On 06/17/2016 07:59 PM, Emil Velikov wrote:
On 17 June 2016 at 16:53, Brian Paul <bri...@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
     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

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

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.


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?


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