[vtkusers] Point picking and ZBuffer problem with vtkFixedPointVolumeRayCastMapper

[Frank Gao]

Greetings!

When trying to select a voxel on 3D volume, I am experiencing some problems
with the vtkWorldPointPicker with Ray cast mappers (both
vtkFixedPointVolumeRayCastMapper and the opengl raycast mapper). Looking at
the code, I could not understand the way vtkFixedPointRayCastMapper handles
the zbuffer.

Problem: with volume rendering with vtkFixedPointVolumeRayCastMapper,
vtkWorldPointPicker::Pick function always returns same Zbuffer value no
matter where you pick on the volume. The Zbuffer value is same as the
(requested) depth value found in
VtkFixedPointVolumeRayCastMapper::DisplayRenderedImage.

Questions:
1. Does anybody succeed in point picking with vtkWorldPointPicker? I tried
to use vtkPointPicker, which seems to give me right result, but is slow, and
cannot correctly deal with the situation that clipping planes are defined.
2. Am I using the right approach to select point?
3. Why would VtkFixedPointVolumeRayCastMapper set a fixed depth value in
DisplayRenderedImage, which call RenderInternalTexture()? The latter only
draw a 2D rectangle to display the image (this is something I really don't
understand as an OpenGL newbie). If this depth has to be set, is there any
place and time that the correct openGL Zbuffer can be generated correctly?
4. If it is not possible to fix VtkFixedPointVolumeRayCastMapper to generate
right Zbuffers, is there any way to fix vtkPointPicker::Pick so that when
slanted clipping planes are given, the selection can take into account the
clipping planes?

I have tried vtkWorldPointPicker with 3D texture mapper and the two ray cast
mappers on Windows. Only texture mapper can work correctly. It seems to be
very frustrating that the point pick problem will prevent the usage of ray
cast mappers.

Your suggestions and feedbacks are highly appreciated.

Best regards,

Frank Gao



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