Experiment · HDR headroom
Brighter than white
Screens composite pixels far brighter than the white of a webpage. That unused headroom is a material: a highlight here isn't a lighter shade, it's a physically brighter light. On a screen that can't show it, the same drawing tone-maps down and still holds its shape.
Capability gate
What this screen can show
The piece detects its own conditions rather than guessing from the browser name. HDR canvas support requires an HDR display and a Chromium browser; Firefox and Safari fall back to the SDR rendition below, which is the honest default. The chips update live if you drag the window onto a different display.
The foil
A sheet of light
Move your pointer, or tilt the phone. The seam divides one drawing into two renditions.
Left, the HDR rendition: the glint carries values above 1.0. Right, its SDR twin: every value clamped to white. On an HDR screen the seam is where the extra light decides whether it exists; on an SDR screen the two halves are identical, which is the point.
How it works
The unused register
UI white is a promise, not a ceiling. HDR displays can push pixels well past it, and web canvas has started to expose that register: a Chromium canvas can be created in an HDR color space with a float16 backing store, and colors written above 1.0 survive compositing instead of clamping. What you see here is drawn three ways at once — a normal base, a bright band, and a hot core with components in the 1.5–2.0 range. On an HDR screen that core is physically brighter than the page around it. On an SDR screen it clamps to white, which is why the same file is a coherent drawing everywhere and a luminous one only where it can be.
The larger idea is the one the file formats already encode: gain-map formats like Ultra HDR JPEG and Apple's HEIC store two renditions of one image, so an image can be honestly different on SDR and HDR displays — hidden highlights, a second reading, a watermark. The Split view is that same trick without the file: one canvas, the HDR rendition on the left and its clamped twin on the right, drawn from the same geometry. Where they differ is the register the screen can actually let through.