[Paraview] One Isosurface always in front?

[W. Bryan Smith]

Hi,

I am running ParaView 2.4.3 or 2.5 and am seeing something unexpected:

I am loading two separate 3-D images saved as legacy vtk files and
attempting to render isosurfaces of the data.  the images are thresholded,
such that all the pixels in image1 are equal to 10, and all the pixels in
image2 are equal to 20 (just for example).  each image defines an object
that is distinct in space from objects in the other image.  so far, this is
working fine: i read in both images, render an isosurface at value 10 for
image 1, at 20 for image 2, and i can color them separately without any
trouble.  problem is, when i set some  transparency values to the
isosurfaces and then rotate the volume to get a different angle, one of the
isosurfaces occludes the other such that it is entirely invisible.  in case
that is not clear, here are a couple images of what i am talking about:

http://ncmir.ucsd.edu/~bryan/PVimage001.jpg
http://ncmir.ucsd.edu/~bryan/PVimage002.jpg
http://ncmir.ucsd.edu/~bryan/PVimage003.jpg

so the gray blob in the top left corner of PVimage001.jpg appears to have
jumped to the foreground in PVimage002.jpg, even though the green one, in
reality, should be in front.

can someone please tell me what i am doing wrong (and/or if you need more
info to help troubleshoot this)?

thanks,
bryan
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