Hi,
[EMAIL PROTECTED] (2007-11-24 at 1903.37 -0000):
> 
> I'm trying to reduce noise using multiple photos of the very same subject, 
> using both a tripod or simply repeating a scan a number of times ( i have a 
> flatbed scanner with a lot of noise especially in dark areas... )
[...]
> I have used the first level, as normal, with 100% opacity, then i have added 
> other 9 levels ( 9 photos ), with opacity 100/9=11,1, and mode set to normal.
> 
> If i use 10 levels, at 10% opacity each one, the brightness of the result is 
> different from the original ( lighter ).

The trick for mixing this way is that the N layer has to be set to 1/N
opacity. The reasoning is that the N layer will contribute that
factor, and the rest ((N-1)/N) has to come from the previous mixed
layers, so the result always totals 100% ((1 + N-1)/N). First (base)
layer 1/1 -> 100%, second layer (the one just above base) 1/2 -> 50%
(and the other 50% comes from the first), third layer 1/3 -> 33.33%
(and the other 66.66% comes from the 1 and 2) and so on. This approach
will probably have rounding errors.

> So what do you think i should use?

Imagemagick. Simpler, faster and maybe even less rounding issues (it
can add all images at once, then do a single multiply by 1/N, at least
that is how I would code such op). :]

convert image1.png ... imageN.png -average result.png

I investigated the topic for mixing different frames time ago to
create more film like renders (lots of other things in the trick, but
finding a fast and nice average op was one of the core issues I had to
solve): http://www.infernal-iceberg.com/blender/mblur/

GSR
 
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