Two Squares in a Grid. The Grid Makes Them Look Different.
Which patch is lighter?
You are looking at the Dungeon illusion, a modern brightness illusion named by Paola Bressan in the 2000s. Two identical grey squares sit embedded in a grid of coloured squares · sometimes black-and-white in a checker pattern, sometimes with coloured bands. The two grey squares land on different positions in the grid such that one is surrounded by primarily lighter neighbours and the other by primarily darker neighbours. The two greys look very different · the one surrounded by dark squares appears lighter, the one surrounded by light squares appears darker.
What you are about to learn. What the Dungeon illusion actually is, why it is essentially a Koffka-ring-style grouping illusion applied to a grid, how the interlocking structure amplifies the effect, what Bressan’s “double-anchoring” theory says about it, and how it relates to brightness anchoring in everyday scenes.
What the Illusion Looks Like
Draw a grid of squares alternating between black and white (a checkerboard), or more loosely, a grid where columns and rows of different colours interlock. Embed two identical grey squares at specific positions in the grid · one where most of its neighbours (sharing edges) are black, and one where most of its neighbours are white.
The grey in the black-neighbour position looks distinctly lighter. The grey in the white-neighbour position looks distinctly darker. The magnitude of the effect can reach 15 percent. And the grey squares are physically identical.
The minimal recipe. A grid with alternating or interlocked luminance structure. Two identical target squares placed at positions where their edge-sharing neighbours differ in average luminance. The target surrounded by dark neighbours reads lighter (contrast with surround), the target surrounded by light neighbours reads darker. This is simultaneous contrast applied in a grid context, with the grid amplifying the effect.
Why It Works: Local Contrast Plus Grouping
The Dungeon effect combines two mechanisms you have seen already.
Local contrast. Each target square is subjected to lateral inhibition from its immediate edge-sharing neighbours. Black neighbours push the target lighter; white neighbours push the target darker. This is pure Chevreul 1839.
Perceptual grouping. Your visual system also parses the grid into larger structures · rows of mostly-dark, rows of mostly-light, interlocking diagonal chains, etc. The target square gets assigned to one of these larger groups, and its brightness is further influenced by the mean luminance of that group.
The two mechanisms stack. Both push the dark-surrounded target toward lighter, and the light-surrounded target toward darker. Because both push in the same direction, the combined effect is considerably larger than local contrast alone would predict.
The Dungeon as a stress-test. The classical simultaneous-contrast illusion (a patch on a uniform background) uses only the local-contrast mechanism. The Dungeon stacks grouping on top, producing an effect that is both stronger and harder to compute. A complete theory of brightness perception has to handle the Dungeon cleanly · it is a canonical test case for any proposed model. If the model gets the Dungeon wrong (too weak, wrong direction, nonsense for some grid variants), it is incomplete.
Bressan’s Double-Anchoring Theory
Paola Bressan’s 2006 “double-anchoring theory” is one of the cleanest accounts of the Dungeon illusion.
Two anchors. Bressan argues your visual system needs to “anchor” brightness perception · it needs at least one patch in the scene to be taken as reference white, against which everything else is scaled. In the Dungeon, the visual system anchors locally (the brightest patch in the target’s immediate neighbourhood is taken as white) and globally (the brightest patch in the whole scene is taken as white). The two anchors compete, and the competition resolves differently for the two target squares depending on their neighbourhoods. This explains why the effect is large · two anchoring systems reinforce each other.
Why the Grid Structure Matters
The Dungeon effect is much stronger than a simple simultaneous-contrast effect would be. Why?
The grid enforces grouping. In the classical simultaneous-contrast figure, the grey patch sits on a uniform background · there is no grouping for the visual system to latch onto beyond “patch + surround”. In the Dungeon, the grid provides rich grouping cues: rows, columns, interlocking chains, similar-luminance clusters. Every one of these cues participates in the brightness computation, and they all pull in the same direction. Remove the grid and keep only the two grey squares on their immediate neighbours, and the effect drops to classical simultaneous-contrast levels · clearly weaker.
