On 2010-01-16 00:11, yahvuu wrote:
> Philip Rhoades wrote:
>> 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) 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 . .
> well firstly, 1.6MB are not that bad in comparison to 9MB of raw RGB data, 
> right?
> (just try saving to uncompressed BMP as Greg suggested).
> Btw, 100% quality for JPG gives very little visible advantage over the
> default 90% setting.
> The relationship between quality value, file size and perceived image quality
> is very delicate. Any assertion has to be made with a lot of weasel words.
> So yes, compression artifacts have a tendency to hinder compression,
> resulting in larger file sizes after re-compression to comparable quality.
> A similar effect is caused by noise. So to get optimal JPG files, it's
> best to use a RAW->XCF->JPG workflow where the JPG is created only once.
> Just have a look at the advanced settings in the JPG save dialog to get
> a first impression of what machinery is at work here. There also was a
> very long thread on gimp.developer on that very quality setting...
> How does image quality compare if you adjust the quality slider
> such that the resulting file size is about 680KB?

To my eye, it doesn't look much different - I guess I see the 
pixellation more quickly when the image is enlarged . .

I guess what is confusing is this:

- there was a loss of information when the first JPG was saved in the 
digital camera memory from the CCD

- when the JPG is uncompressed by GIMP into RAM, there is no loss of 
information (?)

- when GIMP then saves the same image as a new JPG at 100% quality (I 
would have thought that this meant not losing any more information), 
that the second JPG would be compressed/created in much the same way as 
the first and therefore would be about the same size . .

Good to know that this happens anyhow . . of course I have no control 
over the file format that the camera uses and cropping a camera image 
and actually getting a result that is 2.5 times the size of the original 
is a bit annoying . .


Philip Rhoades

GPO Box 3411
Sydney NSW      2001
Gimp-user mailing list

Reply via email to