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Don't use Canny before HoughCircles. HoughCircles needs a grayscale image because it uses gradients for processing. There will be a Canny edge detection inside HoughCircles using param1 as higher threshold. Looking at the tutorial shows that a grayscale image is passed, so why do you think there should be a edge detection before?

Don't use Canny before HoughCircles. HoughCircles needs a grayscale image because it uses gradients for processing. There will be a Canny edge detection inside HoughCircles using param1 as higher threshold. Looking at the tutorial shows that a grayscale image is passed, so why do you think there should be a an edge detection before?