Hi Philip,

On 15 Jan 10 12:53 Philip Rhoades <p...@pricom.com.au> said:
> What still doesn't make sense is that if the original file is JPG 
> and one simply opens it and then saves it as another JPG file with 
> 100% quality - you are saying that introduced artifacts are adding 
> about 150% to the file size? (681 KB to 1.618 MB)

The %age, isn't one of straight file size reduction, but relates to 
the areas of the image to be averaged. The the more you reduce the 
quality the more you increase the potential for larger areas to be 
averaged. A "busy" image will not suffer much averaging regardless of 
the compression requested, but one with large areas of broadly similar
colour, sky, painted walls, car bodywork, etc will have progressively 
larger areas averaged the more you reduce quality and the file size 
will reduce accordingly.

Some images, for example, where there is a load of tumbling water and 
spray will barely reduce in size t all even at higher compression 
levels as no part of the image has a large enough plain area to allow 
it to be averaged.

> How could the compression algorithms be so different as to cause 
> this sort of result?  - At worst I would have expected maybe a 10% 
> increase in size . .

I think you are assuming the whole imaged is compressed equally, 
regardless of the level of detail and colour change from one pixel to 
the next. Only in images with large areas of similar colour will the 
fle size reduce much at higher compression levels. The control is more
one of "reduce this if you can", rather than "reduce it whatever the 

Greg Chapman
Helping new users of KompoZer and The GIMP
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