1. A method of determining a global noise metric for a picture, comprising: estimating a noise-free value for a pixel; calculating a noise metric for the pixel based on the estimated noise-free value; disqualifying, the noise metric if the pixel satisfies at least one spatial condition within the picture; repeating the estimating, the calculating, and the disqualifying for all candidate pixels in a block of pixels; determining a block noise metric based on noise metrics for the candidate pixels; repeating the estimating, the calculating, the disqualifying, the repeating, and the determining for all candidate blocks in the picture; and ascertaining a picture noise metric based on block noise metrics for the candidate blocks.