REVIEW 3 major objections 3 minor 42 references
Evaluating Model Perception of Color Illusions in Photorealistic Scenes
T0 review · 3 major / 3 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read Current vision-language models perceive color illusions much as humans do, a new 19,000-image photorealistic dataset shows—and they can be trained to report both human-like and pixel-based judgments.
desk verdict RCID is a genuinely useful dataset, but the pixel-preservation assumption is load-bearing and unverified, so the human-similarity claim needs softening. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The mechanism that carries the argument is a three-stage dataset-generation pipeline combined with paired-question evaluation. Simple illusion images are first generated procedurally (backgrounds with different brightness, stripes with alternating colors, etc.), then a conditional diffusion model maps them to photorealistic scenes while retaining the intended color relationships; filter illusions are made by shifting hue in HSV space on MS COCO images. For every image, GPT-4o generates a natural-language question asking about color differences or object color, and two prompts are attached: one asks for the judgment 'based on pixel values' and the other 'based on human perception.' Human validation on Prolific decides which images truly deceive at least 3 of 5 viewers. This design lets the paper label each image with two ground truths—pixel truth and human truth—so a model response can be classified as accurate, human-like, or neither.
What would settle it
Compare, on a random sample of RCID contrast and stripe images, the mean RGB values of the two regions named in the question. If a nontrivial fraction of images labeled 'same' actually have different mean pixel colors (or vice versa), the pixel-based ground truth is broken and the reported deception rates would need to be recomputed. One could also re-run the VLM evaluation on the procedural source images versus the ControlNet-realistic versions: if the human-like response rate changes substantially, the photorealistic rendering step is introducing confounds.
Extended reading notes
Core claim
The central claim is that current vision-language models exhibit perceptual biases similar to human vision when confronted with realistic color illusions. The evidence comes from RCID: models fine-tuned to answer color-comparison questions reach over 75% accuracy on non-illusion images, while accuracy on illusion images is significantly lower, and a portion of the errors are 'human-like'—they agree with human judgments rather than with pixel values. The paper also reports that model size increases human-like responding, that external prompts such as chain-of-thought or few-shot examples shift responses only modestly, and that fine-tuning on illusion images can push models toward either pixel-based or human-like answers. A mixed-training baseline lets a single model produce the correct pixel-based answer under a 'pixel values' prompt and the correct human-based answer under a 'human perception' prompt.
Load-bearing premise
The photorealistic images produced by the conditional diffusion model preserve the exact pixel-level color relationships of the procedurally generated source images, so that the 'based on pixel values' ground-truth labels are correct for every illusion image.
Editorial extensions
If this is right
- Because VLMs show human-like biases on photorealistic illusions, downstream applications that rely on color judgments in real images will inherit those systematic errors.
- The success of mixed instruction tuning shows that models can be trained to report pixel-based or human-perception answers on demand, giving application developers a lever to choose the appropriate behavior.
- The small effect of chain-of-thought and few-shot prompting indicates that these biases are not superficial output artifacts but are rooted in visual representations and learned priors.
- The positive correlation between model size and human-like responses suggests that scaling model size alone will not eliminate these perceptual biases.
Reading between the lines
- If the same pattern holds more broadly, color-illusion susceptibility could serve as a probe for how much a VLM's judgments rely on low-level visual processing versus stored world knowledge, complementing other perceptual benchmarks.
- One testable extension is to vary the photorealistic generation process (e.g., different diffusion models or strengths of realism) and check whether human-like response rates track the fidelity of the scene, which would separate the contribution of image realism from the model's internal priors.
- The RCID-style paired-question setup could be adapted to other perceptual constancies, such as lightness, size, or shape illusions, to see whether 'human-like' errors are a general property of VLM visual encoders or specific to color.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes an automated pipeline for generating photorealistic color-illusion images and introduces RCID, a dataset of 19,000 images spanning contrast, stripe, and filter illusions, with human validation labels. The authors evaluate several open-source VLMs after fine-tuning on non-illusion images, reporting strong accuracy on non-illusion images but significantly lower accuracy on illusion images, with a substantial fraction of human-like responses. They also investigate the roles of the visual encoder and linguistic/commonsense priors, and propose a mixed-training method that lets a model answer according to either pixel values or human perception.
Significance. If the results hold, this is a valuable contribution: RCID is much larger and more realistic than prior illusion datasets, the human-annotation protocol is independent and the fine-tuning design helps separate color-comparison ability from illusion susceptibility. The paper also goes beyond benchmark reporting by studying model size, prompt effects, and the visual/linguistic sources of bias. The code and data are promised open-source, which strengthens reproducibility. The main risk is that the pixel-level ground truth of the generated images is never verified; this threatens the validity of the no-illusion/deception metrics and, in turn, the central claim of human-like perceptual bias.
major comments (3)
- [3.1] The generation stage as written is not a standard diffusion process and is inconsistent with the training objective. Equation (1) trains the model to denoise from x0 = G (the quantized image), but the generating-stage formula I' = sum_t (x_t + epsilon_theta(...)) sums noisy samples and predicted noise across all time steps. This is not the usual recursive denoising update and it is unclear what x_t denotes after the first step. Because the paper's central claim depends on the final images preserving the intended color relationships, the actual generation algorithm must be specified precisely and reproducibly.
