fix: nanoVLM processing

docs/source/recap_value_function_experiments.mdx

@@ -52,9 +52,14 @@ lerobot-train \
   --dataset.repo_id=<dataset_repo_id> \
   --output_dir=outputs/vf_nanovlm_probe \
   --steps=5000 \
-  --batch_size=16
+  --batch_size=1
 ```
 
+The released checkpoint's native preprocessing resizes the long image side to
+2048 and creates 512px global/split views. A 480x640 camera therefore produces
+13 vision inputs and roughly 832 image placeholders; use batch size 1 initially
+for a three-camera setup.
+
 Then load the probe checkpoint and selectively fine-tune the projector/decoder
 at a lower learning rate.
 
@@ -107,5 +112,8 @@ uv run python scripts/overfit_vf_variant.py \
   --steps=500
 ```
 
+For `nanovlm_value_function`, start with `--num_samples=2` because all overfit
+samples are held in one batch and native image tiling is memory intensive.
+
 Compare runs using held-out episode NLL/MAE, per-episode return rank correlation,
 terminal success/failure separation, and the matched-versus-shuffled image loss gap.

scripts/overfit_vf_variant.py

@@ -20,6 +20,11 @@ from lerobot.rewards.factory import make_reward_model, make_reward_pre_post_proc
 from lerobot.rewards.nanovlm_value_function.configuration_nanovlm_value_function import (
     NanoVLMVFConfig,
 )
+from lerobot.rewards.nanovlm_value_function.processor_nanovlm_value_function import (
+    NANOVLM_ATTENTION_MASK,
+    NANOVLM_IMAGES,
+    NANOVLM_INPUT_IDS,
+)
 from lerobot.rewards.temporal_siglip_value_function.configuration_temporal_siglip_value_function import (
     TemporalSiglipVFConfig,
 )
@@ -132,6 +137,20 @@ def main():
 
 
 def _collate_processed(samples):
+    if NANOVLM_IMAGES in samples[0]:
+        max_length = max(sample[NANOVLM_INPUT_IDS].shape[1] for sample in samples)
+        input_ids = []
+        attention_masks = []
+        for sample in samples:
+            padding = max_length - sample[NANOVLM_INPUT_IDS].shape[1]
+            input_ids.append(torch.nn.functional.pad(sample[NANOVLM_INPUT_IDS], (padding, 0)))
+            attention_masks.append(torch.nn.functional.pad(sample[NANOVLM_ATTENTION_MASK], (padding, 0)))
+        return {
+            NANOVLM_IMAGES: [sample[NANOVLM_IMAGES][0] for sample in samples],
+            NANOVLM_INPUT_IDS: torch.cat(input_ids),
+            NANOVLM_ATTENTION_MASK: torch.cat(attention_masks),
+        }
+
     keys = {
         *[key for key in samples[0] if key.startswith("observation.images.")],
         OBS_LANGUAGE_TOKENS,
@@ -192,7 +211,10 @@ def _image_shuffle_diagnostic(model, batch, camera_keys):
     matched_loss, _ = model(batch)
     shuffled = dict(batch)
     permutation = torch.roll(torch.arange(batch["mc_return"].shape[0], device=matched_loss.device), 1)
+    if NANOVLM_IMAGES in batch:
+        shuffled[NANOVLM_IMAGES] = [batch[NANOVLM_IMAGES][index] for index in permutation.cpu().tolist()]
     for key in camera_keys:
+        if key in batch:
             shuffled[key] = batch[key][permutation]
     shuffled_loss, _ = model(shuffled)
     print(

src/lerobot/rewards/nanovlm_value_function/configuration_nanovlm_value_function.py

