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protocol: specify the exact form of premultiplication

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There are two ways to do pre-multiplication of the alpha channel into
the color channels: on optical values or on electrical values. While
pre-multiplication with optical values is arguably more correct, because
operations like blending or scaling require pre-multiplied, optical
color channels, wayland and compositors by default work with
pre-multiplied electrical values. This is most likely a convention that
Wayland took from Cairo.

This commit makes sure that the expectation of pre-multiplied electrical
values is properly documented.

Signed-off-by: Sebastian Wick <sebastian.wick@redhat.com>
This commit is contained in:
Sebastian Wick 2023-05-09 13:55:53 +02:00
parent 2aec19ce41
commit f3026c916e
1 changed files with 2 additions and 2 deletions
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4
protocol/wayland.xml
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@ -469,8 +469,8 @@
interface.
If the buffer uses a format that has an alpha channel, the alpha channel
is assumed to be premultiplied in the color channels unless otherwise
specified.
is assumed to be premultiplied in the electrical color channel values
(after transfer function encoding) unless otherwise specified.
Note, because wl_buffer objects are created from multiple independent
factory interfaces, the wl_buffer interface is frozen at version 1.