www/ Specifications/ OpenRaster/ Draft/ PngDataRequirements

This page is part of the Draft OpenRaster specification.

State: Proposal
Affects: ../FileLayout (addenda)

Proposal: PNG Data Requirements

The OpenRaster specification should place certain limitations on image data within .ora containers in order to ensure that the widest range of applications can open them and display them correctly.

General PNG data requirements

Current specification: ../FileLayout
Change: new section

Unless otherwise specified for a particular kind of PNG data stream in an OpenRaster file;

  • PNG data should not be interlaced (OpenRaster is not a streaming format)
  • Applications reading OpenRaster data should read and process colour management chunks present in PNG files as described in the PNG specification.
    • This specification does not place requirements on how applications should use this colour management information internally.
    • It may however express filters or layer blending modes in terms of particular colour spaces.
  • Applications writing OpenRaster files should write appropriate colour management chunks to each PNG file in the wrapper.

Layer data requirements

Current specification: ../FileLayout#data
Change: addendum

Stored data files for layers should be encoded in PNG format

Thumbnails/thumbnail.png

Current specification: ../FileLayout#Thumbnails.2BAC8-thumbnail.png
Change: addendum

Thumbnail images should be encoded in the sRGB colour space. They should be written with an sRGB chunk (and the "Perceptual" rendering intent), and with the canonical cHRM and gAMA chunks and data values for the sRGB encoding colour space.