Re: Seeking Clarification of Procedure for Averaging Images

Dan Margulis

On Jul 9, 2020, at 6:23 AM, Gerald Bakker <gc.bakker@...> wrote:

I don't want to draw many conclusions from just one example, but I noticed something that may be typical. Zooming in to 100%, the "mean" version looks smoother, whereas the "median" shows a bit more color noise and is slightly more splotchy. I think the reason is that averaging pixel values has a smoothing effect. Taking the median however implies that one pixel may come from version A and a nearby pixel from version B, causing a more rough appearance. The more differences between the base versions, the stronger this effect. 

Correct. Median is generally more useful when comparing gross objects, not individual pixels. If, for example, we wanted to know our consensus view on how colorful to make Monument Valley, well I suppose it would be possible to write a routine that evaluates how colorful each individual version is. In determining our consensus we would want to pick the median, because the mean would be too colorful (there are  more extremely colorful versions than extremely dull ones).

On a pixel level the noise occurs in transition areas. For example, everyone had different ideas about how to transition into the shadows of the greenery, some people basically just plugged them and others had a lengthy transition zone. In that transition zone, every change in pixel value from its neighbor affects the mean. It may affect the median but it may not. So the transition in the mean is smooth and in the median not.


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