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this suggests that 512 is the upper limit for cv::Mat: https://docs.opencv.org/3.4/d1/d1b/group__core__hal__interface.html#ga3de14a42631396fe0480be69d5d2363f
cv::imread uses third party libraries. they may or may not support more than four channels. opencv may not be prepared to accept more channels even if the library can do it.
opencv's imread is a simple function. instead you might want to use an appropriate library directly. libtiff may be suitable.
