[vtkusers] vtkImageSlice and depth peeling

[Giuseppe D'Angelo]

Hello,

as reported here http://www.vtk.org/Wiki/VTK/Image_Rendering_Classes , 
it seems that depth peeling can't be used when using vtkImageSlice.

This matches the code in vtkImageSlice.cxx:

> // Does this prop have some translucent polygonal geometry?
> int vtkImageSlice::HasTranslucentPolygonalGeometry()
> {
>   // Always render during opaque pass, to keep the behavior
>   // predictable and because depth-peeling kills alpha-blending.
>   // In the future, the Renderer should render images in layers,
>   // i.e. where each image will have a layer number assigned to it,
>   // and the Renderer will do the images in their own pass.
>   return 0;
> }

However very little explanation is provided about why this is impossible 
(or undesirable) to do. In other words, why doesn't that method do 
something like:

   return GetProperty()->GetOpacity() != 1.0;

?

HasTranslucentPolygonalGeometry is also used in the 
RenderTranslucentPolygonalGeometry method, which has a reasonable 
implementation:

> int vtkImageSlice::RenderTranslucentPolygonalGeometry(vtkViewport* viewport)
> {
>   vtkDebugMacro(<< "vtkImageSlice::RenderTranslucentPolygonalGeometry");
>
>   if (this->HasTranslucentPolygonalGeometry())
>     {
>     this->Render(vtkRenderer::SafeDownCast(viewport));
>     return 1;
>     }
>
>   return 0;
> }

This makes me think that I can safely override 
HasTranslucentPolygonalGeometry and perform the aforementioned check. 
Indeed, by doing so and enabling depth peeling, the results seem to be 
correct.

Is there anything I should be aware of? :)

Cheers,
-- 
Giuseppe D'Angelo | giuseppe.dangelo at kdab.com | Software Engineer
KDAB (UK) Ltd., a KDAB Group company
Tel. UK +44-1738-450410, Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-independent software solutions