I'm writing data into a 3D texture from within a fragment shader, and I need to asynchronously read back said data into system memory. The only means of asynchronously initiating the packing operation into the buffer object seems to be calling glReadPixels() with a NULL pointer. But this function insists on getting passed a rectangle defining the region to read back. Now I don't know if these parameters are ignored when using PBOs, but I assume not. In this case, I have no idea what to pass to this function in order to obtain the whole 3D texture.
Even if have to read back individual slices (which would be kind of stupid IMO), I still have no idea how to communicate to OpenGL which slice to read from. Am I missing something?
BTW, I could use individual 2D textures for every slice, but that would screw up (3D-)mipmapping if I'm not mistaken. I wanted to use the 3D mipmaps in order to efficiently find regions of interest in the resulting 3D texture.
P.S. Sorry for the sub-optimal tags, apparently no one ever asked about 3d textures before and since I'm not allowed to create new tags...
Who says that glReadPixels is the only way to read image data? Maybe in OpenGL ES it is, but if you're using ES, you should say so. The rest of this answer will be assuming you're talking about desktop GL.
If you have a texture, and you want to read its contents, you should use glGetTexImage. The switch that controls whether it reads into a buffer object or not is the same switch that controls it for glReadPixels: whether a buffer is bound to GL_PIXEL_PACK_BUFFER.
Note that glGetTexImage will retrieve the entire texture (for a given mipmap level).