@@ -12,9 +12,9 @@ from lerobot.optim import AdamWConfig, CosineDecayWithWarmupSchedulerConfig
 class NanoVLMVFConfig(RewardModelConfig):
     nanovlm_pretrained_path: str = "lusxvr/nanoVLM-460M-8k"
     nanovlm_code_path: str = "third_party/nanoVLM"
-    tokenizer_path: str = "HuggingFaceTB/SmolLM2-360M-Instruct"
+    # The checkpoint was aligned with an 8k context. Native image tiling can
-    image_resolution: tuple[int, int] = (512, 512)
+    # require thousands of placeholder tokens for several robot cameras.
-    tokenizer_max_length: int = 256
+    tokenizer_max_length: int = 8192
     num_value_bins: int = 201
     value_support_min: float = -1.0
     value_support_max: float = 0.0

src/lerobot/rewards/nanovlm_value_function/modeling_nanovlm_value_function.py

@@ -9,14 +9,14 @@ from typing import Any
 import torch
 from torch import Tensor, nn
 
-from lerobot.configs.types import FeatureType
 from lerobot.rewards.distributional_value_function.common import DistributionalValueMixin
 from lerobot.rewards.distributional_value_function.modeling_distributional_value_function import ValueHead
-from lerobot.rewards.distributional_value_function.processor_distributional_value_function import (
+from lerobot.rewards.nanovlm_value_function.processor_nanovlm_value_function import (
-    IMAGE_MASK_SUFFIX,
+    NANOVLM_ATTENTION_MASK,
+    NANOVLM_IMAGES,
+    NANOVLM_INPUT_IDS,
 )
 from lerobot.rewards.pretrained import PreTrainedRewardModel
-from lerobot.utils.constants import OBS_LANGUAGE_ATTENTION_MASK, OBS_LANGUAGE_TOKENS
 
 from .configuration_nanovlm_value_function import NanoVLMVFConfig
 
@@ -31,9 +31,6 @@ class NanoVLMVFRewardModel(DistributionalValueMixin, PreTrainedRewardModel):
         super().__init__(config)
         self.config = config
         config.validate_features()
-        self.image_keys = [
-            key for key, feature in config.input_features.items() if feature.type == FeatureType.VISUAL
-        ]
         code_path = Path(config.nanovlm_code_path)
         if not code_path.is_absolute():
             code_path = Path(__file__).resolve().parents[4] / code_path
@@ -80,25 +77,33 @@ class NanoVLMVFRewardModel(DistributionalValueMixin, PreTrainedRewardModel):
         return self._distributional_forward(batch)
 
     def _get_value_readout(self, batch: dict[str, Tensor]) -> Tensor:
-        batch_size = batch[OBS_LANGUAGE_TOKENS].shape[0]
+        input_ids = batch[NANOVLM_INPUT_IDS]
-        image_tokens = []
+        attention_mask = batch[NANOVLM_ATTENTION_MASK].bool()
-        image_masks = []
+        batch_size = input_ids.shape[0]
-        for key in self.image_keys:
+        images = self.nanovlm._process_images(batch[NANOVLM_IMAGES], input_ids.device)
-            image = batch[key]
+        text_tokens = self.nanovlm.decoder.token_embedding(input_ids)
-            mask = batch[key + IMAGE_MASK_SUFFIX].bool()
+        if images is not None:
-            features = self.nanovlm.MP(self.nanovlm.vision_encoder(image))
+            image_tokens = self.nanovlm.MP(self.nanovlm.vision_encoder(images))
-            image_tokens.append(features * mask[:, None, None].to(features.dtype))
+            placeholder_count = (input_ids == self.nanovlm.tokenizer.image_token_id).sum().item()
-            image_masks.append(mask[:, None].expand(batch_size, features.shape[1]))
+            image_token_count = image_tokens.shape[0] * image_tokens.shape[1]
-
+            if placeholder_count != image_token_count:
-        text_tokens = self.nanovlm.decoder.token_embedding(batch[OBS_LANGUAGE_TOKENS])
+                raise ValueError(
+                    "nanoVLM image placeholders do not match projected image tokens: "
+                    f"{placeholder_count} placeholders versus {image_token_count} tokens. "
+                    "The prompt may have been truncated; increase tokenizer_max_length."
+                )
+            text_tokens = self.nanovlm._replace_img_tokens_with_embd(
+                input_ids,
+                text_tokens,
+                image_tokens,
+            )
         query = self.value_query(torch.zeros(batch_size, 1, dtype=torch.long, device=text_tokens.device)).to(
             text_tokens.dtype
         )
-        inputs = torch.cat([*image_tokens, text_tokens, query], dim=1)
+        inputs = torch.cat([text_tokens, query], dim=1)
         attention_mask = torch.cat(
             [
-                *image_masks,
+                attention_mask,
-                batch[OBS_LANGUAGE_ATTENTION_MASK].bool(),
                 torch.ones(batch_size, 1, dtype=torch.bool, device=text_tokens.device),
             ],
             dim=1,

