) -> None:
        """Load tensors and convert back to Python types if needed."""
        self.running_mean = state.get("running_mean")
        self.running_var = state.get("running_var")
        if "num_samples_tensor" in state:
            self.num_samples = int(state["num_samples_tensor"].item())
```

## Using Complementary Data for Inter-Step Communication

The `complementary_data` field is perfect for passing information between steps without polluting the info dictionary:

```python
@dataclass
class ImageStatisticsCalculator:
    """Calculate image statistics and pass to next steps."""

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        obs = transition.get(TransitionKey.OBSERVATION)
        comp_data = transition.get(TransitionKey.COMPLEMENTARY_DATA) or {}

        if obs is None:
            return transition

        # Calculate statistics for all images
        image_stats = {}
        for key in obs:
            if key.startswith("observation.images."):
                img = obs[key]
                stats = {
                    "mean": img.mean().item(),
                    "std": img.std().item(),
                    "min": img.min().item(),
                    "max": img.max().item(),
                }
                image_stats[key] = stats

        # Store in complementary_data for next steps
        comp_data = dict(comp_data)  # Make a copy
        comp_data["image_statistics"] = image_stats

        # Return transition with updated complementary_data
        new_transition = transition.copy()
        new_transition[TransitionKey.COMPLEMENTARY_DATA] = comp_data
        return new_transition

@dataclass
class AdaptiveBrightnessAdjuster:
    """Adjust brightness based on statistics from previous step."""

    target_brightness: float = 0.5

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        obs = transition.get(TransitionKey.OBSERVATION)
        comp_data = transition.get(TransitionKey.COMPLEMENTARY_DATA) or {}

        if obs is None or "image_statistics" not in comp_data:
            return transition

        # Use statistics from previous step
        image_stats = comp_data["image_statistics"]

        processed_obs = dict(obs)  # Create a copy
        for key in processed_obs:
            if key.startswith("observation.images.") and key in image_stats:
                current_mean = image_stats[key]["mean"]
                brightness_adjust = self.target_brightness - current_mean

                # Adjust brightness
                processed_obs[key] = torch.clamp(processed_obs[key] + brightness_adjust, 0, 1)

        new_transition = transition.copy()
        new_transition[TransitionKey.OBSERVATION] = processed_obs
        return new_transition

# Use them together
processor = RobotProcessor([
    ImageProcessor(),
    ImageStatisticsCalculator(),  # Calculates stats
    AdaptiveBrightnessAdjuster(), # Uses stats from previous step
])
```

## Loading Processors with Overrides: Handling Non-Serializable Objects

One of the most powerful features of RobotProcessor is the ability to override step configurations when loading from saved checkpoints. This is particularly useful for handling non-serializable objects like environment instances, database connections, or hardware interfaces that can't be saved to JSON.

### The Problem: Non-Serializable Parameters

Imagine you have a processor step that needs a live gym environment:

```python
from dataclasses import dataclass
import gym

@ProcessorStepRegistry.register("action_repeat_step")
@dataclass
class ActionRepeatStep:
    """Step that repeats actions using environment feedback."""

    repeat_count: int = 3
    env: gym.Env = None  # This can't be serialized to JSON!

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        action = transition.get(TransitionKey.ACTION)

        if self.env is not None and action is not None:
            # Repeat action multiple times in environment
            total_reward = 0
            for _ in range(self.repeat_count):
                _, r, d, t, _ = self.env.step(action)
                total_reward += r
                if d or t:
                    break

            # Update reward in transition
            new_transition = transition.copy()
            new_transition[TransitionKey.REWARD] = total_reward
            return new_transition

        return transition

    def get_config(self) -> dict[str, Any]:
        # Note: env is NOT included because it's not serializable
        return {"repeat_count": self.repeat_count}

    def state_dict(self) -> dict[str, torch.Tensor]:
        return {}

    def load_state_dict(self, state: dict[str, torch.Tensor]) -> None:
        pass
```

If you try to save and load this processor normally, the `env` parameter will be lost because it can't be serialized to JSON.

### The Solution: Override Parameters

The `overrides` parameter in `from_pretrained()` allows you to provide non-serializable objects when loading:

