[vtk-developers] transparent foreground polygons occluding background geometry

[Ken Martin]

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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