A Harder Variant
Below is a Dungeon figure at difficulty 3, with a more complex grid. The two target squares remain identical.
Which patch is lighter?
Common misconception: “this is just simultaneous contrast.” The Dungeon effect is quantitatively much larger than a simple simultaneous-contrast effect with the same patch-to-immediate-neighbour luminance ratio. The extra magnitude comes from the grouping-anchoring interaction, which requires a grid (or other extended structure) to take effect. Calling it “just simultaneous contrast” misses the whole point of why Bressan named it separately.
The two-finger proof. Cover everything in the figure except the two target grey squares and their immediate edge-sharing neighbours. The effect collapses to classical simultaneous-contrast size (small, but still present). Uncover the rest of the grid and the effect snaps back to full strength. You are literally watching the grouping mechanism engage and disengage in real time, with nothing changing but the visible context.
Connection to Other Brightness Illusions
The Dungeon sits at the intersection of several illusions we have met:
- Like simultaneous contrast, it uses lateral inhibition as its baseline mechanism
- Like the Koffka ring, it depends on how the visual system groups the figure
- Like White’s illusion, it can run in directions that naïve contrast theory cannot predict
- Like Adelson’s checker shadow, it stacks multiple cues that all agree in direction
The modern brightness-illusion family. If you were teaching a course on brightness perception, you would use: simultaneous contrast for the 19th-century baseline, Mach bands for retinal edge-enhancement, Cornsweet for surface fill-in, Koffka ring for grouping, White’s for perceptual-organisation contrast, and the Dungeon for grid-based anchoring. Together these six illusions span the major mechanisms your visual system uses to compute brightness. The Dungeon is the most recent entry and one of the hardest to fully explain.
Where the Dungeon Mechanism Appears
- Tile floors. A grey-tile kitchen or bathroom floor shows subtle Dungeon-style shifts · tiles near darker grout read as lighter, tiles near lighter grout read as darker. The effect is small but measurable.
- Pixel art and game textures. Pixel artists working with large palettes have known for decades that identical hex-codes can look different in different grid positions. Modern game engines often include “dithering” and “colour-shift” tools that compensate for Dungeon-style grid anchoring.
- Spreadsheet and data visualisation. Heatmap-style visualisations where cells of similar values cluster can produce Dungeon illusions · a given cell’s colour reads differently depending on whether it is in a high-value cluster or a low-value cluster. Careful visualisation designers use colour palettes with perceptually uniform spacing to minimise this.
- Printed patterns. Checkered clothing, tiled backgrounds in packaging, geometric wallpapers · all produce Dungeon-style brightness interactions. The more regular the grid, the stronger the effect.
Test Yourself on 50 More Illusions
The Dungeon illusion is one of more than 50 classical illusions on PlayMemorize. Each round draws a deterministic SVG scene and asks one grounded question: which is larger, which is brighter, which is actually parallel. The reveal overlay shows the true geometry plus a one-line “why it works” caption.
- Keep playing Dungeon → · the standalone game, pinned to this one figure with fresh seeds each round
- Play Illusions → · spot the tricks across size, colour, orientation, and impossible figures
- Play Spatial → · train mental rotation and area estimation
- Play Matrix → · abstract pattern reasoning under time pressure
The takeaway. The Dungeon illusion is a reminder that brightness perception is a team sport: local contrast, extended grouping, scene-level anchoring, and edge computation all participate, and their combined effect can be much larger than any one mechanism would produce alone. It is one of the strongest demonstrations that your visual system is not a simple pixel-reading device but a multi-stage inference engine · and that even “this one grey square” is the output of several layers of contextual computation running simultaneously.
Illusions
Your eyes lie - the math knows the truth. Spot equal lengths, identical greys, and truly parallel lines across 57 classic optical illusions
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