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Regarding scale space extrema

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Hi all, I am developing an application to detect scale space extrema quite similar to how its done in SIFT (scale invariant feature transform). To begin with, i need the Gaussian pyramid. I already have the Gaussian pyramid using the mipmaps. Now i need the Laplacian pyramid that is obtained by subtracting each adjacent Gaussian pyramid image. The thing I want to ask here is what would be the best way of doing this calculation. What I was thinking was to use a fragment shader to find the Laplacian but I dont know how i would store the resulting image. Should i use glReadPixels or would it be too slow. Any ideas? Roughly speaking this is how i m thinking at the moment any input will be greatly apprecited.
AttachFBO
//Should i use glReadPixels or any other thing??? to get the laplacian image?
DetachFBO


Thanks, Mobeen

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hi all,
Currently, I have managed to calculate the Laplacian by using the slice by slice approach i mentioned in my last post. Now I want to store this Laplacian 2D texture into a 3D texture. Currently, I use FBO to calculate the Laplacian in the fragment shader and show the output on screen. How do i read back from a bound FBO. I know about glReadPixels and glGetTexImage. glReadPixels works but its very very slow, while I m unable to make glGetTexImage work. I always get error 1280 I have tried all the formats.
Do i need to detach the FBO and then read ?
Currently, I need to store the Laplacian into another 3D texture, is there a way that i can transfer my data directly (after modifying in the fragment shader) to the 3d texture or do i need to read slice by slice and copy to 3D texture myself?
This is how my flow is
AttachFBO();   SetLaplacianShader();   DrawFullScreenQuad();   //I tried glReadPixels but its very slow don;t know if it works or not   //but the opengl error bit is not set   glGetTexImage(..); //doesnot work at all i tried all the formats i get error ???DetachFBO();

Any help will b appreciated.
Mobeen

[Edited by - mobeen on March 17, 2009 9:21:39 AM]