Brian Paul wrote:
> Ian Romanick wrote:
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>>
>> It looks like adding support for GLX_SGI_make_current_read is almost as
>> easy as just enabling the extension thanks to Mesa's internal
>> framebuffer object changes. Thanks Brian!
>>
>> However, there is one thing that I don't get. Each driver supplies a
>> GetBufferSize callback. This callback is passed a GLframebuffer
>> pointer, but all of the drivers ignore it. They simply return the size
>> of the currently bound drawable. Should these functions determine if
>> the supplied GLframebuffer is the current drawable or readable and
>> return the appropriate size?
>
> GetBufferSize() should return the width/height of the X window (or
> Pbuffer) that corresponds to the given GLframebuffer.
>
> Unfortunately, GetBufferSize() isn't correctly implemented in most
> (any?) DRI drivers. It really needs to be fixed to support
> make_current_read.
Well, actually it is impossible to implement correctly. For any given
window you have a possible implementation:
LOCK_HARDWARE()
(do something to update window dimensions)
UNLOCK_HARDWARE()
The trouble is that as soon as you unlock the hardware, the window
dimensions are no longer trustworthy. If you use those values in such a
way that carries over to another hardware interaction, you end up with
potentially incorrect rendering.
This has lead to real bugs in things like the driver clear functions -
Mesa attempts to pass in some x,y,w,h values to the clear function under
certain circumstances (when the 'all' parameter is non-true). These
values are not trustworthy and must be recalculated once LOCK_HARDWARE
has been called again.
In general, GetBufferSize is not an answerable question. It's been a
vague irritant for a long time, but up until bugs like the above started
surfacing I was sort of ok about ignoring the issue. I need to get a
better understanding of where the buffer size values are used and under
what conditions they need to be updated. As it stands I think there are
any number of latent bugs hidden behind the assumption that we can know
what size a window is when the hardware lock is not held.
Keith
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