About HDR
On an ordinary display, white is as bright as the screen goes, and every tone in the photo sits below it. An HDR display goes brighter than that white. Fotometis calls the room above white HDR Headroom, and measures it in stops, from 0 to 4. One stop is twice the brightness of white, and 4 stops is sixteen times.
The headroom holds the brightest highlights, for example water, cloud with the sun behind it, or a lit window. Everything below the top of the range stays as it was. A photo with no bright highlights gains nothing from headroom.
What shows it
Section titled “What shows it”To see the headroom you need a display that supports HDR and software that reads gain maps. A recent MacBook Pro display reaches 4 stops. Fotometis shows as much of the headroom as your display has, and it shows the SDR photo on a display that does not support HDR.
Fotometis writes the headroom into an exported file as an ISO 21496-1 gain map. JPEG, HEIC, and AVIF carry one. Software that reads the map shows the expanded highlights. Software that does not read the map shows the SDR photo.
A photo with HDR Headroom at 0 gets no gain map. It is an ordinary SDR photo, and every screen shows it.
Gain maps and 10-bit HDR
Section titled “Gain maps and 10-bit HDR”There are two ways to put HDR in a file.
One bakes a wide transfer function into the image, for example Rec. 2100 PQ, usually at 10 bits per channel. The file states the brightness of each pixel directly, and holds no SDR version of the photo.
The other is a gain map. The file holds an ordinary 8-bit SDR image under an ordinary color profile, and the map says, for each pixel, how much brighter an HDR display can show it.
AVIF and HEIC hold either kind. JPEG holds only a gain map. Fotometis writes all three with a gain map. An export is Display P3 or sRGB.
HDR displays are still uncommon. The fallback for a gain map on an SDR display is the SDR image. A 10-bit PQ image has no such fallback, and renders washed out on an SDR display. Gain maps are the most compatible way to deliver HDR content today, so Fotometis supports gain maps only.
HDR and prints
Section titled “HDR and prints”HDR is not a good option for a print. Paper reflects light instead of emitting it, so a print cannot show brightness above white. A print made from an HDR photo shows the SDR photo, and the work you did in the headroom does not appear. If you intend to print a photo, do not develop it with HDR.