This option is used when reducing number of colors used in image (mainly when converting from ARGB image to indexed format).

ImagingColorReductionMask = 128;

ImagingTypes (at line 162)

Mask is 'anded' (bitwise AND) with every pixel's channel value when creating color histogram. If $FF is used all 8bits of color channels are used which can result in very slow proccessing of large images with many colors so you can use lower masks to speed it up (FC, F8 and F0 are good choices). Allowed values are in range <0, $FF> and default is $FE.