Very advanced users can use the custom filter to apply a customizable convolution matrix to a picture.
Use Edit | Effects | Custom… in the Browser or Effects | Custom… in the Editor to reach this function.
The principle behind this function is actually quite simple at its core. The matrix contains 5×5 coefficients, which set how the value of each individual pixel in the future picture will be calculated. Each coefficient multiplies the brightness of the given pixels during processing. The value stored directly in the middle of the matrix represents the value that is used directly for the given pixel itself. The other coefficients are used for processing this pixel's surroundings. For example, the coefficient at the top left is used to multiply the brightness of the pixel two pixels up and to the left from the given pixel; all 25 coefficients are used in a similar manner. The resulting values are summed together one by one, and then their sum is divided by the Divisor value. You can have its value Set automatically by the program to the sum of all coefficients. The Shift is then added to the result of the division, and the result is the final brightness value sent to the given pixel. The whole calculation is repeated for every pixel in the picture and for each of the three color channels.
The function's practical use depends on what matrix you use. For example, a high positive value in the middle and negative values in its immediate surroundings cause sharpening of a picture, doing the inverse blurs it. Using the Shift value, you can lighten or darken the whole picture by a specified amount.