```python
from lerobot.processor.pipeline import RobotProcessor

# Create processor with environment
env = gym.make("CartPole-v1")
action_step = ActionRepeatStep(repeat_count=2, env=env)
processor = RobotProcessor([action_step], name="CartPoleProcessor")

# Save the processor (env won't be saved)
processor.save_pretrained("./cartpole_processor")

# Later, load with environment override
new_env = gym.make("CartPole-v1")
loaded_processor = RobotProcessor.from_pretrained(
    "./cartpole_processor",
    overrides={
        "action_repeat_step": {"env": new_env}  # Provide the environment
    }
)
```

### How Overrides Work

The `overrides` parameter is a dictionary where:

- **Keys** are step identifiers (class names for unregistered steps, registry names for registered steps)
- **Values** are dictionaries of parameter overrides that get merged with saved configurations

```python
overrides = {
    "StepClassName": {"param1": "new_value", "param2": 42},
    "registered_step_name": {"device": "cuda", "learning_rate": 0.01}
}
```

The loading process:

1. Load saved configuration from JSON
2. For each step, check if overrides exist for that step
3. Merge override parameters with saved parameters (overrides take precedence)
4. Instantiate the step with merged configuration
5. Load any saved tensor state

### Real-World Examples

#### Example 1: Environment-Dependent Steps

```python
@ProcessorStepRegistry.register("ik_solver_step")
@dataclass
class InverseKinematicsStep:
    """Convert Cartesian positions to joint angles."""

    robot_model: str = "ur5"
    solver_timeout: float = 0.1
    kinematics_solver: Any = None  # Non-serializable solver instance

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        obs, action, reward, done, truncated, info, comp_data = transition

        if self.kinematics_solver is not None and action is not None:
            # Convert Cartesian action to joint angles
            joint_angles = self.kinematics_solver.solve(action, timeout=self.solver_timeout)
            action = joint_angles

        return (obs, action, reward, done, truncated, info, comp_data)

    def get_config(self) -> dict[str, Any]:
        return {
            "robot_model": self.robot_model,
            "solver_timeout": self.solver_timeout
        }

# Save processor without solver
processor = RobotProcessor([InverseKinematicsStep(robot_model="ur5")])
processor.save_pretrained("./robot_processor")

# Load with solver instance
from robotics_toolbox import URKinematics
solver = URKinematics("ur5")

loaded_processor = RobotProcessor.from_pretrained(
    "./robot_processor",
    overrides={
        "ik_solver_step": {
            "kinematics_solver": solver,
            "solver_timeout": 0.05  # Also override timeout
        }
    }
)
```

#### Example 2: Device and Hardware Configuration

```python
@ProcessorStepRegistry.register("camera_capture_step")
@dataclass
class CameraCaptureStep:
    """Capture images from physical camera."""

    camera_id: int = 0
    resolution: tuple = (640, 480)
    camera_interface: Any = None  # Hardware interface

    def get_config(self) -> dict[str, Any]:
        return {
            "camera_id": self.camera_id,
            "resolution": self.resolution
        }

# Deploy on different robots with different camera setups
# Robot A
camera_a = CameraInterface("/dev/video0")
processor_a = RobotProcessor.from_pretrained(
    "shared/vision_processor",
    overrides={
        "camera_capture_step": {
            "camera_interface": camera_a,
            "resolution": (1920, 1080)  # High-res camera
        }
    }
)

# Robot B
camera_b = CameraInterface("/dev/video1")
processor_b = RobotProcessor.from_pretrained(
    "shared/vision_processor",
    overrides={
        "camera_capture_step": {
            "camera_interface": camera_b,
            "resolution": (640, 480)  # Lower-res camera
        }
    }
)
```

#### Example 3: Multiple Environment Deployment

```python
# Training processor that works with simulation
@ProcessorStepRegistry.register("physics_validator")
@dataclass
class PhysicsValidatorStep:
    """Validate actions against physics constraints."""

