flattened pixel bytes
number of color channels (ie: bytes per pixel). for instance, you'd use 4 for RGBA, 3 for RGB, 1 for L, 2 for AL, etc...
Optionalalpha_channel: numberspecify which channel is the alpha channel, or leave it as undefined to dictate lack of thereof. for instance, you'd use 3 for RGBA, 0 for AL, and undefined for RGB
if alpha is present, you can specify the minimum alpha value required for the pixel to be visible. anything less will make the pixel dull
get a grayscale intensity bitmap of multi-channel
pixel_bufimage buffer, using an optionalalpha_channelnumber to mask off the resulting intensity wherever the masking channel is zero (or less than the optionalalpha_biasparameter).