[Piglit] [waffle] long-standing wgl pixel format issue

Emil Velikov emil.l.velikov at gmail.com
Sat Jun 18 01:59:41 UTC 2016

On 17 June 2016 at 16:53, Brian Paul <brianp at vmware.com> wrote:
> I spent a few hours yesterday pulling out my hair trying to understand why
> the piglit fbo-mipmap-copypix test was failing on Windows.  But it was only
> failing when I ran it directly.  It passed when I ran it via piglit-run.py
> The key difference was the -fbo option.  With -fbo the test used an RGBA8
> framebuffer but without -fbo the test used an RGB565 framebuffer.
> So why is a 565 framebuffer being used?  It boils down to the fact that
> wglChoosePixelFormatARB() does not work like glXChooseFBConfig().
> From the glXChooseFBConfig man page:
> """
>      Each attribute, if present, must be followed by a nonnegative minimum
> size
>     specification or GLX_DONT_CARE.
>     The largest available total RGBA color buffer size (sum of GLX_RED_SIZE,
>     of at least the minimum size specified for each color component is
> preferred.
> """
> So if you specify GLX_RED_SIZE, BLUE_SIZE, etc to be 1 and there are both
> RGB565 and RGBA8 formats available, the _later_ (the largest) will be
> chosen.
> But the wglChoosePixelFormatARB docs say:
> """
> Some attribute values must match the pixel format value exactly when
>     the attribute is specified while others specify a minimum criteria,
>     meaning that the pixel format value must meet or exceed the
>     specified value.
>      Attribute                      Type        Match Criteria
>      WGL_RED_BITS_ARB               integer     minimum
>      WGL_GREEN_BITS_ARB             integer     minimum
>      WGL_BLUE_BITS_ARB              integer     minimum
>      WGL_ALPHA_BITS_ARB             integer     minimum
> """
> So if you specify WGL_RED/GREEN/BLUE_BITS_ARB to be 1 and there are both
> RGB565 and RGBA8 formats available, the _former_ may be chosen.  Note that
> some WGL apps use WGL_COLOR_BITS_ARB=24 and avoid this.
> Piglit's call to piglit_wfl_framework_init() uses an attribute list with
> WAFFLE_RED/GREEN/BLUE_SIZE = 1 and that winds up going directly to
> wglChoosePixelFormatARB and glXChooseFBConfig so this difference in behavior
> effects the window's pixel format.
Thanks for this Brian and apologies I did not spot these differences
as I was writing the WGL backend.

Here's a bit more comprehensive list, listing all the waffle backends
and attributes.

Largest - red, green, blue, alpha plus their accum counterparts + depth
Smallest - buffer, stencil

If requested size is zero - "largest" become "smallest" (but it's not
said it will be zero), "smallest" become "zero".

One that "most closely matches the specified size is preferred"

"At least", meaning that there's not definition if it's the "smallest"
or "largest". Furthermore there's not mention that it will give you
the smallest if you specify 0 :-\

> The Waffle docs for waffle_config_choose() say:
> """
>     The default value for each size attribute is 0. Valid values are the
> non-negative integers and WAFFLE_DONT_CARE. If the requested size for a
> channel is 0, then any surface created with the config will lack that
> channel. If the requested size for a channel is positive, then the number of
> bits in that channel for any surface created with the config will be at
> least the requested size.
> """
> There's some ambiguity here because if several different pixel formats (such
> as RGB565 and RGBA8) both meet the WAFFLE_RED/GREEN/BLUE_SIZE minimums,
> which should be preferred?
> I can fix my Windows Piglit issue by changing Piglit's
> choose_config_attribs() function to specify WAFFLE_RED/GREEN/BLUE_SIZE=8
> instead of 1, but that's not a final solution.
> I propose:
> 1. The Waffle docs should be clarified to specify whether the largest or
> smallest color format should be used when several meet the WAFFLE_*_SIZE
> minimums.  My suggesting is "smallest", like WGL.
> 2. The Waffle code for either GLX or WGL should be modified to follow that
> part of the spec.  Following my suggestion, the GLX format chooser code
> would need to be modified.
> 3. The Piglit code to specify the Waffle pixel format should be updated,
> probably replacing '1' with '8' as above.  And maybe falling back to the
> former if the later fails (though I doubt anyone runs piglit on less than a
> 24-bit display nowadays).
> 4. If Waffle wants to get fancy, we could consider new attributes like
> provide more control over format selection.  But I think my suggestion in
> (1) would avoid this for now.
> Thoughts?
I'm somewhat inclined that the GLX/EGL behaviour might be the better
choice. Then again I don't might if people choose another route -
always smallest, always largest, a combination of the two, minimum or
"most closely matches".
In each case there's the issue (as each spec has a massive table) of
how exactly to determine(sort) the config(s).

Now that I think of it I do recall seeing some heuristics on the topic
in SDL and Xwin. Perhaps it's worth checking what they do and using
something identical/close to it ?


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