    max_force: float = 100.0
    physics_engine: Any = None

    def get_config(self) -> dict[str, Any]:
        return {"max_force": self.max_force}

# Different physics engines for different environments
import pybullet as pb
import mujoco

# Simulation deployment
sim_engine = pb.connect(pb.DIRECT)
sim_processor = RobotProcessor.from_pretrained(
    "shared/control_processor",
    overrides={
        "physics_validator": {
            "physics_engine": sim_engine,
            "max_force": 150.0  # Higher limits in sim
        }
    }
)

# Real robot deployment
real_engine = RealRobotInterface()
real_processor = RobotProcessor.from_pretrained(
    "shared/control_processor",
    overrides={
        "physics_validator": {
            "physics_engine": real_engine,
            "max_force": 50.0  # Conservative limits on real robot
        }
    }
)
```

### Override Key Matching Rules

The override system uses exact string matching:

#### For Registered Steps

Use the registry name (the string passed to `@ProcessorStepRegistry.register()`):

```python
@ProcessorStepRegistry.register("my_custom_step")
class MyStep:
    pass

# Use registry name in overrides
overrides = {"my_custom_step": {"param": "value"}}
```

#### For Unregistered Steps

Use the exact class name:

```python
class MyUnregisteredStep:
    pass

# Use class name in overrides
overrides = {"MyUnregisteredStep": {"param": "value"}}
```

### Error Handling and Validation

The override system provides helpful error messages:

#### Invalid Override Keys

```python
# This will raise KeyError with helpful message
overrides = {"NonExistentStep": {"param": "value"}}

try:
    processor = RobotProcessor.from_pretrained("./processor", overrides=overrides)
except KeyError as e:
    print(e)
    # Output: Override keys ['NonExistentStep'] do not match any step in the saved configuration.
    # Available step keys: ['ActualStepName', 'AnotherStepName']
```

#### Instantiation Errors

```python
# Invalid parameter types are caught
overrides = {"MyStep": {"numeric_param": "not_a_number"}}

try:
    processor = RobotProcessor.from_pretrained("./processor", overrides=overrides)
except ValueError as e:
    print(e)
    # Output: Failed to instantiate processor step 'MyStep' with config: {...}
```

### Multiple Steps with Same Class Name

When you have multiple steps of the same class, all instances get the same override:

```python
step1 = MyStep(param=1)
step2 = MyStep(param=2)
processor = RobotProcessor([step1, step2])

# Both steps will get the override
overrides = {"MyStep": {"param": 999}}
loaded = RobotProcessor.from_pretrained("./processor", overrides=overrides)
# Both steps now have param=999
```

To override steps individually, use different classes or register with different names:

```python
@ProcessorStepRegistry.register("step_1")
class MyStep:
    pass

@ProcessorStepRegistry.register("step_2")
class MyStep:  # Same class, different registry names
    pass

# Now you can override them separately
overrides = {
    "step_1": {"param": 1},
    "step_2": {"param": 2}
}
```

### Best Practices for Overrides

#### 1. Design Steps for Overrides

When creating steps that need non-serializable objects, design them with overrides in mind:

```python
@dataclass
class WellDesignedStep:
    # Serializable configuration
    timeout: float = 1.0
    retry_count: int = 3

    # Non-serializable objects with default None
    database: Any = None
    api_client: Any = None

    def __post_init__(self):
        # Validate that required non-serializable objects are provided
        if self.database is None:
            raise ValueError("database must be provided via overrides")

    def get_config(self) -> dict[str, Any]:
        # Only include serializable parameters
        return {
            "timeout": self.timeout,
            "retry_count": self.retry_count
        }
```

#### 2. Use Registry Names for Clarity

Register steps with descriptive names to make overrides clearer:

```python
@ProcessorStepRegistry.register("robot_arm_controller")
class ArmControlStep:
    pass

@ProcessorStepRegistry.register("gripper_controller")
class GripperControlStep:
    pass

# Clear override keys
overrides = {
    "robot_arm_controller": {"joint_limits": arm_limits},
    "gripper_controller": {"force_limit": gripper_force}
}
```

#### 3. Document Override Requirements

Include clear documentation about what overrides are needed:

```python
@ProcessorStepRegistry.register("vision_processor")
class VisionProcessor:
    """Process camera images for robot vision.

