Stretching a big backbuffer to a small texture

Hi all!

I'm currently trying to implement the now well-kown "Tron Glow Effect" 
under OpenGL
(http://www.gamasutra.com/features/20040526/james_01.shtml)

The algorithm is pretty clear, I'm just suffering of a lack of 
functionatilies with GL.
My glowing effect is performed on a small texture of say, 256x256.  
Since it's out of
question to render the scene twice (i.e.: one time for the normal scene, 
second time with
a small viewport of 256x256 for my texture) because it would be too 
slow, I'm trying
to grab the content of the backbuffer and resize it to a small texture 
of 256x256 where
I can perform the glow effect (two 1-D pass to make a Gaussian filter).

My problem is the D3D's StretchRect
(http://msdn.microsoft.com/library/default.asp?url=/library/en-us/directx9_c/directx/graphics/reference/d3d/interfaces/idirect3ddevice9/StretchRect.asp)
doesn't have any equivalent under GL.

The fallback I've currently found is to make a glCopyTexture2D() of my 
backbuffer to a big texture (big enough to grab the entire backbuffer)
and then draw a small quad of 256x256 on the screen that I 
glCopyTexture2D() back to my 256x256 texture.
Grumpf :-/

Any hint or remark is welcome,

Igor.

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Author: Igor Kravtchenko
  INET: [email protected]

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