Robert Grzeszczuk (rg)
Tue, 13 Jul 1999 08:57:15 -0700 (PDT)
If I understand you correctly, you would like to treat the volume as two
distinct sub-parts and manipulate them individually? If so, one option is to
define separate TetraSets to describe each of the regions (e.g., a simple
threshold-based segmentation will produce the regions which can then be
tesselated into tetras). Given that your regions are not very regular, you may
need to simplify the tetrahedral mesh. Each resulting tetra set would be drawn
separately with or without the clip plane enabled as needed.
Another solution to the multi-part object rendering problem is to use a
multi-pass algorithm. First, set the LUT to make all the entries above 30 to
fully transparent and draw the volume without the clipping plane enabled.
Then, set the values under 30 to fully tranapsrent, enable clip plane and draw
the rest. Depth and Alpha tests may have to be enabled to get the correct
occlusion.
Finally, you can use stecil planes to differenciate between the sub-parts:
create a binary shadow volume that will contain resuls of thresholding at 30.
Every time you're drawing a sampling polygon, draaw it into stencil planes
first to produce a subpart id mask. Then draw the same sampling polygon into
the frambuffer using the stencil test to apply sub-part specific actions (e.g.,
enable clip plane and draw if stencil_id == abov_threshold_id).
Each of the methods has its merits and drawbacks (e.g., the first method would
give best perfomance but requires non-trivial development effort). Take your
pick.
Let me know if any of this helps.
-rg
On Jul 13, 1:27pm, Volker Heid wrote:
> Subject: Clipping parts of a volume
>
> Hi,
> in the attached image I am using a RGBa-LUT with color values < LUT-entry
> 30 and luminance values (R==G==B) > LUT-entry 30. The volume is transformed
> to work with this LUT. I am interested in clipping away only the luminance
> values e.g. above a clipping plane to be able to see the colored part of
> the volume better. So the problem is that the clipping plane should only
> work on the luminance values.
>
> Any ideas?
>
> ~Volker
>
>
> --
> Worst Month of the Year:
> February. February has only 28 days in it, which means that if
> you rent an apartment, you are paying for three full days you
> don't get. Try to avoid Februarys whenever possible.
> --- Steve Rubenstein
>
> [ Attachment (image/gif): "heart.gif" 56299 bytes
> Encoded with "base64" ]
>-- End of excerpt from Volker Heid
This archive was generated by hypermail 2.0b2 on Mon Nov 01 1999 - 14:11:13 PST