[vtk-developers] vtk-developers Digest, Vol 143, Issue 30

Fri Mar 25 09:20:41 EDT 2016

There was a bug in VTK 7 where it was not reporting depth peeling as being
used when really it was being used.

To make a long story long :-) I do not like Renderer having an ivar and
maintaining state about depth peeling. I feel like the render passes should
not pollute the core graphics objects with specifics about them (even
though I myself have done it many times) So I started removing/not updating
the LastRenderUsedDepthPeeling code. One of the recent VTK releases went
out with that method not reporting correctly.  It is fixed in master as of
a while ago.  I still think it is a bad method but at least in master it
should report correctly.

Thanks
Ken

On Thu, Mar 24, 2016 at 1:08 PM, Brad Hollister <behollis at sci.utah.edu>
wrote:

> This should help. However, it looks like depth peeling isn't being enable
> regardless of using custom shaders.
>
> I just saw this old email thread:
>
> http://www.vtk.org/pipermail/vtkusers/2010-August/062106.html
>
> We're using QVTK / Python bindings. Is there a problem with depth peeling
> and the Python modules?
>
> Regards,
> Brad
>
> Date: Thu, 24 Mar 2016 08:26:30 -0400
>> From: Ken Martin <ken.martin at kitware.com>
>> To: Brad Hollister <behollis at sci.utah.edu>
>> Cc: VTK Developers <vtk-developers at vtk.org>
>> Subject: Re: [vtk-developers] transparent foreground polygons
>>         occluding background geometry
>> Message-ID:
>>         <
>> CANXz0Sb_qKZkgny0eymo6xZJuw-w653VtP0nr843NR7MAQ8V9A at mail.gmail.com>
>> Content-Type: text/plain; charset="utf-8"
>>
>> Your custom shader has to be written to work with depth peeling. When in a
>> depth peeling pass it needs to do some texture lookups and discard
>> fragments based on those lookups. I've included the general gist below. I
>> use string replacements to handle it but you are free to use other
>> approaches as long as the end result in a depth peeling pass has something
>> like the following in it.
>>
>> Thanks
>> Ken
>>
>>  // are we using depth peeling?
>>  vtkInformation *info = actor->GetPropertyKeys();
>>  if (info && info->Has(vtkDepthPeelingPass::OpaqueZTextureUnit()))
>>    {
>>    vtkShaderProgram::Substitute(FSSource, "//VTK::DepthPeeling::Dec",
>>      "uniform vec2 screenSize;\n"
>>      "uniform sampler2D opaqueZTexture;\n"
>>      "uniform sampler2D translucentZTexture;\n");
>>    // the .0000001 below is an epsilon.  It turns out that
>>    // graphics cards can render the same polygon two times
>>    // in a row with different z values. I suspect it has to
>>    // do with how rasterization of the polygon is broken up.
>>    // A different breakup across fragment shaders can result in
>>    // very slightly different z values for some of the pixels.
>>    // The end result is that with depth peeling, you can end up
>>    // counting/accumulating pixels of the same surface twice
>>    // simply due to this randomness in z values. So we introduce
>>    // an epsilon into the transparent test to require some
>>    // minimal z separation between pixels
>>    vtkShaderProgram::Substitute(FSSource, "//VTK::DepthPeeling::Impl",
>>      "float odepth = texture2D(opaqueZTexture,
>> gl_FragCoord.xy/screenSize).r;\n"
>>      "  if (gl_FragCoord.z >= odepth) { discard; }\n"
>>      "  float tdepth = texture2D(translucentZTexture,
>> gl_FragCoord.xy/screenSize).r;\n"
>>      "  if (gl_FragCoord.z <= tdepth + .0000001) { discard; }\n"
>>
>>
>>
>>
>> On Wed, Mar 23, 2016 at 7:08 PM, Brad Hollister <behollis at sci.utah.edu>
>> wrote:
>>
>> Hi,
>>>
>>> We have a problem with transparent foreground polys occluding geometry.
>>>
>>> We're trying to use depth peeling to handle foreground polygons with
>>> transparency. However, while depth peeling is enabled,
>>> GetLastRenderingUsedDepthPeeling() returns false.
>>>
>>> (The polygons also intersect other similar polygons so that some parts of
>>> them are in front of others, while other parts of the same poly may be
>>> behind others. Another reason to use depth peeling as opposed to depth
>>> sorting, in our opinion.)
>>>
>>> We also require custom shaders, using
>>> vtkOpenGLPolyDataMapper::SetVertexShader(), etc. From the test code @
>>>
>>> http://www.vtk.org/gitweb?p=VTK.git;a=blob;f=Rendering/OpenGL2/Testing/Cxx/TestUserShader2.cxx
>>> ,
>>> it looks as though custom shaders can't be used with depth peeling in
>>> VTK.
>>>
>>> Thus, is there another way to enable depth peeling or to have transparent
>>> foreground polys in our usage scenario?
>>>
>>> Regards,
>>> Brad
>>> _______________________________________________
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>>>
>>>
>>
>> --
>> Ken Martin PhD
>> Chairman & CFO
>> Kitware Inc.
>> 28 Corporate Drive
>> Clifton Park NY 12065
>> 518 371 3971
>>
> _______________________________________________
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>
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>

-- 
Ken Martin PhD
Chairman & CFO
Kitware Inc.
28 Corporate Drive
Clifton Park NY 12065
518 371 3971

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