The problem is, that you can't (except maybe latest technology) read from and write to the same render target. The disortion effect needs to read back texels from the source image, therefore you can't render it directly to the same render target.

The problem is, that you can't (except maybe latest technology) read from and write to the same render target. The disortion effects need to read back texels from the source image, therefore you can't render it directly to the same render target.