| Re: IDLgrWindow, IDLgrVolume and alpha channel [message #33801] |
Fri, 24 January 2003 16:06 |
Karl Schultz
Messages: 341
Registered: October 1999
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Senior Member |
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"Rick Towler" <rtowler@u.washington.edu> wrote in message
news:b0s6ln$1k30$1@nntp6.u.washington.edu...
>
> "Sebastian" wrote in message
>
>> I am rendering an IDLgrVolume into a IDLgrWindow. When I get the image
>> data from the Window, the image has only 3 channels. How can I get the
>> alpha channel of this image? The alpha channel information surely must
be
>> somewhere, because volume rendering needs it.
>
> <speculation>
>
> I'm sure Karl can give you a technical answer but the short of it is that
I
> am pretty sure you can't get the alpha channel from a window by using the
> Read method. I would guess that by the time the data is written to your
> display buffer the alpha information is long gone. I don't know if this
is
> a "limitation" of OpenGL, the OS, or IDL and while I can think of a few
> reasons not to do this, I can't think of a good reason to do it.
>
> </speculation>
Right.
The only way to get alpha information back out of the frame buffer is if the
graphics system supports "destination alpha". This means that there are
some planes in the frame buffer dedicated for alpha planes. Destination
alpha is primarily used for some advanced rendering techniques such as
compositing. There aren't very many devices that support destination alpha,
and there is currently no way to leverage it with IDL. You'll find
destination alpha more often on really high-end workstations.
The IDL volume renderer generates an RGBA IDLgrImage and draws that to the
screen. There is no way to gain access to this image, and I'm not sure that
would make sense, because the contents of the image would depend on on the
current View, etc. Besides, it get created and destroyed during the draw
anyway. I suppose that a new method that is similar to Draw could be
created that would return an IDLgrImage (like IDLgrWindow::Read), but that
does not exist today.
> You may be able to get the data you need in some other way. Maybe if you
> post some specifics on what you are doing someone might come up with a
> creative alternative.
Yes, knowing some more specifics might help. But here are a couple of
ideas:
1) Try rendering the volume with a greyscale ramp and read it back. The
same data will be in all three channels. Although this is really color
data, it might be close enough to the desired transparency values. I can
look at the validity of this approach more closely if there is interest.
2) If you think that you need the alpha to coordinate with drawing other
objects, like an isosurface, take a look at the ZBUFFER keyword on
IDLgrVolume.
Hope this helps,
Karl
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| Re: IDLgrWindow, IDLgrVolume and alpha channel [message #33808 is a reply to message #33801] |
Fri, 24 January 2003 12:08  |
Rick Towler
Messages: 821
Registered: August 1998
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Senior Member |
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"Sebastian" wrote in message
> I am rendering an IDLgrVolume into a IDLgrWindow. When I get the image
> data from the Window, the image has only 3 channels. How can I get the
> alpha channel of this image? The alpha channel information surely must be
> somewhere, because volume rendering needs it.
<speculation>
I'm sure Karl can give you a technical answer but the short of it is that I
am pretty sure you can't get the alpha channel from a window by using the
Read method. I would guess that by the time the data is written to your
display buffer the alpha information is long gone. I don't know if this is
a "limitation" of OpenGL, the OS, or IDL and while I can think of a few
reasons not to do this, I can't think of a good reason to do it.
</speculation>
You may be able to get the data you need in some other way. Maybe if you
post some specifics on what you are doing someone might come up with a
creative alternative.
-Rick
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