JPEG export without chroma subsampling RRS feed

  • General discussion

  • It would sure be nice if the JPEG export option could be modified to not subsample chroma to 4:2:0 (2x2).  Yes, I realize I could choose a lossless export option (e.g. Photoshop) and then convert that file into a JPEG without chroma subsampling, but that is a lot of work to get to a point that could be accomplished with just a little work by the ICE team to modify the code to not subsample chroma for JPEG export.  I find it a bit ironic that a nice export quality slider if furnished, but this chroma "deficiency" issue has not been addressed.  JPEG export quality could be greatly enhanced if no (further) subsampling were introduced by ICE.  Thank you for considering my suggestion.  Michael
    Wednesday, June 1, 2011 2:05 PM

All replies

  • Thanks for the suggestion.

    This is the first request for this feature that I've heard.  I don't have any experience with the chroma subsampling option in jpeg.  Do you have any example images where this provides noticably better images?  Would you only use this in conjunction with maximum export quality, or is it important to be able to change quality and chroma sub-sampling independently?  I just took a look at the Photoshop CS5 jpeg export options and they don't explicity expose this feature, but perhaps it is done by default with max export quality - do you know?


    Wednesday, June 1, 2011 4:01 PM
  • Matt,

    Sorry for the delay in getting back to you, and thank you for your questions.  4:2:0 chroma subsampling can indeed contribute a visible degradation in color reproduction, specifically color fidelity and resolution, when processing camera original images.  This site demonstrates it quite clearly:  http://www.impulseadventure.com/photo/chroma-subsampling.html

    The author developed quite a sophisticated JPEG decoding utility and so is quite "expert" in these matters.

    Photoshop does not apply chroma subsampling when saving at a quality setting of 7 or higher (unbeknownst to the user).  Many graphics programs, including numerous panorama utilities, e.g. Autopano Pro, save JPEGs in 4:2:0 no matter the save quality setting.  This is unfortunate, as logic would tell you that stripping another 50% of the chroma information away from a camera image is bound to have some visible affect.  While many times casual observation of these 4:2:0 chroma subsampled images does not reflect this chroma deficiency, more careful comparisons to the original images does reflect the degradation in color reproduction.  Saving modified JPEGs without chroma subsampling will actually preserve their (color) fidelity better than saving at a higher quality setting.  This author addressess that issue:  http://www.wfu.edu/~matthews/misc/jpg_vs_gif/JpgCompTest/JpgChromaSub.html

    As JPEG is by far the preferred final file format for the internet and other distribution, it would be nice to see the format offered to its full potential in ICE, i.e. offer 4:4:4 sampling, either by default or as an option.  I hope we ICE users will see that happen.

    All the best to you, and thanks for listening to me preach from my soapbox (ha ha).





    Michael Lee
    Saturday, June 4, 2011 12:24 PM