[osg-users] Use cases of MultipassTechnique?

[Alex Taylor]

All,

I've posted recently with a debugging question related to my exploration of
MultipassTechnique. Robert has provided guidance on how to proceed with my
debugging problem, but I have a conceptual question about what the
practical use cases are when a person would choose to use
MultipassTechnique because its still not totally clear to me.

There are two effects I'm trying to achieve, both are already implemented
in the Medical Image application Slicer that uses VTK as the rendering
backend. I'm wondering if either of these effects are possible using
MultipassTechnique, or really any of the volume rendering methods exposed
in the osgVolume nodekit. I was hoping that if someone knows that either of
both of the things that I want to do are not possible off hand that they
could tell me so that I can save myself some time.

1) If you refer to the two images below, the white bounding box represents
a "region of interest" that can be used to define clipping on the volume.
In the first image, the ROI box includes the entire volume. In the second
image, the upper face of the ROI bounding box has been lowered along the Z
dimension to apply clipping to the volume axially. Everything above the
upper face of the clipping ROI is not rendered. You can see this in the
images below because I've moved the ROI below the 2-D axial slice, where as
the volume structure above the slice is visible in the first image.

*My first question*, is this effect possible by defining a "hull" with
MultipassTechnique where one would specify an osg::Geode and some quads to
define the bounding ROI?

2) In both images, a grayscale 2-D axial "slice" is displayed in the 3-D
rendering demonstrating where a given 2-D slice plane is in the context of
the 3-D structure. *My second question: *Would MultipassTechnique allow me
to accurately render the "slices" as as part of the geometry passed in as
children of the tile and allow me to get the depth right of where the slice
intersects the volume structure?

If no one knows offhand, forgive this rather lengthy post. I've just trying
to get calibrated on the uses of MultipassTechnique and I thought these
concrete examples would help clarify my questions.

[image: volumeROIExample.jpg][image: volumeROIExampleNarrowedROI.jpg]
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