Purple at Fixation: A Distance‑Dependent Color Illusion
I recently read an interesting article on an optical illusion, driven by our visual perception: arxiv.org/abs/2509.11582. In short, purple can look purple at fixation but shift bluish in the periphery. Below are interactive demos and a simple narrative of what I understood from the paper.
Important: To see this illusion properly, turn off any features that alter colors (e.g., Night Shift/Night Light, blue‑light filters). For best results, view on an OLED/AMOLED display.
Which polygon appears more purple?
Let's start via a vote. Look at what is below and vote for which polygon appears more purple to you.
Cast your vote:
By now you should have noticed that everything you see appears more purple once you focus on it!
Why it happens
The human retina contains three types of cone photoreceptors: long‑wavelength (L), medium‑wavelength (M), and short‑wavelength (S). Purple is a non‑spectral hue, perceived mainly from simultaneous L+S activity with relatively less M. At fixation the image falls on the foveola (the central ≈0.3° of the fovea), where S‑cones are sparse (~8–12% overall and nearly absent at the very center). In addition, macular pigment (lutein/zeaxanthin) in the inner retina absorbs short‑wavelength (blue) light before it reaches the photoreceptors. Both effects reduce S‑cone input at fixation, so the same stimulus appears less bluish when you look directly at it.
Outside the foveola (para‑foveal/peripheral retina), S‑cone density is higher and macular‑pigment influence is lower. With stronger S‑cone input, the same stimulus shifts toward a bluer appearance in peripheral vision, so the perceived color depends on where on the retina the light lands.
Viewing distance also matters. Reducing retinal image size (for example by stepping back or using smaller elements) places a larger fraction of each item within the S‑cone‑sparse foveola, so more items appear purple rather than bluish. That's why the grid and the moving shapes change with both fixation and distance.
Background and surround also modulate the effect: a short‑wave‑rich surround (like this blue background) can accentuate the peripheral bluish shift, while different surrounds can reduce it.
TL;DR: Purple is a color your brain invents by mixing red and blue signals. Your eye center is bad at detecting blue, so when you look directly at purple, your brain struggles. But when you look away, your peripheral vision sees more blue, making the same object look bluer. It's all about how your brain interprets the same color information differently depending on where you're looking!
Now you can test how this is distance dependent. For that, you don't have to really move from your monitor (Laziness makes you smart!), but expose more surface area to see the effect.
Adjustable Grid
You can change the grid size and the element size to see how it changes and observe its distance dependence. Try different combinations and notice how more dots appear purple when they're smaller or when the grid is larger!
Vanishing Poem
You can also write your own poem here, save it, and send it to a friend!