Three Spotlights, One Wall
Red, green and blue spotlights overlap on a white wall. The prompt gives the arrangement and the three colours, and stops there. What colour each overlap turns is fixed by one rule the model is never told, and you can check every one of them yourself without leaving the picture.
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Input & prompt
Prompt template
A photograph of a white wall in a darkened room. Three round spotlights of the same size are projected onto the wall, one red, one green and one blue, arranged in a triangle so that each circle overlaps the other two, with a small area at the centre falling inside all three.
Before you scroll
What the correct answer looks like
Every patch of wall shows the sum of the lamps that reach it. Light adds. It does not mix like paint.
Overlapping lamps are the rare image-generation test with one right answer you can compute rather than argue about. Both renders below were produced by adding the lamp colours together — no artistic licence, no grading. Use them as the key when you scroll down to the model results.
Red, green, blue lamps
The version almost everyone has seen before. Pairs make yellow, cyan and magenta; all three make white.
Cyan, magenta, yellow lamps
The same rule with lamps nobody has memorised. Pairs make blue, red and green — and the centre is still white, because these are lamps, not inks.
The part almost nobody knows
Getting the colours right is the easy half, and it is the half that can be recalled rather than reasoned. The harder half is brightness: each time another lamp lands on a patch of wall, that patch gets brighter. The bars below are the actual measured brightness of each region in the renders above.
- One lamp85
- Two lamps overlapping170
- All three — white255
Identical on both palettes, because the palette never mattered. This is also why a correct render looks dim: if three lamps must add up to white, each one can only be a third of the way there on its own.
What wrong looks like
All three of these show cyan, magenta and yellow lamps. Only one of them is light. The centre settles it on its own.
Correct — light adds
White centre, and every overlap brighter than the circles around it.
Wrong — mixed like paint
The centre goes black and vanishes into the dark wall. This is ink on paper: it gets darker where more light lands, which is backwards.
Wrong — averaged, not added
Grey centre. The hues are right, so this one survives a glance — which is exactly why brightness is the test.
Every region in these figures is arithmetic on the lamp colours, not an artist's impression.
Open the full reference figure →ChatGPT (GPT Image 2)
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