From: jason allen bryan (jbryan@cis.ohio-state.edu)
Date: 10/26/2000 11:48:00
it sounds like linear interpolation is the culprit.
i can only assume you're using bilinear or trilinear interpolation if
you're using volumizer to view datasets (although you may not, and if not
then i don't know what the problem is). i suspect if you switch to nearest
neighbor interpolation the problem will go away, but then your image
quality is severely reduced, and nobody wants that.
suppose you have 2 adjacent voxels, one with a value of x, the other
with y. it probably doesn't matter what channel the assignment is in, so
lets assume it's all of them. when those 2 voxels get rasterized, a pixel
on the screen gets a color which is a weighted sum of the colors of the
voxels it is closest to. assuming bilinear interpolation in this case,
this results in some linear combination of x and y. so what happens to a
pixel which is between the 2 voxels? it gets some of one color and some of
the other. now for colors this is good b/c it reduces aliasing in the
resulting image. but for mutually exclusive regions like in your dataset,
this is very bad. pixels on the edge of a voxel are blended with
surrounding voxels, which changes their region index. during a subsequent
lookup, the pixels are actually getting a value from the wrong region.
one solution is to use nearest neighbor interpolation, but your images
won't look that great. another is to enable the color lookup only during
texture download. if you've got a copy of the volume in main memory, the
values are mapped (with no interpolation) during texture download and
subsequently rendered correctly. the only draw back is you now have to
download the entire volume with each change in the lookup table. this may
or may not be a problem, it depends on your application.
if you'd like to discuss this more offline, i'm available. i've actually
got a prototype algorithm going which allows a 4-channel volume to have
arbitrary opacity and region information encoded in the alpha channel, and
i can use a lookup table to modify the opacity of whole regions of the
volume in real time. for example, i have a medical dataset of a human head
with the skin, brain, bone, blood vessels, and a tumor segmented out, and
i can change the opacity of every region in any combination. the algorithm
also allows each voxel to have its own opacity independent of its region,
so edges in the volume are rendered with less aliasing. i make use of the
lookup during texture download as mentioned above. frame rates will drop
if you are constantly changing the opacities of different regions, but for
the application i'm working towards, this is acceptable.
hope this helps!!
-jason
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|
| Jason Bryan
| Director of Software Development
| Interface Lab - Ohio Supercomputer Center
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| www.osc.edu/Biomed
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+--
|
| Discovery is seeing what everyone else has seen
| and thinking what no one else has thought.
| - Albert Szent-Gyorgi
|
| Anyone who has never made a mistake has never tried
| anything new.
| - Albert Einstein.
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