[Mesa-dev] [PATCH 2/4] mesa: Add helper function to see if we have a float depth format.

[Roland Scheidegger]

Am 15.09.2014 19:04, schrieb Mathias Fröhlich:
> 
> Hi again,
> 
> On Monday, September 15, 2014 18:11:51 Roland Scheidegger wrote:
>> btw I'm not entirely sure everything is really correct with llvmpipe wrt
>> that extension (the rasterization stuff, gallium isn't involved in the
>> texture conversion). There's some very annoying differences how things
>> work between d3d10 and GL there (I suspect there's actually bugs wrt
>> d3d10 too especially when outputting depth from the pixel shader but so
>> far we got away with it).
>> So, with old style GL rendering, you can't actually produce depth values
>> outside 0/1, due to clipping. But this is no longer true since a while
>> for various reasons - fragment shader writing depth, ARB_depth_clamp,
>> clip control... There's various places such values are clamped later -
>> with ARB_depth_clamp if enabled they are clamped to min/max depth range
>> (hence not outside 0/1 unless using the nv extension), I'm pretty sure
>> there's also a 0/1 clamp elsewhere if you output depth from a fragment
>> shader.
>> With d3d10, you can likewise disable clipping, but there's no 0/1 clamp
>> anywhere in sight (just to value range reproducible by depth buffer
>> format), and the depth clamp is always enabled, regardless any depth
>> clip enable / disable bits (clamping to the zmin/max range, which is not
>> limited).
>> In any case, I think there's a lot of possiblities of subtle bugs when
>> trying to support this extension.
> 
> You are actually mentioning clip control.
> I knew that there is something in the queue, but I thought that this is
> not available as an extension and only available with new enough OpenGL
> versions. But there actually is ARB_clip_control.
> 
> This one actually fits the bill.
> Am I right that you would not object against ARB_clip_control?
I don't have anything against this at all, though I think it's only
partly related (and as far as I can tell, issue 8 of ARB_clip_control is
totally bogus wrt the changed behavior of GL for depth range, as this
was a retroactively fixed spec error). gallium (and the software drivers
via draw) should be able to do that already I believe. It won't do
anything against the weird fragment shader depth output clamp, for instance.
Maybe someone else can provide some more input about this stuff, I bet
there were some discussions at some point about 0/1 depth clamps at some
point in the ARB/Khronos.

Roland