[crazygringo]

> Grain is independent frame-to-frame. It doesn't move with the objects in the scene (unless the video's already been encoded strangely). So long as the synthesized noise doesn't have an obvious temporal pattern, comparing stills should be fine.

Sorry if I wasn't clear -- I was referring to the underlying objects moving. The codec is trying to capture those details, the same way our eye does.

But regardless of that, you absolutely cannot compare stills. Stills do not allow you to compare against the detail that is only visible over a number of frames.