A method of measuring a test video frame comprising: providing a test video input; providing a reference video input; performing a gradient change measurement based upon the test video input and the reference video input; and providing a gradient change measurement map, characterised in that the gradient change measurement comprises, for each pixel of a frame from the test video input and the reference video input: determining four gradient amplitudes using four basic gradient orientation filters; determining a gradient magnitude of each of the four amplitudes; determining the largest of the four magnitudes as the gradient magnitude; and determining a gradient direction corresponding to the largest magnitude, wherein the gradient direction is determined by the corresponding gradient orientation filter and the sign of the corresponding amplitude; and classifying a gradient change as either: detail added if the gradient magnitude of the test image is larger than the gradient magnitude of the reference image, detail removed if the gradient magnitude of the test image is smaller than the gradient magnitude of the reference image, or edges rotated if the gradient direction of the test image is not the same as the gradient direction of the reference image and the direction of the test image is horizontal or vertical. providing a test video input; providing a reference video input; performing a gradient change measurement based upon the test video input and the reference video input; and providing a gradient change measurement map, characterised in that the gradient change measurement comprises, for each pixel of a frame from the test video input and the reference video input: determining four gradient amplitudes using four basic gradient orientation filters; determining a gradient magnitude of each of the four amplitudes; determining the largest of the four magnitudes as the gradient magnitude; and determining a gradient direction corresponding to the largest magnitude, wherein the gradient direction is determined by the corresponding gradient orientation filter and the sign of the corresponding amplitude; and classifying a gradient change as either: detail added if the gradient magnitude of the test image is larger than the gradient magnitude of the reference image, detail removed if the gradient magnitude of the test image is smaller than the gradient magnitude of the reference image, or edges rotated if the gradient direction of the test image is not the same as the gradient direction of the reference image and the direction of the test image is horizontal or vertical. determining four gradient amplitudes using four basic gradient orientation filters; determining a gradient magnitude of each of the four amplitudes; determining the largest of the four magnitudes as the gradient magnitude; and determining a gradient direction corresponding to the largest magnitude, wherein the gradient direction is determined by the corresponding gradient orientation filter and the sign of the corresponding amplitude; and classifying a gradient change as either: detail added if the gradient magnitude of the test image is larger than the gradient magnitude of the reference image, detail removed if the gradient magnitude of the test image is smaller than the gradient magnitude of the reference image, or edges rotated if the gradient direction of the test image is not the same as the gradient direction of the reference image and the direction of the test image is horizontal or vertical. detail added if the gradient magnitude of the test image is larger than the gradient magnitude of the reference image, detail removed if the gradient magnitude of the test image is smaller than the gradient magnitude of the reference image, or edges rotated if the gradient direction of the test image is not the same as the gradient direction of the reference image and the direction of the test image is horizontal or vertical.