[vtkusers] vtkImageActor, transparency, and depth peeling

[David C Thompson]

> I had some transparency issues when using a vtkImageActor (generated by 
> vtkImageViewer2) and depth peeling at the same time.  Some digging 
> revealed that my vtkImageActor was not getting rendered into the initial 
> opaque layer, but rather somewhere in the transparent layers.  The 
> offending line of code is tagged below:
> 
> int vtkImageActor::RenderTranslucentPolygonalGeometry(vtkViewport* 
> viewport)
> {
> ...
>    if (!(this->Opacity >= 1.0) && input->GetNumberOfScalarComponents() % 
> 2)) ********
>      {
>      this->Render(vtkRenderer::SafeDownCast(viewport));
>      return 1;
>      }
> ...  return 0;
> }
> 
> 
> The tagged line appears multiple times throughout the file.  When I 
> comment out all portions related to scalar components and rely strictly 
> on opacity values, depth peeling works wonderfully. What does the number 
> of scalar components of the input vtkImageData have to do with 
> transparency?

I believe 4 components is assumed to be RGBA data (A=alpha transparency)
and 2 components would be greyscale data plus transparency. But if so,
the number of components mod 2 should be 0 for the test to be correct.

	David