Downsampling explanation please

Started Jul 3, 2012 | Discussions thread
zhir
New MemberPosts: 17
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Re: Downsampling explanation please
In reply to thubleau7, Jul 4, 2012

thubleau7 wrote:

down sampling is the same as what happens when you create a small jpeg.

The program or camera throws away pixels which are alike.that way its merely reducing the size of the file not merging pixels, it has to discard something to get a smaller size and does not reduce noise it merely takes adjacent pixels which are the same and discards them.

If you play with Photoshop and the like, you will notice that there are far better downsizing techniques (bilinear, bicubic, etc...). The technique you mention (discarding pixels or scan lines) was the first one that existed. For instance, the first Monkey Island videogame used that technique (visible when your character was walking from the foreground towards the horizon). Hopefully, that kind of downsizing hasn't been used for a decade.

Basically, downsizing (using serious techniques, not by discarding pixels) reduces noise by virtue of this theorem: http://www.stat.yale.edu/Courses/1997-98/101/sampmn.htm

Think of every pixel in the downsized image as a sample mean. The noise (variance) decreases as the number of pixels (sample size) increases, as explained in the link above.

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