The Custom Filter, or Convolution Filter, allows you to create your own custom filters. A convolution filter loops through every pixel in an image and performs the calculations you tell it to perform. These calculations can result in anything from a custom emboss filter, to a high-pass filter, to nearly anything you can dream up. The convolution filter in Images: In Context!™ allows you to convolve a 5x5 grid. Most filters require you to set only a few of the fields for calcuation.
For instance, here is a typical emboss filter:
1, 0, 0
0, 0, 0
0, 0, -1
Here is a typical sharpening matrix:
1, 1, 1
1, -8, 1
1, 1, 1
Here is a more complex Laplacian edge detector
-1, -1, -1, -1, -1
-1, -1, -1, -1, -1
-1, -1, 24, -1, -1
-1, -1, -1, -1, -1
-1, -1, -1, -1, -1
I came up with the following sample in about 60 seconds:
The art of constructing convolution filters is well beyond the scope of this document, however much can be learned about them by searching the web.