[vtkusers] Blending two volumes

[Hämäläinen Janne]

Hi all,

I have a following situation: I have two vtkImageData objects (tomographic
medical data) that I want to display using volume rendering. I would want to
blend them (as with vtkImageBlend), so that both objects would have
different colors, e.g., so that I had a gray translucent data set and inside
that an opaque green area. 

I have done the same thing for 2D slices extracted from the volume by
mapping the original intensity levels of the images to colors with
vtkImageMapToColors (using a different vtkLookupTable for both) and
combining the results with vtkImageBlend. The results can be shown easily
with 2D texture mapping. For 3D rendering this doesn't seem a possible way
to do things. I can of course map the volumes to different colors and blend
them, but volume rendering seems to assume that the mapped data has no
colors, but instead a vtkColorTransferFunction is used to map the intensity
levels. Different color transfers can be specified for each component of the
input data, but I don't understand how this is interpreted. Is it simply
that you can specify what component to use for volume rendering? And is
there any way to use the original color components of the input vtkImageData
in volume rendering to get the desired effect? 

I also tried rendering (ray casting) the two volumes to a same scene using a
different color transfer for both, but the results weren't satisfactory. The
volume that was rendered last (it is inside the other volume) is not veiled
by the other translucent volume, but instead is shown as though it were
outside it. The toggle IntermixIntersectingGeometry is on, although I would
guess that it hasn't any effect on volumes, right?

If anyone 1. knows a way to volume render colored image data (or knows that
it is not possible), or 2. knows if there is a way to render multiple
volumes to a same scene so that overlapping parts are blended, I would
gladly accept any information,

-Janne



--
Janne Hämäläinen
Research Assistant
Helsinki University Central Hospital 
X-ray Department				

email   janne.hamalainen at hus.fi