simpleseganonymous/SimpleSeg

This is an anonymous version for peer review.

Paper: Towards Pixel-level VLM Perception via Simple Points Prediction

Introduction

We present SimpleSeg, a strikingly simple yet highly effective approach to endow Multimodal Large Language Models (MLLMs) with native pixel-level perception. Our method reframes segmentation as a simple sequence generation problem: the model directly predicts sequence of points (textual coordinates) delineating object boundaries, entirely within its language space. To achieve high fidelity, we introduce a two-stage SFT→RL training pipeline, where Reinforcement Learning with an IoU-based reward refines the point sequences to accurately match ground-truth contours. We find that the standard MLLM architecture possesses a strong, inherent capacity for low-level perception that can be unlocked without any specialized architecture. On segmentation benchmarks, SimpleSeg achieves performance that is comparable to, and often surpasses, methods relying on complex, task-specific designs. This work lays out that precise spatial understanding can emerge from simple point prediction, challenging the prevailing need for auxiliary components and paving the way for more unified and capable VLMs.

Method

In this work, we explore the limits of MLLM pixel-level perception by predicting the next point in a contour with the simplest approach possible. Without introducing any complex architectures or special patterns, we show how even minimalistic point prediction can achieve effective segmentation at the pixel level.

Key Benefits

• Simplicity: SimpleSeg requires no specialized modules and adheres to the standard MLLM architecture, it can be seamlessly and efficiently integrated as a new, core pre-training task for foundation models, similar to visual grounding.
• Task Generality: By framing segmentation as a text-generation problem, our approach is inherently flexible. The model can be easily adapted to a wide range of vision-language tasks that require precise spatial localization.
• Interpretable Output: The model generates explicit, human-readable coordinate sequences instead of dense pixel masks. This transparency simplifies debugging and makes the output directly usable for downstream applications like interactive editing or tool use.

Performance

• Referring Expression Segmentation results

Methods	refCOCO			refCOCO+			refCOCOg		Avg.
	val	testA	testB	val	testA	testB	val	test
Decoder-based Models
NEXT-Chat	74.7	78.9	69.5	65.1	71.9	56.7	67.0	67.0	68.9
LISA	74.9	79.1	72.3	65.1	70.8	58.1	67.9	70.6	69.9
PixelLM	73.0	76.5	68.2	66.3	71.7	58.3	69.3	70.5	69.2
AnyRef	76.9	79.9	74.2	70.3	73.5	61.8	70.0	70.7	72.2
GSVA	77.2	78.9	73.5	65.9	69.6	59.8	72.7	73.3	71.4
LaSagNA	76.8	78.7	73.8	66.4	70.6	60.1	70.6	71.9	71.1
Groundhog	78.5	79.9	75.7	70.5	75.0	64.9	74.1	74.6	74.2
Text4Seg (w/ SAM)	79.2	81.7	75.6	72.8	77.9	66.5	74.0	75.3	75.4
Decoder-free Models
Text4Seg	74.7	77.4	71.6	68.5	73.6	62.9	70.7	71.6	71.4
SimpleSeg	76.9	78.9	73.6	71.1	75.2	66.1	72.8	74.3	73.6

• Referring Expression Comprehension results

Methods	refCOCO			refCOCO+			refCOCOg		Avg.
	val	testA	testB	val	testA	testB	val	test
Decoder-based Models
LISA	85.4	88.8	82.6	74.2	79.5	68.4	79.3	80.4	79.8
GSVA	86.3	89.2	83.8	72.8	78.8	68.0	81.6	81.8	80.3
NEXT-Chat	85.5	90.0	77.9	77.2	84.5	68.0	80.1	79.8	80.4
PixelLM	89.8	92.2	86.4	83.2	87.0	78.9	84.6	86.0	86.0
Text4Seg (w/ SAM)	90.3	93.4	87.5	85.2	89.9	79.5	85.4	85.4	87.1
Decoder-free Models
Text4Seg	88.3	91.4	85.8	83.5	88.2	77.9	82.4	82.5	85.0
SimpleSeg	90.5	92.9	86.8	85.3	89.5	80.2	86.1	86.5	87.2

Model Usage

Inference with 🤗 Hugging Face Transformers

It is recommended to use python=3.10, torch>=2.1.0, and transformers=4.48.2 as the development environment.

from PIL import Image
from transformers import AutoModelForCausalLM, AutoProcessor
model_path = "simpleseganonymous/SimpleSeg"
model = AutoModelForCausalLM.from_pretrained(
    model_path,
    torch_dtype="auto",
    device_map="auto",
    trust_remote_code=True,
)
processor = AutoProcessor.from_pretrained(model_path, trust_remote_code=True)
image_path = "./figures/octopus.png"
image = Image.open(image_path)
messages = [
    {"role": "user", "content": [{"type": "image", "image": image_path}, {"type": "text", "text": "Output the polygon coordinates of octopus in the image."}]}
]
text = processor.apply_chat_template(messages, add_generation_prompt=True, return_tensors="pt")
inputs = processor(images=image, text=text, return_tensors="pt", padding=True, truncation=True).to(model.device)
generated_ids = model.generate(**inputs, max_new_tokens=512)
generated_ids_trimmed = [
    out_ids[len(in_ids) :] for in_ids, out_ids in zip(inputs.input_ids, generated_ids)
]
response = processor.batch_decode(
    generated_ids_trimmed, skip_special_tokens=True, clean_up_tokenization_spaces=False
)[0]
print(response)

Decode the polygons and masks from the response string

import re
import json
import pycocotools.mask as mask_utils

class RegexPatterns:
    BOXED_PATTERN = r'\\boxed\{([^}]*)\}'
    BLOCK_PATTERN = r'^```$\r?\n(.*?)\r?\n^```$'

    NON_NEGATIVE_FLOAT_PATTERN = (
        r'(?:[1-9]\d*\.\d+|0\.\d+|\d+)'
    )

    BBOX_PATTERN = rf'\[\s*({NON_NEGATIVE_FLOAT_PATTERN})\s*,\s*({NON_NEGATIVE_FLOAT_PATTERN})\s*,\s*({NON_NEGATIVE_FLOAT_PATTERN})\s*,\s*({NON_NEGATIVE_FLOAT_PATTERN})\s*\]'
    POINT_PATTERN = (
        rf'\[\s*({NON_NEGATIVE_FLOAT_PATTERN})\s*,\s*({NON_NEGATIVE_FLOAT_PATTERN})\s*\]'
    )

    POLYGON_PATTERN = rf'\[\s*{POINT_PATTERN}(?:\s*,\s*{POINT_PATTERN})*\s*\]'

polygon_matches = [
    m.group(0) for m in re.finditer(RegexPatterns.POLYGON_PATTERN, response, re.DOTALL)
]
pred_polygons = []
for polygon_match in polygon_matches:
    polygon = json.loads(polygon_match)
    pred_polygons.append(polygon)

pred_masks = []
for pred_polygon in pred_polygons:
    pred_polygon = np.array(pred_polygon) * np.array([width, height])
    rle = mask_utils.frPyObjects(pred_polygon.reshape((1, -1)).tolist(), height, width)
    mask = mask_utils.decode(rle)
    mask = np.sum(mask, axis=2, keepdims=True)
    pred_masks.append(mask)
pred_mask = np.sum(pred_masks, axis=0)
pred_mask = pred_mask.sum(axis=2)
pred_mask = (pred_mask > 0).astype(np.uint8)