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Picture compression

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My guess is, it wasn't originally at 100% quality.  When you open a compressed image, the program has to make all sorts of assumptions about the compressed data and fill in every pixel with color information; then when you save it at 100%, it saves those exact pixels as if they were the original image you were trying to save.


Imagine it like a 500-page book.  When you compress an image to save it, it's like going through a book and ripping out all the pages you don't need.  Maybe you get it from 500 pages down to 250.  Then, when you open it again, you know you have to deliver a 500-page book, so you guess about the missing pages to fill in all the gaps.  Saving the image instantly at 100% is like saving that newly-reconstructed book perfectly; you're saving the 250 original pages and the 250 pages you just made up to fill in the gaps.  (Plus, Paint.NET adds metadata to the file, etc. which will increase the size)


The best way to prevent it is to use a lossless image format like PNG.  It'll be bigger to start with, but you won't lose information.  It's like saving and reopening all 500 pages.

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  • 2 weeks later...

For me, Paint.Net's doubling of file size is a possible deal breaker. I can't see why it is necessary, or why there doesn't seem to be a way to choose not to save metadata/history. When I made an overall color correction to a JPG photo (originally 4.3 Mb) from my camera, and wind up with an 8+ Mb file, I thought I must have done something wrong. But, now it looks like that is the way Paint.Net works. Am I missing something?


If this is the only way P.N works, I'll just have to move on to another program.



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I would be interested in analyzing this further.  Is there any chance you could provide access to the source image?

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Thank you EER. I just uploaded the original photo and two corrected versions to Flickr:



One version uses mild color correction, the other uses about the same color correction and also lightens the scene.


The photos were made in a cave restaurant with some sort of very warm fluorescent lighting.


Thanks for your help. I hope there is a way to control file size other than reducing the size or resolution of a photo



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