src/lerobot/rewards/nanovlm_value_function/processor_nanovlm_value_function.py

@@ -1,30 +1,180 @@
-"""Processor for the nanoVLM value-function experiment."""
+"""Processor using nanoVLM's native image splitting and chat-token layout."""
 
+import json
+import sys
+from dataclasses import dataclass, field
+from pathlib import Path
 from typing import Any
 
 import torch
+from torchvision.transforms.functional import to_pil_image
 
-from lerobot.configs import FeatureType
+from lerobot.configs import FeatureType, PipelineFeatureType, PolicyFeature
 from lerobot.processor import (
     AddBatchDimensionProcessorStep,
+    ComplementaryDataProcessorStep,
     DeviceProcessorStep,
     NormalizerProcessorStep,
     PolicyAction,
     PolicyProcessorPipeline,
+    ProcessorStepRegistry,
     RenameObservationsProcessorStep,
-    TokenizerProcessorStep,
     batch_to_transition,
     policy_action_to_transition,
     transition_to_batch,
 )
-from lerobot.rewards.distributional_value_function.processor_distributional_value_function import (
+from lerobot.types import TransitionKey
-    DistributionalVFImagePreprocessorStep,
-    DistributionalVFPrepareTaskPromptStep,
-)
 from lerobot.utils.constants import POLICY_POSTPROCESSOR_DEFAULT_NAME, POLICY_PREPROCESSOR_DEFAULT_NAME
 
