Any protocol documentation about premultiplied alpha?

[Hoosier, Matt]

Hi there,



Is there any definitive word about whether or not clients should be expected or allowed to premultiply alpha the alpha channels in the normal EGL and linux-dmabuf buffers they send to a compositor?


I have never seen this practice used with Wayland -- the shaders in the gl-renderer certainly don't seem to expect clients to premultiply. But I stumbled on a comment or two that maybe gnome-calculator [1] does this and that Sway settled on that convention [2].



I see hardly any mention of premult in the Weston codebase. In fact, the only real reference to it occurs at a site dealing with Pixman color formats. In this location, it's a prominent warning to the reader that Pixman does use premultiplication; this seems to carry the meaning that Pixman's premult usage stands apart from the rest of Weston.


To me, it seems that Wayland's adoption of the DRM_FORMAT_xxx identifiers in the wire protocols (e.g., dmabuf) combined with drm_fourcc.h's complete stonewalled silence on the subject, seems to be a fairly strong indicating that premultiplication should not be used.



-Matt


[1] https://github.com/swaywm/wlroots/issues/984#issue-324334642


[2] https://github.com/swaywm/wlroots/issues/984#issuecomment-390221778



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