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"What am I missing?" -- a cv2.imshow("lala", img) call before the cv2.waitKey() one.

then, this is the most creative ( or absurd, can't make up my mind :} ) way to draw a rectangle, please revisit:

>>> help(cv2.rectangle)
Help on built-in function rectangle:

rectangle(...)
    rectangle(img, pt1, pt2, color[, thickness[, lineType[, shift]]]) -> img

(you could just use that, with tl and br points, and use -1 for lineType, to have it "filled")

"What am I missing?" -- a cv2.imshow("lala", img) call before the cv2.waitKey() one.

then, this is the most creative ( or absurd, can't make up my mind :} ) way to draw a rectangle, please revisit:

>>> help(cv2.rectangle)
Help on built-in function rectangle:

rectangle(...)
    rectangle(img, pt1, pt2, color[, thickness[, lineType[, shift]]]) -> img

(you could just use that, with tl and br points, and use -1 for lineType, thickness, to have it "filled")