 from .configuration_nanovlm_value_function import NanoVLMVFConfig
 
+NANOVLM_IMAGES = "observation.nanovlm.images"
+NANOVLM_INPUT_IDS = "observation.nanovlm.input_ids"
+NANOVLM_ATTENTION_MASK = "observation.nanovlm.attention_mask"
+
+
+@ProcessorStepRegistry.register(name="nanovlm_native_processor")
+@dataclass
+class NanoVLMNativeProcessorStep(ComplementaryDataProcessorStep):
+    pretrained_path: str
+    code_path: str
+    image_keys: tuple[str, ...]
+    max_length: int
+    _tokenizer: Any = field(default=None, init=False, repr=False)
+    _image_processor: Any = field(default=None, init=False, repr=False)
+    _get_image_string: Any = field(default=None, init=False, repr=False)
+    _mp_image_token_length: int = field(default=64, init=False, repr=False)
+
+    def __post_init__(self):
+        code_path = Path(self.code_path)
+        if not code_path.is_absolute():
+            code_path = Path(__file__).resolve().parents[4] / code_path
+        if str(code_path) not in sys.path:
+            sys.path.insert(0, str(code_path))
+
+        from data.processors import get_image_processor, get_image_string, get_tokenizer
+
+        config_path = _resolve_checkpoint_file(self.pretrained_path, "config.json")
+        config = json.loads(Path(config_path).read_text())
+        if self.max_length > config["lm_max_length"]:
+            raise ValueError(
+                f"tokenizer_max_length={self.max_length} exceeds nanoVLM's "
+                f"lm_max_length={config['lm_max_length']}"
+            )
+        self._tokenizer = get_tokenizer(
+            config["lm_tokenizer"],
+            config["vlm_extra_tokens"],
+            config["lm_chat_template"],
+        )
+        self._image_processor = get_image_processor(
+            config["max_img_size"],
+            config["vit_img_size"],
+            config["resize_to_max_side_len"],
+        )
+        self._get_image_string = get_image_string
+        self._mp_image_token_length = config["mp_image_token_length"]
+
+    def complementary_data(self, complementary_data):
+        raw_tasks = complementary_data.get("task")
+        if raw_tasks is None:
+            raise ValueError("Task is required for nanoVLM value processing")
+        observation = self.transition[TransitionKey.OBSERVATION]
+        present_image_keys = [key for key in self.image_keys if key in observation]
+        if not present_image_keys:
+            raise ValueError("No configured nanoVLM image key is present in the observation")
+        batch_size = observation[present_image_keys[0]].shape[0]
+        tasks = [raw_tasks] * batch_size if isinstance(raw_tasks, str) else list(raw_tasks)
+        if len(tasks) != batch_size:
+            raise ValueError(f"Received {len(tasks)} tasks for an image batch of size {batch_size}")
+
+        processed_batch = []
+        input_rows = []
+        attention_rows = []
+        for batch_index in range(batch_size):
+            processed_images = []
+            split_counts = []
+            for key in self.image_keys:
+                if key not in observation:
+                    continue
+                image = observation[key][batch_index]
+                if image.ndim != 3:
+                    raise ValueError(f"nanoVLM expects CHW images, got {tuple(image.shape)} for {key}")
+                if image.shape[0] not in (1, 3, 4) and image.shape[-1] in (1, 3, 4):
+                    image = image.permute(2, 0, 1)
+                if image.dtype != torch.uint8:
+                    image = image.float()
+                    if image.min() < -1e-6 or image.max() > 1.0 + 1e-6:
+                        raise ValueError(
+                            f"nanoVLM expects uint8 [0,255] or float [0,1] images; "
+                            f"{key} has range [{image.min().item()}, {image.max().item()}]"
+                        )
+                    image = image.clamp(0, 1)
+                pil_image = to_pil_image(image.cpu()).convert("RGB")
+                processed, split_count = self._image_processor(pil_image)
+                processed_images.append(processed)
+                split_counts.append(split_count)
+
+            image_string = self._get_image_string(
+                self._tokenizer,
+                split_counts,
+                self._mp_image_token_length,
+            )
+            prompt = self._tokenizer.apply_chat_template(
+                [{"role": "user", "content": image_string + f"Task: {tasks[batch_index]}."}],
+                tokenize=False,
+                add_generation_prompt=True,
+            )
+            tokenized = self._tokenizer(
+                prompt,
+                truncation=False,
+                add_special_tokens=False,
+            )
+            if len(tokenized["input_ids"]) > self.max_length:
+                raise ValueError(
+                    f"nanoVLM prompt has {len(tokenized['input_ids'])} tokens, exceeding "
+                    f"tokenizer_max_length={self.max_length}. The native nanoVLM collator "
+                    "discards over-length examples instead of truncating image placeholders."
+                )
+            input_rows.append(tokenized["input_ids"])
+            attention_rows.append(tokenized.get("attention_mask", [1] * len(tokenized["input_ids"])))
+            processed_batch.append(processed_images)
+
+        max_length = max(map(len, input_rows))
+        for input_ids, attention_mask in zip(input_rows, attention_rows, strict=True):
+            padding = max_length - len(input_ids)
+            input_ids[:0] = [self._tokenizer.pad_token_id] * padding
+            attention_mask[:0] = [0] * padding
+
+        observation = dict(observation)
+        observation[NANOVLM_IMAGES] = processed_batch
+        observation[NANOVLM_INPUT_IDS] = torch.tensor(input_rows, dtype=torch.long)
+        observation[NANOVLM_ATTENTION_MASK] = torch.tensor(attention_rows, dtype=torch.bool)
+        self.transition[TransitionKey.OBSERVATION] = observation
+        return complementary_data
+
+    def transform_features(
+        self,
+        features: dict[PipelineFeatureType, dict[str, PolicyFeature]],
+    ):
+        return features
+
+    def get_config(self):
+        return {
+            "pretrained_path": self.pretrained_path,
+            "code_path": self.code_path,
+            "image_keys": self.image_keys,
+            "max_length": self.max_length,
+        }
+
+
+def _resolve_checkpoint_file(repo_id_or_path: str, filename: str) -> str:
+    local_path = Path(repo_id_or_path) / filename
+    if local_path.exists():
+        return str(local_path)
+    from huggingface_hub import hf_hub_download
+
+    return hf_hub_download(repo_id=repo_id_or_path, filename=filename)
+
 
