PNG Data Requirements
=====================

*Status*: proposal

*Author*: unknown

*Submission date*: unknown

*Implementations*: unknown

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*: File Layout

*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 <http://www.w3.org/TR/PNG/#4Concepts.ColourSpaces>`__.
-  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*: File Layout ('Data' section)

*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.