    Required overrides when loading:
        camera_interface: Hardware camera interface object

    Optional overrides:
        resolution: Camera resolution tuple (default: (640, 480))
        fps: Camera frame rate (default: 30)
    """
    camera_interface: Any = None
    resolution: tuple = (640, 480)
    fps: int = 30
```

#### 4. Environment-Specific Configuration Files

Create configuration helpers for different deployment environments:

```python
# config/simulation.py
def get_simulation_overrides():
    return {
        "camera_step": {"camera_interface": SimCamera()},
        "physics_step": {"engine": SimPhysics()},
        "control_step": {"safety_limits": False}
    }

# config/production.py
def get_production_overrides():
    return {
        "camera_step": {"camera_interface": RealCamera("/dev/video0")},
        "physics_step": {"engine": RealPhysics()},
        "control_step": {"safety_limits": True}
    }

# Usage
from config.production import get_production_overrides
processor = RobotProcessor.from_pretrained(
    "shared/processor",
    overrides=get_production_overrides()
)
```

### Integration with Hub Sharing

Overrides work seamlessly with Hugging Face Hub sharing:

```python
# Save processor without non-serializable objects
processor.push_to_hub("my-lab/robot-processor")

# Anyone can load and provide their own environment
import gym
local_env = gym.make("MyRobotEnv-v1")

processor = RobotProcessor.from_pretrained(
    "my-lab/robot-processor",
    overrides={"env_step": {"env": local_env}}
)
```

This enables sharing of preprocessing logic while allowing each user to provide their own environment-specific dependencies.

## Complete Example: Device-Aware Processing Pipeline

Here's a complete example showing proper device management and all features:

```python
from lerobot.processor.pipeline import RobotProcessor, ProcessorStepRegistry, TransitionKey
import torch
import torch.nn.functional as F
import numpy as np

@ProcessorStepRegistry.register("device_mover")
@dataclass
class DeviceMover:
    """Move all tensors to specified device."""

    device: str = "cuda"

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        obs = transition.get(TransitionKey.OBSERVATION)

        if obs is None:
            return transition

        # Move all tensor observations to device
        processed_obs = dict(obs)  # Create a copy
        for key, value in processed_obs.items():
            if isinstance(value, torch.Tensor):
                processed_obs[key] = value.to(self.device)

        # Also handle action if present
        action = transition.get(TransitionKey.ACTION)
        if action is not None and isinstance(action, torch.Tensor):
            action = action.to(self.device)
            new_transition = transition.copy()
            new_transition[TransitionKey.OBSERVATION] = processed_obs
            new_transition[TransitionKey.ACTION] = action
            return new_transition

        new_transition = transition.copy()
        new_transition[TransitionKey.OBSERVATION] = processed_obs
        return new_transition

    def get_config(self) -> Dict[str, Any]:
        return {"device": str(self.device)}

@ProcessorStepRegistry.register("running_normalizer")
@dataclass
class RunningNormalizer:
    """Normalize using running statistics with proper device handling."""

    feature_dim: int
    momentum: float = 0.1

    def __post_init__(self):
        # Initialize as None - will be created on first call with correct device
        self.running_mean = None
        self.running_var = None
        self.initialized = False

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        obs = transition.get(TransitionKey.OBSERVATION)

        if obs is None or "observation.state" not in obs:
            return transition

        state = obs["observation.state"]

        # Initialize on first call with correct device
        if not self.initialized:
            device = state.device
            self.running_mean = torch.zeros(self.feature_dim, device=device)
            self.running_var = torch.ones(self.feature_dim, device=device)
            self.initialized = True

