[vtkusers] alpha missing f/vtkWindowToImageFilter

[Eric LaMar]

Hello:

I have the following problem: I want to read a rendered image, which
includes an alpha component, from the display buffer and use it for
further processing.  The problem is that vtkWindowToImageFilter only
handles 'vtkWindow's, which has a 'ReadPixelData' method, but no
'ReadRGBAPixelData' method (as found in most of the derived classes).

Since there is no common parent class that has a 'ReadRGBAPixelData' as
a method, where is the best place to put it?  'vtkWindow' seems the most
obvious place, but what are the more general design considerations that
would make this a bad idea?  What is the least amount of work that I
could do to allow this, that would conform to the spirit of VTK?

For comparison, is one does a glReadPixels, and requests 4 components
(RGBA), and there is no alpha channel in the currently selected visual,
OpenGL returns a '255' in the alpha channel.

thanks,
Eric LaMar
Research, LLNL