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What Is Ultra HDR? Gain Map JPEGs Explained
Ultra HDR is a JPEG with a gain map: a normal photo for every screen, brighter highlights on HDR screens. How it works, who supports it, and its limits.
Ultra HDR is Google's image format for HDR photos: an ordinary JPEG with a second, small image inside called a gain map. Any app shows the normal JPEG, so the photo works everywhere. On an HDR-capable screen, a supporting app uses the gain map to show the brightest parts of the photo brighter than normal white, closer to how the scene looked. Android introduced it in Android 14, and the gain map approach is now an ISO standard that Apple also supports.
How a gain map works
A normal photo is SDR (standard dynamic range): its brightest white is the screen's normal white. HDR screens can go much brighter than that for small areas, like a sunlit cloud or a lamp. An Ultra HDR file uses that headroom without breaking compatibility:
- Base image: a regular SDR JPEG, what every viewer shows.
- Gain map: a grayscale or color map, usually smaller than the photo, saying for each area how much to brighten the base image to reach the HDR version. Dark areas get little or no boost; highlights get the most.
- Metadata: numbers that describe the range of the gain map and how much display headroom it was made for.
A supporting viewer checks how much extra brightness the screen has right now, then applies the gain map in proportion. A screen with a lot of headroom shows the full effect; a screen with less shows part of it; an SDR screen shows the base image unchanged.
What's inside the file
Google's Ultra HDR specification (version 1.1) defines the details:
| Part | How it's stored |
|---|---|
| Base image | Primary JPEG, readable by any JPEG decoder |
| Gain map | Second JPEG, 8-bit, appended using the CIPA Multi-Picture Format (MPF) |
| Ultra HDR metadata | XMP in the hdrgm namespace |
| ISO metadata | ISO 21496-1 gain map metadata, which can sit alongside the XMP |
Version 1.1 of the spec added no changes to the format itself, only guidance on ISO 21496-1: encoders should write both kinds of metadata for compatibility, and decoders should prefer the ISO metadata when both are present. Android 15 added ISO 21496-1 support for encoding and decoding JPEG bitmaps.
The ISO 21496-1 standard
ISO 21496-1:2025, "Digital photography: Gain map metadata for image conversion, Part 1: Dynamic range conversion", was published in July 2025. It standardizes the gain map metadata and the math for applying it, so a file made on one platform renders the same way on another. Google's Ultra HDR, Apple's gain map work and other vendors' formats now converge on it.
Apple and gain maps
Apple has captured iPhone photos with embedded gain maps since 2020, in its own format. At WWDC 2024 Apple presented Adaptive HDR, its gain map approach brought into the ISO standardization effort, and added APIs in iOS 18 and macOS 15 to read and write ISO gain maps in HEIC and JPEG files. In iOS 18 and macOS 15, HDR photo display also expanded beyond Photos to Messages, Quick Look and Preview.
In practice: a gain map JPEG made with ISO 21496-1 metadata can show its highlights on a recent iPhone or Mac with an HDR display, and on older systems it shows the base image.
Browser support
- Chromium browsers (Chrome, Edge and others) display gain map JPEGs in HDR on HDR-capable screens.
- Safari 26 (iOS 26, iPadOS 26, macOS 26) added HDR image support on the web, along with the CSS
dynamic-range-limitproperty that lets a page tone HDR images down. - Other browsers may show only the base image. That's the point of the format: nothing breaks, you just see the SDR version.
To see the effect, you need both an HDR screen (most recent phones, many laptops, HDR monitors with HDR turned on in the operating system) and a supporting app. Low power mode and reduced brightness settings can shrink the headroom.
When converting an SDR photo to Ultra HDR helps
You can take an ordinary JPG, PNG or WebP and turn it into an Ultra HDR JPEG by generating a gain map from the image itself. Our Ultra HDR converter does this in your browser, with a slider for how much the highlights glow; the photo isn't uploaded.
Be clear about what this does:
- It helps when a photo has bright elements that look flat on an HDR phone next to native HDR shots: sunsets, city lights, snow, reflections, a product on a white background. The highlights get extra brightness on HDR screens and the rest of the image stays the same.
- It doesn't add captured dynamic range. An SDR file has already lost the detail in clipped highlights and crushed shadows. The converter estimates which areas should glow; it can't bring back a sky that was recorded as flat white.
- It doesn't fix a dark photo. For that, brighten it first; see how to brighten a dark photo or use the image brightener, then convert.
- Too much looks fake. Strong boosts make whole areas glare. A moderate setting usually looks better.
Keeping the HDR part intact
A gain map is extra data in the file, so it's easy to lose:
- Re-encoding in an editor or a compressor that doesn't know about gain maps saves only the base image.
- Metadata removal can drop the XMP or the second image. If a privacy tool strips everything, expect an SDR result.
- Chat apps and social networks often recompress uploads; some keep HDR, many don't.
- Screenshots of an HDR photo are captured in SDR or in the screen's own format, not as a gain map file.
To check a file, open it in the photo metadata viewer and look for the hdrgm XMP fields. If you need a smaller file, see how to compress an image without losing quality, and test the output on an HDR screen afterward.
Ultra HDR vs other HDR photo formats
| Format | Backward compatible | Where you'll meet it |
|---|---|---|
| Ultra HDR JPEG | Yes, base image is a normal JPEG | Android 14+ cameras, Pixel, Samsung "Super HDR" |
| HEIC with gain map | Yes, on devices that read HEIC | iPhone photos |
| AVIF / JPEG XL with gain map | Depends on the viewer | Web and pro tools |
| PQ or HLG HDR image | Not always; can look dull or off in SDR-only apps | Video-style HDR stills |
Gain map formats win for everyday sharing because they degrade gracefully.
Privacy and HDR photos
An Ultra HDR file carries the same EXIF fields as any JPEG, including location if your camera recorded it. sigil. keeps photos in an encrypted vault on your phone, opened only by the sign you draw on a 5×5 grid, and strips EXIF and location metadata when photos are imported and exported. Photos you take with its private camera stay out of the Camera Roll.
Bottom line
Ultra HDR is a regular JPEG plus a gain map. Everyone sees the photo; people with an HDR screen and a supporting app see brighter highlights. The metadata now follows ISO 21496-1, which both Android and Apple support. Converting an SDR photo can make its highlights glow, but it adds display brightness, not detail the camera never captured.
Sources
Frequently asked questions
What is an Ultra HDR photo?
A JPEG that holds a normal (SDR) image plus a small gain map. Any viewer shows the normal image; HDR-capable phones, browsers and screens use the gain map to show brighter highlights.
Is Ultra HDR the same as HDR mode on my camera?
No. Camera HDR mode merges several exposures to fit a wide scene into one photo. Ultra HDR is a file format that tells HDR screens how much brighter each part of that photo can be shown.
Do Ultra HDR photos work on iPhone?
Apple supports gain map images built on the same ISO 21496-1 standard since iOS 18 and macOS 15. On older systems or SDR screens you simply see the normal image.
Why does my Ultra HDR photo look normal on some screens?
The screen or app doesn't support HDR display, or HDR brightness is limited, for example in low power mode. The gain map is ignored and the base image is shown.
Can converting an old photo to Ultra HDR add dynamic range?
No. It can make the brightest parts glow on HDR screens, but it can't recover detail in shadows or highlights that the original file never captured.