OpenGL - how to render object to 3D texture as a volumetric billboard

Question

I'm trying to implement volumetric billboards in OpenGL 3.3+ as described here and video here. The problem I'm facing now (quite basic) is: how do I render a 3D object to a 3D texture (as described in the paper) efficiently? Assuming the object could be stored in a 256x256x128 tex creating 256*256*128*2 framebuffers (because it's said that it should be rendered twice at each axis: +X,-X,+Y,-Y,+Z,-Z) would be insane and there are too few texture units to process that many textures as far as I know (not to mention the amount of time needed).

Does anyone have any idea how to deal with something like that?

Answer 1

A slice of 3D texture can be directly attached to the current framebuffer. So, create a frame buffer, a 3D texture and then do rendering like:

glFramebufferTexture3D( GL_FRAMEBUFFER, Attachment, GL_TEXTURE_3D,
                        TextureID, 0, ZSlice );
...render to the slice of 3D texture...

So, you need only 1 framebuffer that will be iterated by the number of Z-slices in your target 3D texture.