| Re: IDLgrVolume related [message #34396 is a reply to message #34337] |
Mon, 10 March 2003 07:27   |
Karl Schultz
Messages: 341
Registered: October 1999
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Senior Member |
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"lyubo" <lzagorch@cs.wright.edu> wrote in message
news:b4577v$8ma$1@proxy1.wright.edu...
>
> I have a grayscale volume with intensities between 0 and 255
> and I use an IDLgrVolume object to display it.
>
> I want to draw particular points inside the volume with different
> color, how do I do that? Is it at all possible when I am using the
> average-intensity projection (COMPOSITE_FUNCTION=3)?
> Any examples or web links will be highly appreciated.
>
Are your "points" defined as volumetric data or as separate geometry?
If volumetric, you could rescale your grayscale volume to use, say, 250
colors and change the color table so that the grayscale ramp runs from
0:249. Then, use the leftover color table values for your point colors.
Finally, insert your points into your volume by changing the desired voxels
to the appropriate color indices for your point colors. There may be
interpolation problems, though.
As another poster suggested, you might be able to use a 2-channel volume and
put all your point data in the second volume channel. The second volume
channel has its own color table.
Or if you have separate geometry, look at the ZBUFFER keyword.
Example:
pro tvol
vol = congrid(bytscl(randomu((seed=0), 4, 4, 4)), 40, 40, 20)
oVolume = OBJ_NEW('IDLgrVolume', vol, /ZBUFFER,
OPACITY_TABLE0=BINDGEN(256) / 4)
oOrb = OBJ_NEW('orb', POS=[20,20,10], RADIUS=15, COLOR=[255,0,0])
XOBJVIEW, [oOrb, oVolume]
end
IDLgrVolume ends up creating a 2D image when it is done rendering the
volume. Usually, images are just blitted to the screen without much regard
for the Z buffer. But if the ZBUFFER keyword is set, the IDLgrVolume object
will read the Z buffer back from the frame buffer and change the pixels in
the image so that volume image pixels that are "behind" the frame buffer
pixels are not drawn. It does this by setting the volume image pixel alpha
to zero and telling GL to not draw pixels that have an alpha of zero. The
volume object can do this because it maintains the "voxel depth" for each
volume image pixel. That is, it knows how deep the first visible voxel is
for each volume image pixel.
Note that it is important to draw your geometry (the orb in this case)
first.
Karl
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