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Lossy compression

troron
 
Posts: 219

Lossy compression

Post Sun Feb 06, 2011 4:12 pm


Hello and sorry if the answer to my question is here somewhere already. I'll just have to say I cant find it.
Jpg compression has two parts. Lossy compression and lossless compression.
Lossy is the kicking out and throwing away some parts of an image that is not easily observable (this according to the ISO subcommittee of experts Joint Photographic Experts Group.)
The lossy compression is permanent.
However after the first compression there is another file compression that loses nothing.
My question is can anyone tell me what percentage of the final file size is due to the lossy compression and how much should be given to the lossless compression.
I have google myself silly trying to resolve this.

Any explanation, or link, or anything will be highly appreciated.

Thanks in advance
Troron

thelund
 
Posts: 45

Re: Lossy compression

Post Fri Mar 04, 2011 8:35 am


I don't think you can ever give a clear answer to this question in percent since the percentage of the compression is dependant on compression level AND very much the detail level in the picture!

With a LOT of details the file compression will have a much smaller part in the final file size than with a very fuzzy picture without any details!

vglass
 
Posts: 8
Location: Bedminster NJ USA

Re: Lossy compression

Post Sun Apr 10, 2011 7:39 am


Do you want this info out of curiousity or is there something you want to solve?

In any case jpeg compression is very destructive resulting in a huge (high %) is the image data being lost (it's gone baby gone). Detail is lost but much much worse than that further work in PS or whatever will destroy even more data. By the way jpeg is considered a terminal format meaning an image file is converted to jpeg for the purpose of sending it to a printer or put on the web.

I'm not sure from whence you come with your question so I'm not sure if this info will be useful to you:
Capture and keep the maximum amount of image data because then you can choose to create a jpeg or create a fine art quality print.
To capture and keep the maximum amount of data shoot only RAW.


I

troron
 
Posts: 219

Re: Lossy compression

Post Sat May 14, 2011 4:09 am


vglass wrote:Do you want this info out of curiousity or is there something you want to solve?

In any case jpeg compression is very destructive resulting in a huge (high %) is the image data being lost (it's gone baby gone). Detail is lost but much much worse than that further work in PS or whatever will destroy even more data. By the way jpeg is considered a terminal format meaning an image file is converted to jpeg for the purpose of sending it to a printer or put on the web.

I'm not sure from whence you come with your question so I'm not sure if this info will be useful to you:
Capture and keep the maximum amount of image data because then you can choose to create a jpeg or create a fine art quality print.
To capture and keep the maximum amount of data shoot only RAW.


I



My question comes from this:
Loss-less compression is well known. A zip file compresses a bunch and then decompresses losslessly.
Yes, you can compress an image file using zip and other algorithms and lose nothing upon decompressing with the same codec that compressed it.
Loss-less compression works as a ZIP file does on the principle of redundancy. By collecting hundreds of exactly repeating pixel data within an image and converting them into a simple list of Cartesian coordinates all containing the exact same information. This can save a substantial amount of redundant information and upon decompression lose nothing whatsoever.
See "lossless data compression" http://en.wikipedia.org/wiki/Lossless_data_compression. See further Huffman encoding.

A jpg compression has two parts. The first part defined by the Joint Photographers Experts Group (JPEG) created by the International Standards Organization (ISO) is lossy. The second part of the compression is not lossy as discussed above in Huffman encoding.

What I would like to knwo is if anyone knows what percentage of a jpg compression is lossy and what percentage is loss-less. It is important to know and I cannot find it withing the ISO papers.

troron
 
Posts: 219

Re: Lossy compression

Post Sat May 14, 2011 8:09 pm


thelund wrote:I don't think you can ever give a clear answer to this question in percent since the percentage of the compression is dependant on compression level AND very much the detail level in the picture!

With a LOT of details the file compression will have a much smaller part in the final file size than with a very fuzzy picture without any details!


I think you gave me the answer!

I just thought about it some more and was thinking along the lines of an image of a red brick wall and cloudless blue sky above it. Its a boring shot but has so much redundancy that loss-less Huffman encoding would account for far more compression than lossy jpg compression. A shot with lots of color and detail... like a landscape of Smokey Mountain's in the fall would be primarily jpg compression I'd guess.

I was originally trying to equate file size differences. Sometimes a jpg can be 5% the size of the raw file and other times as high as 40%. It would lead some to assume that, in the case of 5%, that there would have to be a 95% loss of image data. You can't say that with any accuracy at all.

Thanks for the response people

Tro


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