        # Update statistics
        with torch.no_grad():
            batch_mean = state.mean(dim=0)
            batch_var = state.var(dim=0, unbiased=False)

            self.running_mean = (1 - self.momentum) * self.running_mean + self.momentum * batch_mean
            self.running_var = (1 - self.momentum) * self.running_var + self.momentum * batch_var

        # Normalize
        state_normalized = (state - self.running_mean) / (self.running_var + 1e-8).sqrt()
        processed_obs = dict(obs)  # Create a copy
        processed_obs["observation.state"] = state_normalized

        new_transition = transition.copy()
        new_transition[TransitionKey.OBSERVATION] = processed_obs
        return new_transition

    def get_config(self) -> Dict[str, Any]:
        return {
            "feature_dim": self.feature_dim,
            "momentum": self.momentum,
            "initialized": self.initialized
        }

    def state_dict(self) -> Dict[str, torch.Tensor]:
        if not self.initialized:
            return {}
        return {
            "running_mean": self.running_mean,
            "running_var": self.running_var
        }

    def load_state_dict(self, state: Dict[str, torch.Tensor]) -> None:
        if state:
            self.running_mean = state["running_mean"]
            self.running_var = state["running_var"]
            self.initialized = True

    def reset(self) -> None:
        # Don't reset statistics - they persist across episodes
        pass

# Create a complete pipeline
processor = RobotProcessor([
    ImageProcessor(),      # Convert images to float32 [0,1]
    StateProcessor(),      # Convert states to torch tensors
    ImagePadder(target_height=224, target_width=224), # Pad images to standard size
    DeviceMover("cuda"),   # Move everything to GPU
    RunningNormalizer(7),  # Normalize states
], name="CompletePreprocessor")

# The processor handles device transfers automatically
processor = processor.to("cuda")  # Moves all stateful components to GPU

# Use it
obs = {
    "pixels": {"cam": np.random.randint(0, 255, (200, 300, 3), dtype=np.uint8)},
    "agent_pos": np.random.randn(7).astype(np.float32)
}
transition = {
    TransitionKey.OBSERVATION: obs,
    TransitionKey.ACTION: None,
    TransitionKey.REWARD: 0.0,
    TransitionKey.DONE: False,
    TransitionKey.TRUNCATED: False,
    TransitionKey.INFO: {},
    TransitionKey.COMPLEMENTARY_DATA: {}
}

# Everything is processed and on GPU
processed = processor(transition)
print(processed[TransitionKey.OBSERVATION]["observation.images.cam"].device)  # cuda:0
```

## Solving Real-World Problems with RobotProcessor

Let's see how RobotProcessor elegantly solves the scenarios we discussed earlier:

### Renaming Observation Keys

When your environment and policy speak different "languages":

```python
@ProcessorStepRegistry.register("key_remapper")
@dataclass
class KeyRemapper:
    """Rename observation keys to match policy expectations."""

    key_mapping: Dict[str, str] = field(default_factory=lambda: {
        "rgb_camera_front": "observation.images.wrist",
        "joint_positions": "observation.state",
        "gripper_state": "observation.gripper"
    })

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        obs = transition.get(TransitionKey.OBSERVATION)
        if obs is None:
            return transition

        # Create new observation with renamed keys
        processed_obs = dict(obs)  # Create a copy
        renamed_obs = {}
        for old_key, new_key in self.key_mapping.items():
            if old_key in processed_obs:
                renamed_obs[new_key] = processed_obs[old_key]

        # Keep any unmapped keys as-is
        for key, value in processed_obs.items():
            if key not in self.key_mapping:
                renamed_obs[key] = value

        new_transition = transition.copy()
        new_transition[TransitionKey.OBSERVATION] = renamed_obs
        return new_transition
```

### Workspace-Focused Image Processing

When you need to crop and resize images to focus on the manipulation workspace:

```python
@ProcessorStepRegistry.register("workspace_cropper")
@dataclass
class WorkspaceCropper:
    """Crop and resize images to focus on robot workspace."""

