On 2010-01-15 23:33, yahvuu wrote:
> Hi Philip,
> Philip Rhoades wrote:
>> - When saving as JPG with 85% quality am I losing information?
> JPG utilizes lossy compression, which means you'll loose information
> every time you save as JPG, even at 100% quality setting.
> That value does not specify the percentage of information stored
> in the JPG. It is just a number which allows to choose a trade-off
> between subjective image quality and file size.
> In consequence, the workflow recommendation is to routinely save as XCF
> and only create a JPG when the (finished) work leaves your system.
>> - How can saving as JPG with 100% quality increase information (file size)?
> As said above, it is wrong to assume you were saving
> 100% of the 85% of the original image's information here.
> In practice, one has to look at the compression artifacts to
> be able to adjust for minimum file size at acceptable quality.
> There's no way to just rely on the numbers.
> What happens in detail:
> when opening the JPG it gets decompressed to 2048x1536 RGB pixels of 3 bytes
> each, a whopping total of 9437184 bytes of RAM. (This holds true for any
> color JPG of 2048x1536 size, regardless of file size).
> Now when saving this image as JPG, it's these 9437184 byte of image that
> get compressed, regardless from where this data originated.
> Compressing an image of 9437184 bytes at 100% gives a larger file
> size  than compressing the same image at 85%. There's no memory of
> previously used compression rates.
> And regardless of file size, each new JPG compression step adds new artifacts
> to the image, degrading quality.

Firstly, thanks for the replies!

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 . .


Philip Rhoades

GPO Box 3411
Sydney NSW      2001
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