/libultrahdr

Ultra HDR is a true HDR image format, and is backcompatible. libultrahdr is the reference codec for the Ultra HDR format. The codecs that support the format can render the HDR intent of the image on HDR displays; other codecs can still decode and display the SDR intent of the image.

Primary LanguageC++Apache License 2.0Apache-2.0

Introduction

libultrahdr is an image compression library that uses gain map technology to store and distribute HDR images. Conceptually on the encoding side, the library accepts SDR and HDR rendition of an image and from these a Gain Map (quotient between the two renditions) is computed. The library then uses backward compatible means to store the base image (SDR), gain map image and some associated metadata. Legacy readers that do not support handling the gain map image and/or metadata, will display the base image. Readers that support the format combine the base image with the gain map and render a high dynamic range image on compatible displays.

For additional information, see android hdr-image-format guide.

Build from source using CMake

This software suite has been built and tested on platforms:

  • Android
  • Linux
  • macOS
  • Windows

Refer to building.md for complete instructions.

Using libultrahdr

A detailed description of libultrahdr encode and decode api is included in ultrahdr_api.h and for sample usage refer demo app.

libultrahdr includes two classes of APIs, one to compress and the other to decompress HDR images:

Encoding api outline:

Scenario Hdr intent raw Sdr intent raw Sdr intent compressed Gain map compressed Quality Exif Use Case
API - 0 P010 or rgba1010102 or rgbaf16 No No No Optional Optional Used if, only hdr raw intent is present. 1
API - 1 P010 or rgba1010102 or rgbaf16 YUV420 or rgba8888 No No Optional Optional Used if, hdr raw and sdr raw intents are present.2
API - 2 P010 or rgba1010102 or rgbaf16 YUV420 or rgba8888 Yes No No No Used if, hdr raw, sdr raw and sdr compressed intents are present.3
API - 3 P010 or rgba1010102 or rgbaf16 No Yes No No No Used if, hdr raw and sdr compressed intents are present.4
API - 4 No No Yes Yes No No Used if, sdr compressed, gain map compressed and GainMap Metadata are present.5

Decoding api outline:

Configure display device characteristics (display transfer characteristics, max display boost) for optimal usage.

Input Usage
max_display_boost (optional, >= 1.0) the maximum available boost supported by a display.
supported color transfer format pairs
color transferColor format
SDR32bppRGBA8888
HDR_LINEAR64bppRGBAHalfFloat
HDR_PQ32bppRGBA1010102 PQ
HDR_HLG32bppRGBA1010102 HLG

Footnotes

  1. Tonemap hdr to sdr. Compute gain map from hdr and sdr. Compress sdr and gainmap at quality configured. Add exif if provided. Combine sdr compressed, gainmap in multi picture format with gainmap metadata. ↩

  2. Compute gain map from hdr and sdr. Compress sdr and gainmap at quality configured. Add exif if provided. Combine sdr compressed, gainmap in multi picture format with gainmap metadata. ↩

  3. Compute gain map from hdr and raw sdr. Compress gainmap. Combine sdr compressed, gainmap in multi picture format with gainmap metadata. ↩

  4. Decode compressed sdr input. Compute gain map from hdr and decoded sdr. Compress gainmap. Combine sdr compressed, gainmap in multi picture format with gainmap metadata. ↩

  5. Combine sdr compressed, gainmap in multi picture format with gainmap metadata. ↩