- [3.1 and 4.1] The pixel-value ground truth used for No-Illusion and Deception rates in Section 4 rests on the assumption that the ControlNet-generated images I' preserve the mean-color relationships of the procedural images S (e.g., the two target squares have identical pixel colors in 'same but look different' contrast illusions, and differ in the intended direction in 'different but look same' cases). This is never verified after generation. Human validation in Section 3.4 only confirms that participants were deceived; it does not confirm that the final images satisfy the intended pixel comparisons. ControlNet is conditioned on text and noise, so local color shifts are plausible; if one target region shifts more than another, the reported >75% non-illusion accuracy and the accuracy drop on illusion images could partly be an artifact of mislabeled pixel comparisons rather than genuine perceptual bias. The authors should measure the target-region mean colors in the final images, filter or re-label images that fail the intended comparisons, and rerun the affected metrics.
- [Abstract and Appendix B.4] The abstract's claim that 'all studied VLMs exhibit perceptual biases similar human vision' is stronger than the evidence in the paper. Appendix B.4 shows that without fine-tuning, models perform near random guessing on contrast and stripe tasks even on non-illusion images, so their 'human-like' behavior on illusions is not distinguishable from chance at the base-model level. The human-like bias is convincingly demonstrated only for models after fine-tuning on non-illusion images. The claim should be qualified to the fine-tuned setting, or the base-model analysis should be used to support a weaker claim about susceptibility rather than similarity to human perception.
minor comments (3)
- [Abstract and 6] There are typos: 'similar human vision' should be 'similar to human vision' in the abstract; 'an simple training method' in Section 1; 'fileds' in Section 6.
- [3.1] The notation is inconsistent: x0 is defined as the quantized grid G in the training stage but as the procedural image S in the generating stage, and the symbol C is used both for colors and for text captions. Please use distinct symbols.
- [5.2] The analysis in Figure 10 reports correlations between color descriptor diversity and pixel-value accuracy, but the figure would benefit from error bars or a clearer statement of the number of color terms per language and the significance of the correlation.
Circularity Check
No significant circularity: RCID labels come from independent human annotators and procedural pixel definitions, and VLM scores are measured against those external labels.
full rationale
The paper's central comparison is not circular: human-perception ground truth is obtained from independent Prolific annotators (Section 3.4), pixel-value ground truth is defined by the procedural color assignments (Section 3.1), and VLM responses are then scored against these external labels (Section 4.1). The fine-tuning experiments are standard supervised learning with held-out test splits; the 'mixed training' model is trained on both label types and evaluated on a separate test set, so its ability to follow pixel versus perception prompts is not forced by construction. The main validity threat, that ControlNet may not preserve exact pixel equalities or differences in the generated photorealistic images, would make some external labels incorrect, but it is an empirical assumption about image fidelity rather than a derivation that reduces to the paper's own inputs. Self-citations (e.g., [9], [35]) are contextual and carry no load in the evaluation chain. Accordingly, no circular step is identified.
Assumptions & free parameters
free parameters (4)
- Contrast illusion foreground/background brightness scales (µb1, µb2, µf1, µf2) =
randomly sampled ranges (not reported exactly)
- Stripe illusion color scales (µs1, µs2) =
based on initial human-perception estimates
- Human deception threshold (3 of 5 annotators) =
3/5
- Fine-tuning epochs (5) and data mixture sizes =
5 epochs; 6000 simple + 5500 realistic non-illusion images
assumptions (4)
- domain assumption ControlNet training on MS COCO produces a mapping that can turn the simple procedural illusion images into photorealistic images while preserving the illusion and the relative pixel colors.
- domain assumption The majority vote of five Prolific participants is a reliable estimate of human perception for each image.
- domain assumption GPT-4o-generated questions accurately and unambiguously refer to the intended image regions without leaking the correct answers.
- domain assumption The color illusions studied (contrast, stripe, filter) are representative of real-world color perception phenomena.
Cite this review
Pith. "Pith review of Evaluating Model Perception of Color Illusions in Photorealistic Scenes." pith.science (2026). https://pith.science/paper/OBYCAJZH
@misc{pith2026241206184,
author = {Pith},
title = {Pith review of: Evaluating Model Perception of Color Illusions in Photorealistic Scenes},
year = {2026},
howpublished = {\url{https://pith.science/paper/OBYCAJZH}},
note = {Machine review of arXiv:2412.06184}
}
read the original abstract
We study the perception of color illusions by vision-language models. Color illusion, where a person's visual system perceives color differently from actual color, is well-studied in human vision. However, it remains underexplored whether vision-language models (VLMs), trained on large-scale human data, exhibit similar perceptual biases when confronted with such color illusions. We propose an automated framework for generating color illusion images, resulting in RCID (Realistic Color Illusion Dataset), a dataset of 19,000 realistic illusion images. Our experiments show that all studied VLMs exhibit perceptual biases similar human vision. Finally, we train a model to distinguish both human perception and actual pixel differences.
Figures
Figures from the paper (15 more)
Reference graph
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