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Convert an 8-bit deep greyscale image to a 1-bit deep image by thresholding all the pixels. The resulting image is tightly packed and is probably only useful as an argument to mono2grey(). imageop.dither2mono (image, width, height) ¶ Convert an 8-bit greyscale image to a 1-bit monochrome image using a (simple-minded) dithering algorithm. Aug 28, 2017 · Have you ever had to loop over an image pixel-by-pixel using Python and OpenCV? If so, you know that it’s a painfully slow operation even though images are internally represented by NumPy arrays. So why is this? Why are individual pixel accesses in NumPy so slow? You see, NumPy operations are implemented in C.