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cogl: Pick glReadPixels format by target, not source

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Presumably glReadPixels itself can be more performant with pixel format
conversions than doing a fix-up conversion on the CPU afterwards. Hence,
pick required_format based on the destination rather than the source, so
that it has a better chance to avoid the fix-up conversion.

With CoglOnscreen objects, CoglFramebuffer::internal_format (the source
format) is also wrong. It is left to a default value and never set to
reflect the reality. In other words, read-pixels had an arbitrary
intermediate pixel format that was used in glReadPixels and then fix-up
conversion made it work for the destination.

The render buffers (GBM surface) are allocated as DRM_FORMAT_XRGB8888.
If the destination buffer is allocated as the same format, the Cogl
read-pixels first converts with glReadPixels XRGB -> ABGR because of the
above default format, and then the fix-up conversion does ABGR -> XRGB.
This case was observed with DisplayLink outputs, where the native
renderer must use the CPU copy path to fill the "secondary GPU"
framebuffers.

This patch stops using internal_format and uses the desired destination
format instead.

_cogl_framebuffer_gl_read_pixels_into_bitmap() will still use
internal_format to determine alpha premultiplication state and multiply
or un-multiply as needed. Luckily all the formats involved in the
DisplayLink use case are always _PRE and so is the default
internal_format too, so things work in practise.

Furthermore, the GL texture_swizzle extension can never apply to
glReadPixels. Not even with FBOs, as found in this discussion:
https://gitlab.gnome.org/GNOME/mutter/issues/72
Therefore the target_format argument is hardcoded to something that can
never match anything, which will prevent the swizzle from being assumed.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/313
This commit is contained in:
Pekka Paalanen 2018-11-06 16:27:52 +02:00 committed by Ray Strode
parent 6502735f01
commit 981b045459
1 changed files with 5 additions and 1 deletions
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6
cogl/cogl/driver/gl/cogl-framebuffer-gl.c
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@ -4,6 +4,7 @@
* A Low Level GPU Graphics and Utilities API
*
* Copyright (C) 2007,2008,2009,2012 Intel Corporation.
* Copyright (C) 2018 DisplayLink (UK) Ltd.
*
* Permission is hereby granted, free of charge, to any person
* obtaining a copy of this software and associated documentation
@ -1273,9 +1274,12 @@ _cogl_framebuffer_gl_read_pixels_into_bitmap (CoglFramebuffer *framebuffer,
if (!cogl_is_offscreen (framebuffer))
y = framebuffer_height - y - height;
/* Use target format ANY, because GL texture_swizzle extension cannot
* ever apply for glReadPixels.
*/
required_format = ctx->driver_vtable->pixel_format_to_gl_with_target (ctx,
framebuffer->internal_format,
format,
COGL_PIXEL_FORMAT_ANY,
&gl_intformat,
&gl_format,
&gl_type);