A geometry string containing LOWxHIGH thresholds. The string is in the form `XxY'. The Y value may be omitted, in which case it is assigned the value QuantumRange-X. If an % appears in the string then the values are assumed to be percentages of QuantumRange. If the string contains 2x2, 3x3, or 4x4, then an ordered dither of order 2, 3, or 4 will be performed instead.
The affected channels.
Changes the value of individual pixels based on the intensity of each pixel compared to a random threshold. The result is a low-contrast, two color image.