This question is concerned with how one can enable this "pixel filter" to work within an Ogre based app. Simply put, one can write two passes, the first without writing any colour values to the frame buffer

lighting off
colour_write off
shading flat

The second pass is the one that employs heavy pixel shader computations, hence it would be really nice to get rid of those hidden surface patches and not process them pixel-wise. This approach works, except for one thing: objects with alpha, such as billboard trees suffer in a peculiar way - from one side, they seem to capture the sky/background within their alpha region and ignore other trees/houses behind them, while viewed from the other side, they exhibit the desired behavior. To tackle the issue, I thought I could write a custom vertex shader in the first pass and offset the projected Z component of the vertex a little further away from its actual position, so that in the second pass there is a need to recompute correctly the pixels of the objects closest to the camera. This doesn't work at all, all surfaces are processed in the pixel shader and there is no performance gain. So, if anyone has done a similar trick with Ogre and alpha objects, kindly please help.