On 09/30/2014 12:35 PM, Derek Foreman wrote:
Argh - thanks. Why isn't Z scale relevant? I'm worried about making
assumptions about the transformations these matrices represent and
having those assumptions violated in the future... For Z not to matter
are we assuming projection will always be orthographic?
A projection matrix will have non-zero at [3,2] so that Z contributes to
the output W. The output Z is only used to set the Z in the depth buffer
(which I doubt you are using).
I recommend instead of checking the rotation to instead look for zeros
in the correct locations in the matrix. Matrix must be of the form:
|sx 0 0 tx|
|0 sy 0 ty|
|? ? ? ?|
|0 0 0 1|
or
|0 sx 0 tx|
|sy 0 0 ty|
|? ? ? ?|
|0 0 0 1|
sx and sy must be ±1, and tx and ty must be integers. The ? can be any
value.
That could save us the very expensive matrix decomposition. I'll try
this out. Thanks.
I think this may be better than decomposition for deciding to use video
planes in compositor-drm as well.
Most code just looks at the matrix for this. The Cairo and Pixman code
works this way, for instance.
This is also used for the gl renderer, so I don't think I can count on
that short circuit there... Though bilinear vs nearest doesn't have
anywhere near the same performance impact there.
It may be worthwhile to check the Mesa code to see if it does this sort
of optimization when using software renderings.
I believe the hardware for bilinear sampling is so fast and optimized
that you do not save anything by asking for impulse. And I got some
indications that NVidia's driver does impulse sampling for any samples
sufficiently close to the center. This was in a floating point image,
where a math error of about 1/100 of a pixel still produced the expected
identity image. An equivalent OpenCL program mixed in 1/100 of the
neighboring pixel, which unfortunately had a very large floating point
value, so it made the math error visible. But the NVidia driver hid it.
This seems to indicate some optimization is done somewhere.
Are you suggesting pixman always be set to use bilinear, and it'll just
pick nearest automatically when the image transform doesn't require it?
I am currently working on pixman and cairo code, and both of them
already do tests (ie before any changes I am submitting) for this and
use nearest filter if possible. You can actually use the default "good"
filter all the time as it is the same as bilinear (for scaling up).
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