    crop_bbox: Tuple[int, int, int, int] = (400, 200, 1200, 800)  # (x1, y1, x2, y2)
    output_size: Tuple[int, int] = (224, 224)

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        obs = transition.get(TransitionKey.OBSERVATION)
        if obs is None:
            return transition

        processed_obs = dict(obs)  # Create a copy
        for key in list(processed_obs.keys()):
            if key.startswith("observation.images."):
                img = processed_obs[key]
                # Crop to workspace
                x1, y1, x2, y2 = self.crop_bbox
                img_cropped = img[:, :, y1:y2, x1:x2]
                # Resize to expected dimensions
                img_resized = F.interpolate(
                    img_cropped,
                    size=self.output_size,
                    mode='bilinear',
                    align_corners=False
                )
                processed_obs[key] = img_resized

        new_transition = transition.copy()
        new_transition[TransitionKey.OBSERVATION] = processed_obs
        return new_transition
```

### Building Complete Pipelines for Different Robots

Now you can compose these steps into robot-specific pipelines:

```python
# Pipeline for Robot A with wrist camera
robot_a_processor = RobotProcessor([
    # Observation preprocessing
    KeyRemapper({
        "wrist_rgb": "observation.images.wrist",
        "arm_joints": "observation.state"
    }),
    ImageProcessor(),
    WorkspaceCropper(crop_bbox=(300, 100, 900, 700), output_size=(224, 224)),
    StateProcessor(),
    VelocityCalculator(state_key="observation.state"),
    DeviceMover("cuda"),

    # Action postprocessing
    ActionSmoother(alpha=0.2),
], name="RobotA_ACT_Processor")

# Pipeline for Robot B with overhead camera
robot_b_processor = RobotProcessor([
    # Different key mapping for Robot B
    KeyRemapper({
        "overhead_cam": "observation.images.wrist",  # Reuse same policy!
        "joint_encoders": "observation.state"
    }),
    ImageProcessor(),
    WorkspaceCropper(crop_bbox=(100, 50, 1100, 950), output_size=(224, 224)),
    StateProcessor(),
    VelocityCalculator(state_key="observation.state"),
    DeviceMover("cuda"),

    ActionSmoother(alpha=0.3),  # Different smoothing
], name="RobotB_ACT_Processor")

# Share processors with the community
robot_a_processor.push_to_hub("my-lab/robot-a-act-processor")
robot_b_processor.push_to_hub("my-lab/robot-b-act-processor")

# Now anyone can use the same preprocessing
processor = RobotProcessor.from_pretrained("my-lab/robot-a-act-processor")
```

The beauty of this approach is that:

1. **Each transformation is tested independently** - The IK solver can be validated separately from image cropping
2. **Configurations are shareable** - Other labs can use your exact preprocessing setup
3. **Pipelines are composable** - Mix and match steps for different robots
4. **Everything is version controlled** - Use Hub revisions to track changes

## Best Practices for Processor Steps

### 1. Always Check for None

```python
def __call__(self, transition: EnvTransition) -> EnvTransition:
    obs = transition.get(TransitionKey.OBSERVATION)

    # Always check if observation exists
    if obs is None:
        return transition

    # Also check for specific keys
    if "observation.state" not in obs:
        return transition
```

### 2. Preserve Transition Structure

```python
# Good - preserve all elements
return (modified_obs, *transition[1:])

# Bad - losing information
return (modified_obs, None, 0.0, False, False, {}, {})
```

### 3. Clone When Storing State

```python
def __call__(self, transition: EnvTransition) -> EnvTransition:
    obs = transition.get(TransitionKey.OBSERVATION)

    if self.store_previous:
        # Good - clone to avoid reference issues
        self.previous_state = obs["observation.state"].clone()

        # Bad - stores reference that might be modified
        # self.previous_state = obs["observation.state"]
```

### 4. Handle Device Transfers in state_dict

```python
def state_dict(self) -> Dict[str, torch.Tensor]:
    if self.buffer is None:
        return {}

