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assuming, your frame has 3 channels,

you could use a triple copy of one of the channels (e.g. green) to make it look grayscale:

# x,y,w,h are selected, get a numpy slice:
roi = frame[y:y+h, x:x+w]

# replace b and r channel with g, so they're all the same:
roi[:,:,0] = roi[:,:,1]
roi[:,:,2] = roi[:,:,1]

assuming, your (numpy) frame has 3 channels,

you could use a triple copy of one of the channels (e.g. green) to make it look grayscale:

# x,y,w,h are selected, get a numpy slice:
roi = frame[y:y+h, x:x+w]

# replace b and r channel with g, so they're all the same:
roi[:,:,0] = roi[:,:,1]
roi[:,:,2] = roi[:,:,1]

assuming, your (numpy) frame has 3 channels,

you could copy one of the channels (e.g. green) to make it look grayscale:

# x,y,w,h are selected, get a numpy slice:
roi = frame[y:y+h, x:x+w]

# replace b and r channel in that area with g, so they're all the same:
roi[:,:,0] = roi[:,:,1]
roi[:,:,2] = roi[:,:,1]