1. A method of operating a noise reduction circuit, the method comprising: calculating, in the noise reduction circuit, an edge metric for a target pixel; comparing, in said noise reduction circuit, a value of said target pixel with a respective value of each of a plurality of pixels located in an area that includes the target pixel; for each pixel of said plurality of pixels that has a value different by at least a threshold amount from the value of the target pixel, replacing in the noise reduction circuit the value of said each pixel with the value of the target pixel; and applying a filter function in the noise reduction circuit to a set of pixel values, said set of pixel values including the value of the target pixel and current values, after said replacing, of said plurality of pixels; wherein said threshold amount is a function of said edge metric for said target pixel.