    # Good - clone to avoid memory sharing issues
    return {"buffer": self.buffer.clone()}
```

## Complete Policy Example with Pre and Post Processing

Here's how to use RobotProcessor in a real robot control loop, showing both tuple and batch formats:

```python
from lerobot.processor.pipeline import RobotProcessor, ProcessorStepRegistry, TransitionKey
from lerobot.policies.act.modeling_act import ACTPolicy
from pathlib import Path
import time

# Create preprocessing pipeline
preprocessor = RobotProcessor([
    ImageProcessor(),
    StateProcessor(),
    ImagePadder(target_height=224, target_width=224),
    DeviceMover("cuda"),
    RunningNormalizer(feature_dim=14),  # For 14-DOF robot
], name="ACTPreprocessor")

# Create postprocessing pipeline for actions
@ProcessorStepRegistry.register("action_clipper")
@dataclass
class ActionClipper:
    """Clip actions to safe ranges."""
    min_value: float = -1.0
    max_value: float = 1.0

    def __call__(self, transition: EnvTransition) -> EnvTransition:
        action = transition.get(TransitionKey.ACTION)

        if action is not None:
            action = torch.clamp(action, self.min_value, self.max_value)
            new_transition = transition.copy()
            new_transition[TransitionKey.ACTION] = action
            return new_transition

        return transition

    def get_config(self) -> Dict[str, Any]:
        return {"min_value": self.min_value, "max_value": self.max_value}

postprocessor = RobotProcessor([
    ActionClipper(min_value=-0.5, max_value=0.5),
    ActionSmoother(alpha=0.3),
], name="ACTPostprocessor")

# Load policy
policy = ACTPolicy.from_pretrained("lerobot/act_aloha_sim_transfer_cube_human")

# Move everything to GPU
preprocessor = preprocessor.to("cuda")
postprocessor = postprocessor.to("cuda")
policy = policy.to("cuda")

# Control loop using EnvTransition format
env = make_robot_env()
obs, info = env.reset()

for episode in range(10):
    print(f"Episode {episode + 1}")

    for step in range(1000):
        # Create transition with raw observation
        transition = {
            TransitionKey.OBSERVATION: obs,
            TransitionKey.ACTION: None,
            TransitionKey.REWARD: 0.0,
            TransitionKey.DONE: False,
            TransitionKey.TRUNCATED: False,
            TransitionKey.INFO: info,
            TransitionKey.COMPLEMENTARY_DATA: {"step": step}
        }

        # Preprocess - works with dictionary format
        processed_transition = preprocessor(transition)
        processed_obs = processed_transition.get(TransitionKey.OBSERVATION)

        # Get action from policy
        with torch.no_grad():
            action = policy.select_action(processed_obs)

        # Postprocess action
        action_transition = {
            TransitionKey.OBSERVATION: processed_obs,
            TransitionKey.ACTION: action,
            TransitionKey.REWARD: 0.0,
            TransitionKey.DONE: False,
            TransitionKey.TRUNCATED: False,
            TransitionKey.INFO: info,
            TransitionKey.COMPLEMENTARY_DATA: {"raw_action": action.clone()}  # Store raw action in complementary_data
        }
        processed_action_transition = postprocessor(action_transition)
        final_action = processed_action_transition.get(TransitionKey.ACTION)

        # Execute action
        obs, reward, terminated, truncated, info = env.step(final_action.cpu().numpy())

        if terminated or truncated:
            # Reset at episode boundary
            preprocessor.reset()
            postprocessor.reset()
            obs, info = env.reset()
            break

    # Save preprocessor with learned statistics
    preprocessor.save_pretrained(f"./checkpoints/preprocessor_ep{episode}")

# Alternative: Using batch dictionary format
# This is useful when integrating with existing LeRobot training code
def control_loop_with_batch_format():
    """Example using batch dictionary format."""
    obs, info = env.reset()

    for step in range(1000):
        # Create batch dictionary
        batch = {
            "observation.image": torch.from_numpy(obs["pixels"]).unsqueeze(0),
            "observation.state": torch.from_numpy(obs["agent_pos"]).unsqueeze(0),
            "action": torch.zeros(1, 7),  # Placeholder
            "next.reward": torch.tensor([0.0]),
            "next.done": torch.tensor([False]),
            "next.truncated": torch.tensor([False]),
            "info": info,
        }

