Random JPEG Questions

Discussion in 'Digital Darkroom' started by bill_tuthill, Sep 20, 2005.

  1. These may be questions for Gordon Richardson, but I thought others might be interested.
    1. Why do Canon digicams save JPEG with 4:2:2 chroma subsampling? According to Gordon's JPEG article, "The 4:2:2 option is an old standard from the television industry which is seldom used." It's 2x1x1 horizontally but 1x1x1 vertically.
    2. Why does Photoshop SaveForWeb use high quality factors like 98 in conjunction with 2x2 chroma subsampling? Mere stupidity, like the failure to warn about non-sRGB colorspace, perhaps.
    3. Is there evidence that different Q factors for qtable 1 and 2 helps? My IJG-based software can't do that; Canon and Photoshop do.
    4. At approximately what quality factor (on the IJG) scale should one switch from 2x2 to 1x1 chroma subsampling?
    5. Do the new JPEG encoding routines with PaintShopPro work better than IJG libraries, or has nobody evaluated this yet? Comparisons between Photoshop 7 and IJG showed that Photoshop performed better at lower Q factors, while IJG was tighter (smaller) at high Q factors with equal or better quality.
  2. jbq


    2. Because the huge majority of JPEG files out there are encoded like that, to the point that some decoders don't work well (or at all) with any other encoding.

    (I have personally found that geocities had problem with some files that had no subsampling).
  3. I've sometimes wondered about 1. myself, Jean-Baptiste's explanation seems plausible, but it might also be (at elast originally) some kind of space-vs-quality optimization made just by gut feeling.

    In general, I find that quality settings depends surprisingly much on subject matter, but tend to go with 1x1 sampling as far as possible.
  4. >>the failure to warn about non-sRGB colorspace, perhaps.
    I can't answer your questions, but if anyone from Adobe is reading this thread, PLEASE FIX THIS!
  5. If Jean-Baptiste is correct about #2, as I suspect he is, then #1
    is even harder to explain! My own answer to #4 would be around
    85-90 on the IJG scale. Photoshop goes from 86 2x2 at their Q6
    setting to 83 1x1 at Q7.
  6. ted_marcus|1

    ted_marcus|1 Ted R. Marcus

    The JPEG Optimizer in PSP X now defaults to 1x1x1. I usually found that produced the best-looking JPEGs with earlier versions, so I guess I'm not the only one.
  7. >>the failure to warn about non-sRGB colorspace, perhaps.
    Heck...let's take it one step further Adobe...It's called Save for Web, why not make the sRGB conversion either automatic or at least a choice in the Save for Web dialog!

Share This Page