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[OM] Re: Jpeg editing - or was it - do they still use cameras?

Subject: [OM] Re: Jpeg editing - or was it - do they still use cameras?
From: Moose <olymoose@xxxxxxxxx>
Date: Sun, 16 Jul 2006 01:33:14 -0700
Brian Swale wrote:
> Other replies did not point out that, recognising that jpg  is a lossy format 
> (ie, 
> if you keep editing the same jpeg image it gets screwed up horribly)
Any JPEG contribution to the problem you describe, as opposed to the 
other effect we've also been discussing*, will be limited to edit, save, 
reload, edit more, etc. cycles. If you are having this problem in a 
single editing session, without that kind of cycle, it has nothing to do 
with JPEG, per se. It is rather the cumulative effect of many edits on 
an 8-bit image, regardless of compressed or uncompressed source (or 
eventual destination), as Chuck and Chris and I and others have been 
discussing.

An original JPEG output from a camera or scanner, which has not already 
accumulated errors from ___, may be converted to 16 bit after loading, 
edited wildly, then converted back to 8-bit without this unnamed effect. 
The editing may or may not enhance the picture, but ____ won't mess it 
up. (If I had done the extensive editing on your mountain pics in 8-bit, 
they would indeed have looked horrible, maybe worse.)
> , there are some image editing programs that always convert the jpeg image to 
> something else (a not lossy format) while the editing is being done.
>   
This is true of all image editing programs. They uncompress the image 
file into full size in memory. Edits can't be done while an image is 
compressed,; you have to know the color/intensity values of each pixel 
as it is processed, which you don't know unless the file is 
uncompressed. So any other approach would require uncompressing the 
image, applying the edit, then recompressing it for each thing you did. 
Leaving aside other, very significant, issues, the processor overhead 
would be enormous and entirely impractical. Performance would be abysmal.

Moose

*What does one call this? The visual result often looks like 
posterization, but not always. Color granularization? Tonal value 
clumping? Tonal coalescing?

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