Average/Mean Filter

The 2D Median Filter from the Altera video and image processing suite basically can achieve the said function. The arithmetic operation itself is almost easy, but the data organisation isn't to my opinion. There are various possible variants, e. g. operating on a frame buffer or performing a stream conversion with only line buffers. You also didn't tell, if you intend a scaling operation with less output pixels or a filtering that keeps the image size.