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 Post subject: Jpeg options.....
PostPosted: Fri Mar 14, 2008 3:58 pm 
I have been editing levels and contrast on a few old shots (Jpeg. No RAW option on my camera), and found that when I adjust the slider to maximum the file's end up bigger sometimes a whole MB bigger. Which is confusing as its started off as Jpeg so you would think the picture would take up the same amount of space after editing.

Can someone explain why this is happening as I would of thought once the camera compresses the file it cannot be reversed. So where is this extra data coming from.

I can only assume that adjusting the pictures levels and contrast adds something?

:shock:


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 Post subject:
PostPosted: Sat Mar 15, 2008 1:25 am 
The somewhat befuddling increase in file-size has to do with the compression algorithm that is used to assemble the image-information into a a JPEG-file.

The algorithms can be different, although they all output in the same file-format.

A very simplified example of the compression can be illustrated thus:

BMP (non-compressed image format):
00000000000000000000001111111111111111100000000000000

In a compressed format that can translate into
15X012X115X0

I.e. a notation that that specifies that the next 15 bits are all 0, next 12 bits are all 1 and next 15 bits are 0 again.

When you edit the image and save it again, the algorithm may choose to record notations about image-data that is no longer there. It's a "syntax" if you will for how the algorithm stores data about an image.

Gobbelygook yep..lol. You can - for example - convert a JPEG back to a BMP-image and it will become a bigger file - but there will not be more image data. Same principle.

Cheers :-)


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 Post subject:
PostPosted: Sat Mar 15, 2008 9:41 am 
So if I save in the default settings the program wont recompress my files any greater than the camera had shooting in Jpeg?


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 Post subject:
PostPosted: Sat Mar 15, 2008 12:01 pm 
That may or may not be true - it all comes down to the specific algorithm used, stores data about the image.

The algorithm in your camera used to store the image data into a JPEG format may be different than the one used in the program you save it in - although they both output in the same file-format.

Not unlike the typically 3 different JPEG settings in cameras: fine, basic and small for example.

Only experimentation can give you the answer.

Cheers :-)


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 Post subject:
PostPosted: Sat Mar 15, 2008 12:27 pm 
Okay, thanks for your help :)


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 Post subject:
PostPosted: Sat Mar 15, 2008 1:20 pm 
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Joined: Sun Nov 05, 2006 11:08 pm
Posts: 7972
Location: Germany
I have another idea:
When postprocessing your images, you may (e.g. by sharpening and contrast-increase) amplify details in a way that the compression algorithm has more difficulty in compressing :?
((-----maybe this is just plain B.S.-----))
But you can clearly notice that images with lots of "plain" background can be compressed much further than images with lots of details in it.

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