[vtk-developers] OSMesa depth buffer issues

[Francois Bertel]

Usually OpenGL implementations pack the depth buffer and the stencil
buffer in the same chunk of memory
24 of for depth+8 for stencil=32.

If you are using X window, type glxinfo and you'll see that most of
the visuals provide this kind of format.

Nothing odd about 16-bit depth buffer, as I said before there is no
API in vtkRenderWindow that allow you to specify the number of bits in
any buffer so it is let to the system to provide whatever it has as a
default configuration.
I agree it sucks and it should be a VTK feature request.

On Thu, Jan 28, 2010 at 7:18 PM, burlen <burlen.loring at gmail.com> wrote:
> Hi Guys,
>
> Yes I did try the environment variable. Just to be clear, we are talking
> about OSMesa. OSMesa doesn't honor that environment variable. To check me on
> this you can take a look at the mesa source code where OSMesaCreateContext
> is defined. Mesa-7.7/src/mesa/drivers/osmesa/osmesa.c:1068:
>
> To get a default value for the depth buffer it uses the compile time
> constant DEFAULT_SOFTWARE_DEPTH_BITS, which is conditionally #defined as 16
> at Mesa-7.7/src/mesa/main/config.h:280
>
> I guess this does present another solution, that I didn't see before, namely
> to add a -D DEFAULT_SOFTWARE_DEPTH_BITS=32 in my CFLAGS when I build mesa.
> at the same time it doesn't seem right for VTK to default to a 16 bit depth
> buffer...
>
> 24 bits of depth also solves the problem. but I would guess that 32 might be
> advantageous form a performance standpoint given this is a natural size of
> the intel registers... just a guess. How come you want 24?
>
> Francois Bertel wrote:
>>
>> Excellent point!
>>
>> On Thu, Jan 28, 2010 at 4:33 PM, Jeff Lee <jeff at cdnorthamerica.com> wrote:
>>
>>>
>>> I am surprised it doesn't honor
>>>
>>>  MESA_GLX_DEPTH_BITS=24
>>>  export MESA_GLX_DEPTH_BITS
>>>
>>> - this seems to work fine for me.
>>> -Jeff
>>>
>>> On Thu, Jan 28, 2010 at 4:30 PM, Francois Bertel
>>> <francois.bertel at kitware.com> wrote:
>>>
>>>>
>>>> Hi Burlen,
>>>>
>>>> I just looked at vtkWindow/vtkRenderWindow and too bad, there is no
>>>> API to let the user specify the render window settings regarding the
>>>> depth in bit of each type of buffer (color buffer, depth buffer,
>>>> stencil buffer, accumulation buffers) in the framebuffer.
>>>> That would have be better than hardcoding the values in
>>>> vtkXOpenGLRenderWindow.
>>>>
>>>> Anyway the typical values for on-screen are 24 bits for depth buffer,
>>>> 8 bits for stencil buffer, 16 bits per channel for accumulation
>>>> buffer.
>>>> What happen in your case if you use these values instead the one you
>>>> provided in your patch?
>>>>
>>>> On Thu, Jan 28, 2010 at 4:14 PM, burlen <burlen.loring at gmail.com> wrote:
>>>>
>>>>>
>>>>> Hi Guys,
>>>>>
>>>>> I'm having some issues in ParaView that relate back to VTK's OSMesa
>>>>> rendering context. Specifically VTK asks OSMesa for the default RGBA
>>>>> context, which only has a 16 bit depth buffer. That's causing some
>>>>> artifacts
>>>>> and compositing issues in certain situations. for example:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> http://nashi-submaster.ucsd.edu/movies/PV/tube-filter-artifact/mesa-artifacts.png
>>>>>
>>>>>
>>>>>
>>>>> http://nashi-submaster.ucsd.edu/movies/PV/tube-filter-artifact/mesa-artifacts-decomp.png
>>>>>
>>>>> Explicitly requesting a 32 bit depth buffer solves the problems.
>>>>>
>>>>> I attached a small patch that makes this change in VTK. It replaces the
>>>>> call
>>>>> to OSMesaCreateContext with one to OSMesaCreateContextExt specifying a
>>>>> 32
>>>>> bit depth buffer and carries forward the current default values for
>>>>> accumulation and stencil buffers. I guess this function was The
>>>>> OSMesaCreateContextExt was added to in mesa release 3.5, the current
>>>>> release
>>>>> is 7.7.
>>>>>
>>>>> Is this the right fix? Or is something more sophisticated necessary?
>>>>> e.g.
>>>>> run time configurable. And while we're on the subject, are OSMesa's
>>>>> other
>>>>> buffer defaults reasonable? accum (8 bits per channel) and stencil
>>>>> buffers
>>>>> (16 bits).
>>>>>
>>>>> Thanks
>>>>> Burlen
>>>>>
>>>>>
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>>>>>
>>>>>
>>>>>
>>>>
>>>> --
>>>> François Bertel, PhD  | Kitware Inc. Suite 204
>>>> 1 (518) 371 3971 x113 | 28 Corporate Drive
>>>>                     | Clifton Park NY 12065, USA
>>>> _______________________________________________
>>>> Powered by www.kitware.com
>>>>
>>>> Visit other Kitware open-source projects at
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>>>>
>>>
>>>
>>
>>
>>
>>
>
>



-- 
François Bertel, PhD  | Kitware Inc. Suite 204
1 (518) 371 3971 x113 | 28 Corporate Drive
                      | Clifton Park NY 12065, USA