[osg-users] Use cases of MultipassTechnique?

[Robert Osfield]

HI Alex,

MultiPassTechnique will be able to handle rendering of 3D elements that
intersect the volume, this includes arbitrary slice geometry like in the
example.  Simply create the slice geometries yourself with the appropriate
texture mapping and then place them in the scene.  When the
MiltiPassTechnique renders it will render the 3d scene to a colour buffer
and depth buffer then use this when rendering the volume.

I don't recall off the top of my head how you pass on the 3d subgraph, it's
probably just added as a child.  Sorry I can't be more specific, it's been
a few years since I wrote it and am away from my dev machine right now.

Robert.

On 27 April 2016 at 18:52, Alex Taylor <alextaylor at gmail.com> wrote:

> 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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