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

Brian Paul brianp at vmware.com
Mon Jun 20 14:46:33 UTC 2016

On 06/17/2016 07:59 PM, Emil Velikov wrote:
> 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).

I think the "choose smallest that exceeds the user-specified minimum" is 
the way to go.

Suppose you have a system that has 3 color formats: RGB10_A2, RGBA8 and 
R5G6B5.  An app could specify WAFFLE_RED/GREEN/BLUE_SIZE=10 to get the 
first one, or WAFFLE_RED/GREEN/BLUE_SIZE=8 to get the second, etc.

> 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 ?

Sounds good. Do you want to do that?


More information about the Piglit mailing list