Convolves the second using the first, and writes a portable anymap as output.
Convolution means replacing each pixel with a weighted aver- age of the nearby pixels.
The weights and the area to aver- age are determined by the convolution matrix. The possible range of values is from 0 to 18, and after the offset that's -9 to 9. The equivalent matrix for 5x5 smoothing would have maxval 50 and be filled with 26. The convolution file will usually be a graymap, so that the same convolution gets applied to each color component.