[RFC weston 13/16] compositor: Add a function to test if images transformed by a matrix should be bilinearly filtered
spitzak at gmail.com
Fri Oct 3 13:13:23 PDT 2014
On 10/03/2014 11:05 AM, Pekka Paalanen wrote:
>> The really lovely part about pixman is that (if I'm "getting" it
>> correctly) you transform the source image instead of transforming
>> destination vertices. So pixman needs things exactly backwards.
> Yeah, backwards sounds familiar. Just reverse the polarity... err,
> invert the matrix. It's only 3x3 at that point, and pixman can
> probably do it. We can optimize it later if needed. Or if it looks
> like the inverse form is more popular in general, we could just
> construct that to begin with. Later. :-)
Since pixman is trying to figure out what color to make a particular
output pixel, it needs the inverse matrix because that is how it
translates the xy of an output pixel to a location in the input image.
OpenGL and other libraries invert the matrix internally as they need
this same information when doing texture mapping.
I think you will have to keep track of both the matrix and the inverse
one. The inverse is needed to translate input events to client space.
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