With the Custom Filter you can design your own convolution operation in which neighboring pixels are mixed together. This operation can be a variation on blurring, sharpening, embossing or a dozen other effects.
The filter passes over every pixel in the selection and for each pixel being evaluated the filter multiplies the pixel's current brightness by the number in the center of the matrix. Then the filter multiplies the brightness values of the surrounding pixels by the surrounding values in the matrix. Finally the filter totals the products of the multiplied pixels, divides the sum by the value in the Scale option and adds the Offset value to calculate the new brightness of the evaluated pixel.
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