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jpg lossy format, I have a question.

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Old 01-05-2006, 10:22 AM
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creeduk creeduk is offline
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Question jpg lossy format, I have a question.

I was lead to accept that if you saved the jpg at the same setting (which PS usually defaults to the jpegs quality setting) then the loss would be at the most negligible and should not lose anything. I personally always have either tif or psd of any edits so that is not a worry but I decided I would like to know more on this. I have always assumed increasing quality i.e. an image saved at 8 (or 80%) than saved at 10 (100%) is not helping and just increases file size. That leads me to another question, what is added. If you open a 500kb jpg and then increase quality on the next save it will just about double the image size but it is impossible for it to add more detail so why the larger size, the compression has already taken place (in the original jpg file) nothing has been edited no meta data is added.

so to sum up, if one opens a file quality 8 and keeps saving quality 8 does loss occur? Compression is not compounded or image size would keep dropping. two why the size increase taking an 8 to a 10 when nothing can be added

I get asked and I answer best I can, decided I should try and find out for sure
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Old 01-05-2006, 02:15 PM
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byRo byRo is offline
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creeduk, I agree with your opinions, here are some comments.....

1) Saving at the same jpg level: If you open an image, do nothing, and then resave it at the same jpg level, yes, loss does occur. JPG is a lossy process, if all the losses when saving were exactly the inverse of the losses when opening then, thoretically, there wouldn't be any loss. But they ain't, so there is! Although the loss very small and basically confined to the difficult parts (for jpg, that means high contrast 45 degree lines);

2) Saving at a higher level: If you open an image, do nothing, and then resave it at a higher jpg level the resultant file will be slightly larger but there will be hardly any useful information there - it will be just a bit more like the previous file was. (observe that the size increases at a slower rate than would for a non-jpeg'ed image.)

3) Do nothing? Dunno why you'd open and resave a file without doing anything. If you did do something, the that something will suffer normal jpeg loss.

Note. Cropping is much more dangerous than just resaving.
JPEG works by chopping the image into 8 x 8 chunks (for luminosity, 16 x 16 for colours). If you crop the top left corner then (unless you chopped off exact multiples of 8 / 16) the boundaries will fall in different places and JPEG losses will accumulate.

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Old 01-05-2006, 08:18 PM
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Janet Petty Janet Petty is offline
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Thank you Ro. Maybe certain others (and you know who you are ) I've been unable to convince regarding the .jpeg loss will actually believe me now. Thank you again!!!

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Old 01-07-2006, 10:26 PM
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creeduk creeduk is offline
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Thanks byro, glad I was mostly correct always good to get some input here.
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