 def make_nanovlm_vf_pre_post_processors(
     config: NanoVLMVFConfig,
@@ -45,16 +195,11 @@ def make_nanovlm_vf_pre_post_processors(
                 norm_map=config.normalization_mapping,
                 stats=dataset_stats,
             ),
-            DistributionalVFImagePreprocessorStep(
+            NanoVLMNativeProcessorStep(
-                image_resolution=config.image_resolution,
+                pretrained_path=config.nanovlm_pretrained_path,
+                code_path=config.nanovlm_code_path,
                 image_keys=image_keys,
-            ),
-            DistributionalVFPrepareTaskPromptStep(),
-            TokenizerProcessorStep(
-                tokenizer_name=config.tokenizer_path,
                 max_length=config.tokenizer_max_length,
-                padding_side="right",
-                padding="max_length",
             ),
             DeviceProcessorStep(device=config.device or "cpu"),
         ],

tests/rewards/test_nanovlm_value_function.py

@@ -1,7 +1,9 @@
+import json
 import sys
 from types import ModuleType, SimpleNamespace
 
 import torch
+from PIL import Image
 from torch import nn
 
 from lerobot.configs import FeatureType, PolicyFeature
@@ -9,7 +11,13 @@ from lerobot.rewards.factory import get_reward_model_class, make_reward_model_co
 from lerobot.rewards.nanovlm_value_function.configuration_nanovlm_value_function import (
     NanoVLMVFConfig,
 )
-from lerobot.utils.constants import OBS_LANGUAGE_ATTENTION_MASK, OBS_LANGUAGE_TOKENS
+from lerobot.rewards.nanovlm_value_function.processor_nanovlm_value_function import (
+    NANOVLM_ATTENTION_MASK,
+    NANOVLM_IMAGES,
+    NANOVLM_INPUT_IDS,
+    NanoVLMNativeProcessorStep,
+)
+from lerobot.types import TransitionKey
 
 CAMERA = "observation.images.top"
 
@@ -17,6 +25,7 @@ CAMERA = "observation.images.top"
 def test_config_and_factory_registration():
     config = make_reward_model_config("nanovlm_value_function")
     assert isinstance(config, NanoVLMVFConfig)
+    assert config.tokenizer_max_length == 8192
     assert get_reward_model_class("nanovlm_value_function").__name__ == "NanoVLMVFRewardModel"
 
 
@@ -56,6 +65,15 @@ def test_nanovlm_model_forward(monkeypatch):
             self.vision_encoder = FakeVision()
             self.MP = FakeProjector()
             self.decoder = FakeDecoder()
+            self.tokenizer = SimpleNamespace(image_token_id=99)
+
+        def _process_images(self, images, device):
+            return torch.cat([image for sample in images for image in sample]).to(device)
+
+        def _replace_img_tokens_with_embd(self, input_ids, token_embd, image_embd):
+            token_embd = token_embd.clone()
+            token_embd[input_ids == self.tokenizer.image_token_id] = image_embd.flatten(0, 1)
+            return token_embd
 
