Can two identical colors look different?

Look before the reveal

The two center squares may look different, similar, or somewhere in between. Keep your own observation. It is not a test of attention or a puzzle with a required visual experience.

Change one thing

Swap the surrounding fields while leaving both center patches in place. We repaint only the context. An experiment is more informative when its changing and fixed parts are clear.

Remove the difference in context

The neutral cover leaves two cutouts for the original center patches. At full coverage, both have the same surrounding gray. This is an isolation control. The joining strip offers another comparison, but also changes grouping.

Ask the image directly

Our pixel lens reads the actual canvas image buffer. It can inspect a center, a surround or an edge. The full check counts every pixel inside both target rectangles, including its opacity.

Give the effect its name

Simultaneous brightness contrast describes a shift in appearance in relation to nearby regions. It names a phenomenon; it does not by itself specify every optical or neural mechanism involved.

Challenge a promising idea

Perhaps appearance depends only on the average lightness around a patch’s border. We make that proposal precise, then try a striped display associated with White’s effect. A rule that helps with one example can fail with another.

A wider scene matters

Researchers vary nearby and more remote regions separately. Spatial arrangement, scale and relationships matter. One informal screen demonstration cannot locate the effect in a particular part of the visual system.

Use the discovery

Check a paint sample in its intended context. Inspect actual pixel values when editing an image. When explaining a chart, keep the colors, their surroundings and the viewer’s task in mind.

Three quantities

RGB is a digital specification. Luminance is a physical photometric quantity, measured in candelas per square meter (cd/m²). Brightness describes visual appearance. The app measures its image buffer, not the emitted light of your screen or your experience.

Lightness has another meaning

In color science, lightness is a relative appearance judgment involving a similarly illuminated area regarded as white. CSS HSL “lightness” is a color-coordinate parameter, not a calibrated measurement of that perceptual quantity.

Exact authored stimulus

The equality fixture is an 800 × 360 opaque sRGB raster. Each square contains 12,544 pixels, all RGBA(128,128,128,255). Baseline surrounds are codes 32 and 224. The full neutral mask uses 176. Integer source rectangles avoid edge ambiguity in the exact check.

sRGB encoding is nonlinear

For a channel c = code/255, use c/12.92 at c ≤ 0.04045, otherwise ((c + 0.055)/1.055)^2.4. Gray codes 32, 128 and 224 give normalized linear-light values 0.01444384, 0.21586050 and 0.74540421. These are not measured cd/m² or perceived-brightness scores.

An explicit, limited hypothesis

Our original stripe targets are 40 × 136 pixels. Short ends contact 80 units of aligned bars; long sides contact 272 units of side bars. The candidate rule subtracts their length-weighted linear-light mean from the target level. It is auditable arithmetic, not a biological simulation.

Why White’s effect is useful

Blakeslee, Padmanabhan and McCourt (2016) manipulated aligned and flanking bars independently. The research challenges a universal explanation based only on total light/dark border contact. Our original display is informed by that phenomenon, not a calibrated replication or a guarantee of each viewer’s report.

Controlled evidence

Blakeslee and McCourt (2023) kept target luminance at 64 cd/m², independently changed ring width/luminance and remote background, and measured matching judgments. Four observers used a calibrated display. The shown group graph uses mean matching luminance with ±1 standard error of the mean; it is not a percentile chart.

Geometry belongs to its protocol

That study’s target radius was 0.5° of visual angle. Ring widths ranged from 0.1° to 24.5°; the largest outer radius was 25°. A browser pixel has no universal angular size. Copying 64 cd/m² into RGB code 64 would confuse two different quantities.

Mechanisms need more than a catchy story

Sinha and colleagues (2020) used special experimental displays and observations after sight-restoring treatment to constrain explanations of brightness contrast. Their results argue against treating learned scene interpretation as the only cause. Our page does not reproduce those procedures or settle one exclusive neural pathway.

A model is a proposal

Troscianko and Osorio (2023) tested an efficient-coding and spatial-filtering account of color appearance. Their cube figure places an input stimulus above the model’s transformed image. A successful transformation is not a photograph of a thought, and our border arithmetic is not their SBL model.

Display and observer limits

Color management, local dimming, nonuniform output, viewing angle, room light and glare can affect the viewing condition. A screenshot checks part of an image pipeline; a physical light measurement requires suitable instrumentation. The same source numbers do not establish identical photons at the eye.

A fair null observation

If you do not see an effect, keep that result. There is no score, timing target, age norm or attention diagnosis here. The lesson remains useful through the numeric controls, explicit hypothesis and source evidence.

Choose colors in context

Place the same sample beside the surrounding colors you plan to use. Keep an isolated reference so that changing context is visible.

Read and edit images

Distinguish what a region looks like from what its pixels specify. Numeric inspection answers a different question from a viewing report.

Build better explanations

Write down a rule’s prediction before choosing a second example. A counterexample can improve the explanation rather than end the investigation.

Both centers still read RGB(128,128,128). What stayed fixed?

• The specified center colors
• Exactly how everyone sees them
• The surroundings

Which control best checks the role of the different surrounds?

• Change one center too
• Keep both centers on one neutral surround
• Require identical observations

What does this pixel lens establish?

• Retinal signals
• Exact light emitted by every monitor
• The sampled image-buffer values

A rule fits one display but conflicts with a second. What next?

• Revise or limit the rule and test more evidence
• Delete conflicting results
• Conclude context never matters

A friend sees no difference. What can this page diagnose?

• Poor attention
• Nothing diagnostic from this observation
• Superior brain accuracy

What is the lower panel in the research cube figure?

• A brain photograph
• The color every person must see
• A computational model’s transformed image

Why is RGB(64,64,64) not automatically a 64 cd/m² laboratory replication?

• Digital code and luminance are different quantities
• Every monitor is identical
• 64 was the number of observers

You know the values are equal, but they still look different. What follows?

• The pixels must have changed
• Knowledge and appearance can differ
• You deliberately chose the wrong answer

Nearby and remote context influence controlled matching judgments

2023 primary study; four observers, calibrated display, constant 64 cd/m² targets. Original Figures 1 and 3 reused with CC BY 4.0 credit.

Aligned and flanking bars challenge a simple border account

2016 primary study of White’s effect. We author our own geometry; no source figures are redistributed.

Evidence constraining learned-scene-only explanations

Special displays and sight-restoration observations; these procedures are not reproduced in this lesson.

A computational account and recognizable cube stimulus

Input and SBL-model transformation, original Figure 5. CC BY 4.0. Not a neural recording or our pixel-equality reference.

Brightness terminology

Perceptual attribute, distinct from an image code or physical meter reading.

Lightness terminology

Relative appearance definition, not CSS HSL coordinate arithmetic.

Physical luminance terminology

Photometric quantity with cd/m² units; cannot be inferred from a code alone.

sRGB transfer function and digital interpretation

Exact color-space arithmetic used in the explicit hypothesis. No personal brightness prediction is derived.

Original source bytes and reuse records

Local unaltered figures, source XML, hashes, authors and CC BY 4.0 attribution.