Jeff's AppsBegin with a dependable SDR picture
The base image is the version older software can understand. It should look complete on its own, with sensible contrast and no missing highlights. That matters because plenty of apps will never ask for the HDR rendition.
The gain map describes a difference
The map is not a second full photograph. Think of it as a per-pixel brightness recipe. Dark areas usually receive little or no lift. A lamp, reflection, white logo, or patch of snow may receive much more. Compatible software combines the base picture with that recipe according to the display's available headroom.
Base image
The complete SDR fallback. It travels well and remains readable on ordinary screens.
Gain map
A compact description of where and how strongly brightness can expand.
Display headroom
The brightness a device can produce above its standard white level at that moment.
HDR result
The base and map combined, adapted to the particular screen instead of assuming every display is identical.
Why the fallback matters
A good gain-map file fails gracefully. If a browser ignores the map, the viewer still sees the base image. If an HDR display has only modest headroom available, the software can apply a restrained version instead of clipping the picture.
What can break the effect
Recompression can discard auxiliary data. Editing software may decode only the SDR base. A sharing service may create a smaller copy without carrying the map forward. The file extension alone does not prove that the extra information is still present.
Easy HDR Glow is designed around this dual-rendering idea: keep a solid ordinary image, then add controlled brightness where it earns its place.