        # Preprocess - works with batch format
        processed_batch = preprocessor(batch)

        # Get action from policy
        with torch.no_grad():
            action = policy.select_action({
                k: v for k, v in processed_batch.items()
                if k.startswith("observation.")
            })

        # Add action to batch for postprocessing
        processed_batch["action"] = action

        # Postprocess
        final_batch = postprocessor(processed_batch)
        final_action = final_batch["action"]

        # Execute action
        obs, reward, terminated, truncated, info = env.step(final_action.cpu().numpy())

        if terminated or truncated:
            break

# Push final version to hub
preprocessor.push_to_hub("my-username/act-preprocessor")
postprocessor.push_to_hub("my-username/act-postprocessor")
```

## Debugging and Monitoring

Use the full power of `RobotProcessor` for debugging:

```python
# Enable detailed logging
def log_observation_shapes(step_idx: int, transition: EnvTransition):
    obs = transition.get(TransitionKey.OBSERVATION)
    if obs:
        print(f"Step {step_idx} observations:")
        for key, value in obs.items():
            if isinstance(value, torch.Tensor):
                print(f"  {key}: shape={value.shape}, device={value.device}, dtype={value.dtype}")
    return None

processor.register_after_step_hook(log_observation_shapes)

# Monitor complementary data flow
def monitor_complementary_data(step_idx: int, transition: EnvTransition):
    comp_data = transition.get(TransitionKey.COMPLEMENTARY_DATA)
    if comp_data:
        print(f"Step {step_idx} complementary_data: {list(comp_data.keys())}")
    return None

processor.register_before_step_hook(monitor_complementary_data)

# Validate data integrity
def validate_tensors(step_idx: int, transition: EnvTransition):
    obs = transition.get(TransitionKey.OBSERVATION)
    if obs:
        for key, value in obs.items():
            if isinstance(value, torch.Tensor):
                if torch.isnan(value).any():
                    raise ValueError(f"NaN detected in {key} after step {step_idx}")
                if torch.isinf(value).any():
                    raise ValueError(f"Inf detected in {key} after step {step_idx}")
    return None

processor.register_after_step_hook(validate_tensors)
```

## Summary

RobotProcessor provides a powerful, modular approach to data preprocessing in robotics:

- **Dual format support**: Works seamlessly with both EnvTransition dictionaries and batch dictionaries
- **Automatic format conversion**: Converts between dictionary and batch formats as needed
- **LeRobot integration**: Native support for LeRobotDataset and ReplayBuffer formats
- **Clear separation of concerns**: Each transformation is a separate, testable unit
- **Proper state management**: Clear distinction between config (JSON) and state (tensors)
- **Device-aware**: Seamless GPU/CPU transfers with `.to(device)`
- **Inter-step communication**: Use `complementary_data` for passing information
- **Easy sharing**: Push to Hugging Face Hub for reproducibility
- **Type safety**: Use `TransitionKey` instead of magic numbers
- **Debugging tools**: Step through transformations and add monitoring hooks
- **Flexible conversion**: Customize `to_transition` and `to_output` functions for specific needs

Key advantages of the dual format approach:

- **Environment interaction**: Use dictionary format for real-time robot control
- **Training/evaluation**: Use batch format for dataset processing and model training
- **Seamless integration**: Same processor works with both formats automatically
- **Backward compatibility**: Existing code using either format continues to work
- **Output format control**: Use `to_output` to control return format regardless of input format
- **Performance optimization**: Avoid unnecessary format conversions during time-critical operations

By following these patterns, your preprocessing code becomes more maintainable, shareable, and robust while being compatible with the entire LeRobot ecosystem.

For the full API reference, see the [RobotProcessor API documentation](/api/processor).