         @classmethod
         def from_pretrained(cls, path):
@@ -72,13 +90,67 @@ def test_nanovlm_model_forward(monkeypatch):
     config.input_features = {CAMERA: PolicyFeature(type=FeatureType.VISUAL, shape=(3, 16, 16))}
     model = NanoVLMVFRewardModel(config)
     batch = {
-        CAMERA: torch.rand(1, 3, 16, 16),
+        NANOVLM_IMAGES: [[torch.rand(1, 3, 16, 16)]],
-        CAMERA + ".mask": torch.ones(1, dtype=torch.bool),
+        NANOVLM_INPUT_IDS: torch.tensor([[99, 99, 99, 99, 1]]),
-        OBS_LANGUAGE_TOKENS: torch.ones(1, 4, dtype=torch.long),
+        NANOVLM_ATTENTION_MASK: torch.ones(1, 5, dtype=torch.bool),
-        OBS_LANGUAGE_ATTENTION_MASK: torch.ones(1, 4, dtype=torch.bool),
         "mc_return": torch.tensor([-0.5]),
         "is_terminal": torch.tensor([False]),
     }
     loss, metrics = model(batch)
     assert torch.isfinite(loss)
     assert -1.0 <= metrics["predicted_value_mean"] <= 0.0
+
+
+def test_native_processor_uses_checkpoint_layout_and_left_padding(monkeypatch, tmp_path):
+    config = {
+        "lm_tokenizer": "fake",
+        "vlm_extra_tokens": {},
+        "lm_chat_template": "fake",
+        "lm_max_length": 8192,
+        "max_img_size": 2048,
+        "vit_img_size": 512,
+        "resize_to_max_side_len": True,
+        "mp_image_token_length": 4,
+    }
+    (tmp_path / "config.json").write_text(json.dumps(config))
+
+    class FakeTokenizer:
+        pad_token_id = 0
+        image_token_id = 99
+
+        def apply_chat_template(self, messages, tokenize, add_generation_prompt):
+            assert not tokenize and add_generation_prompt
+            return messages[0]["content"]
+
+        def __call__(self, prompt, truncation, add_special_tokens):
+            assert not truncation and not add_special_tokens
+            suffix = [1, 2] if "long" in prompt else [1]
+            return {"input_ids": [99] * 4 + suffix, "attention_mask": [1] * (4 + len(suffix))}
+
+    def fake_image_processor(image):
+        assert isinstance(image, Image.Image) and image.mode == "RGB"
+        return torch.rand(1, 3, 512, 512), (1, 1)
+
+    processors = ModuleType("data.processors")
+    processors.get_tokenizer = lambda *args: FakeTokenizer()
+    processors.get_image_processor = lambda *args: fake_image_processor
+    processors.get_image_string = lambda *args: "<image>"
+    monkeypatch.setitem(sys.modules, "data.processors", processors)
+
+    step = NanoVLMNativeProcessorStep(
+        pretrained_path=str(tmp_path),
+        code_path="third_party/nanoVLM",
+        image_keys=(CAMERA,),
+        max_length=8192,
+    )
+    transition = {
+        TransitionKey.OBSERVATION: {CAMERA: torch.rand(2, 3, 16, 16)},
+        TransitionKey.COMPLEMENTARY_DATA: {"task": ["short", "long"]},
+    }
+    output = step(transition)[TransitionKey.OBSERVATION]
+
+    assert len(output[NANOVLM_IMAGES]) == 2
+    assert output[NANOVLM_INPUT_IDS].shape == (2, 6)
+    assert output[NANOVLM_INPUT_IDS][0, 0] == 0
+    assert not output[NANOVLM_ATTENTION_MASK][0, 0]
+    assert output[NANOVLM_ATTENTION_MASK][1].all()