I have a photo that I want to pixelate. I don't want to apply the filter over the whole image, just the left half. Now comes the tricky part: I want the filter to be very coarse (i.e. big pixels) near the left edge and gradually go to finer pixels towards the middle of the photo and transition smoothly to the unfiltered half. Is there a fairly easy way to do this?
If I'm reading your question correctly, I think what you're going for is something like:
Which I achieved like I said in the comment, just masked layers:
I just used the mosaic filter on the separate layers and then masked out what I didn't need.