What Is Ultra HDR?

Ultra HDR stores an ordinary authored SDR base image together with a gain map and metadata. A compatible HDR renderer combines them to expand brightness and color on a suitable display. Older software ignores the extra data and shows the normal SDR JPEG.

This backwards-compatible design means that making an SDR copy usually does not require inventing a tone curve. Extracting the authored SDR base preserves the image the file already provides for non-HDR readers.

A bright image or HDR word in metadata does not prove that a usable gain map exists. Detection should check recognized XMP or ISO metadata, the primary and secondary image relationship, sizes, offsets, and the actual secondary image. Malformed files deserve an incomplete result, not a false positive.

Whether a valid file is displayed in HDR is a second question controlled by the browser, operating system, display, and color pipeline. A web page should label an original preview as browser-managed rather than promising that true HDR is active.

A lossless removal path keeps the primary JPEG scan bytes, removes gain-map MPF or container signals, and discards trailing secondary data. Re-encoding the visible SDR pixels is unnecessary and can add another generation of JPEG loss.

HEIC and AVIF gain maps use different container rules. A tool should report them as unsupported until its browser codec has been validated against reference fixtures instead of treating the JPEG method as universal.