HDR stands for High Dynamic Range. "Dynamic range" is just the ratio between the brightest and darkest parts of a scene an image can represent. Your eyes handle an enormous range — a sunlit window and a shadowed room in the same glance. Most screens, historically, couldn't come close. HDR is the effort to close that gap.
It's about brightness, not just color
A lot of people assume HDR means "more colorful" or "more contrasty-looking." Sometimes it correlates with both, but the actual technical claim is narrower: an HDR image can specify luminance values — how bright a pixel really is — well beyond what a standard (SDR) image can. SDR tops out around 100 nits of reference brightness. HDR content can specify highlights at 1,000, 4,000, even 10,000 nits, while the shadows stay just as dark as before.
Your phone's "HDR" photo mode is a different thing
This is the confusion we run into constantly. Most phone camera "HDR" modes merge multiple exposures into a single, better-balanced SDR photo — no highlight ever actually exceeds normal brightness, it's just tone-mapped to look punchier. That's a legitimate and useful technique, but it produces an SDR file. A genuinely HDR file carries real information about which pixels should render brighter than SDR white on a display that can show it.
Where the "extra" brightness information lives
Different formats do this differently. Some embed a completely different transfer function (the curve that maps stored code values to light output) that's built for absolute high brightness — that's PQ. Others use a gain map: a normal SDR image plus a second, smaller image that says how much brighter each pixel should get on a capable display. That second approach is what our Encode tool produces, because it's the one format Chromium-based browsers can genuinely build and render locally today, verified end to end.
Why this matters practically
If you're publishing images, HDR only helps the people viewing them on HDR-capable screens — and even then, only if the platform they're viewing it on (browser, app, social network) actually respects the HDR data instead of flattening it. For everyone else, a good HDR file should still degrade gracefully to a normal-looking SDR image. That "falls back safely" property is one of the reasons gain-map HDR is appealing: the base image underneath is just a regular JPEG.