RGB to YUV converison was previously baked into every scale shader, but
this work has been moved to the YUV packing shaders. The scale shaders
now write RGBA instead. In the case where base and output resolutions
are identical, the render texture is forwarded directly to the YUV pack
step, skipping an entire fullscreen pass.
Intel GPA, SetStablePowerState, Intel HD Graphics 530, NV12
1920x1080, Before:
RGBA -> UYVX: ~321 us
UYVX -> Y: ~480 us
UYVX -> UV: ~127 us
1920x1080, After:
[forward render texture]
RGBA -> Y: ~487 us
RGBA -> UV: ~131 us
1920x1080 -> 1280x720, Before:
RGBA -> UYVX: ~268 us
UYVX -> Y: ~209 us
UYVX -> UV: ~57 us
1920x1080 -> 1280x720, After:
RGBA -> RGBA (rescale): ~268 us
RGBA -> Y: ~210 us
RGBA -> UV: ~58 us
There are cases where alpha is multiplied unnecessarily. This change
attempts to use premultiplied alpha blending for composition.
To keep this change simple, The filter chain will continue to use
straight alpha. Otherwise, every source would need to modified to output
premultiplied, and every filter modified for premultiplied input.
"DrawAlphaDivide" shader techniques have been added to convert from
premultiplied alpha to straight alpha for final output. "DrawMatrix"
techniques ignore alpha, so they do not appear to need changing.
One remaining issue is that scale effects are set up here to use the
same shader logic for both scale filters (straight alpha - incorrectly),
and output composition (premultiplied alpha - correctly). A fix could be
made to add additional shaders for straight alpha, but the "real" fix
may be to eliminate the straight alpha path at some point.
For graphics, SrcBlendAlpha and DestBlendAlpha were both ONE, and could
combine together to form alpha values greater than one. This is not as
noticeable of a problem for UNORM targets because the channels are
clamped, but it will likely become a problem in more situations if FLOAT
targets are used.
This change switches DestBlendAlpha to INVSRCALPHA. The blending
behavior of stacked transparents is preserved without overflowing the
alpha channel.
obs-transitions: Use premultiplied alpha blend, and simplify shaders
because both inputs and outputs use premultiplied alpha now.
Fixes https://obsproject.com/mantis/view.php?id=1108
Currently several shaders need "DrawMatrix" techniques to support the
possibility that the input texture is a "YUV" format. Also, "DrawMatrix"
is overloaded for translation in both directions when it is written for
RGB to "YUV" only.
A cleaner solution is to handle "YUV" to RGB up-front as part of format
conversion, and ensure only RGB inputs reach the other shaders. This is
necessary to someday perform correct scale filtering without the cost of
redundant "YUV" conversions per texture tap.
A necessary prerequisite for this is to add conversion support for
VIDEO_FORMAT_I444, and that is now in place. There was already a hack in
place to cover VIDEO_FORMAT_Y800. All other "YUV" formats already have
conversion functions.
"DrawMatrix" has been removed from shaders that only supported "YUV" to
RGB conversions. It still exists in shaders that perform RGB to "YUV"
conversions, and the implementations have been sanitized accordingly.
