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Mats Peterson
Currently Being Moderated

Chroma sub-sampling in JPEG Exif

Jan 28, 2014 12:37 PM

Hello.

 

On page 14 in the Exif version 2.3 specification, under 4.4.3, "Pixel Composition and Sampling", the following is stated:

 

"When compressed data is used, the combination shall be Y:Cb:Cr = 4:2:2 or Y:Cb:Cr = 4:2:0."

 

Why isn't a combination of 4:4:4 (i.e. no sub-sampling) allowed? This will produce the highest quality JPEG compressed images, and many times there is a noticeable difference (especially in red areas) between using 4:4:4 (no sub-sampling) versus using 4:2:2 or 4:2:0. And Photoshop happily saves Exif files with no sub-sampling when using higher quality levels. I have already mailed CIPA in Japan about this, but I'm not very hopeful about a response from them.

 
Replies
  • Currently Being Moderated
    Jan 28, 2014 12:51 PM   in reply to Mats Peterson

    We can't do much about the EXIF specification, we just work with it as best we can.

    You have to remember that EXIF is written by camera makers, who are more interested in compressed JPEG images than high quality (more photos on a card).

     
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  • Trevor Dennis
    5,829 posts
    May 24, 2010
    Currently Being Moderated
    Jan 28, 2014 12:56 PM   in reply to Mats Peterson

    Wikipedia has an excellent article on JPG.  Best idea is for you to read it, and come back and say you now understand it and am completely happy.  The alternative is that our Noel and other folk will get into a long and pointless argument, when they have other things they should be getting on with. 

     

    http://en.wikipedia.org/wiki/Jpg

     

    It is a very good article though.

     
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  • Trevor Dennis
    5,829 posts
    May 24, 2010
    Currently Being Moderated
    Jan 28, 2014 9:10 PM   in reply to Mats Peterson

    Good gracious. Have you ever heard of time zones, or that folk have work to do?  Do you even realise who Chris is?  I am sorry for my glib and factious reply, but I was trying to warn you that  pixel-peeping type threads tend to end in tears.  I was also being completely serious about the Wikipedia article.  It’s an excellent resource, and has a ton of reference links.

     
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