Chroma in Downsampled 2x1 JPEG?

Discussion in 'Digital Photography' started by Bill Tuthill, Sep 24, 2007.

  1. Bill Tuthill

    Bill Tuthill Guest

    When 50% downsampling digital camera images with 2x1 chroma subsampling,
    (for sending a CD to friends) should I use 1x1 or 2x1 chroma subsampling?

    Theoretically it seems like there is enough color information for 1x1,
    but I've never done any side-by-side comparisons.
    Bill Tuthill, Sep 24, 2007
    #1
    1. Advertising

  2. Bill Tuthill

    Scott W Guest

    Bill Tuthill wrote:
    > When 50% downsampling digital camera images with 2x1 chroma subsampling,
    > (for sending a CD to friends) should I use 1x1 or 2x1 chroma subsampling?
    >
    > Theoretically it seems like there is enough color information for 1x1,
    > but I've never done any side-by-side comparisons.
    >

    So do a side by side. FWIW I Don't think you will see much of a difference.

    Scott
    Scott W, Sep 24, 2007
    #2
    1. Advertising

  3. -----BEGIN PGP SIGNED MESSAGE-----
    Hash: SHA1

    Bill Tuthill wrote:
    > When 50% downsampling digital camera images with 2x1 chroma subsampling,
    > (for sending a CD to friends) should I use 1x1 or 2x1 chroma subsampling?
    >
    > Theoretically it seems like there is enough color information for 1x1,
    > but I've never done any side-by-side comparisons.
    >


    I don't find much difference aside from when I'm doing graphics work and
    have _very_ saturated reds.

    - --
    Brendan Gillatt
    brendan {at} brendangillatt {dot} co {dot} uk
    http://www.brendangillatt.co.uk
    PGP Key: http://pgp.mit.edu:11371/pks/lookup?op=get&search=0xBACD7433
    -----BEGIN PGP SIGNATURE-----
    Version: GnuPG v1.4.3 (MingW32)

    iD8DBQFG+B8FkA9dCbrNdDMRAjf/AKCEViFsUEyShXJO85mpT1bkZwB4mgCfcHCI
    at/myrNcTrU+PD3YLZccHAw=
    =v9OS
    -----END PGP SIGNATURE-----
    Brendan Gillatt, Sep 24, 2007
    #3
  4. Bill Tuthill

    Paul Furman Guest

    Bill Tuthill wrote:
    > When 50% downsampling digital camera images with 2x1 chroma subsampling,
    > (for sending a CD to friends) should I use 1x1 or 2x1 chroma subsampling?
    >
    > Theoretically it seems like there is enough color information for 1x1,
    > but I've never done any side-by-side comparisons.


    Hmm, I'd never heard of that... what program gives you that option? Ah,
    OK I tried both ways with irfanview and there is a little improvement
    turning off color subsampling.
    http://www.wfu.edu/~matthews/misc/jpg_vs_gif/JpgCompTest/JpgChromaSub.html
    "Chroma subsampling may be a reasonable compression strategy for high
    resolution images, but for color web images, one is better served by
    turning off chroma subsampling and compressing more aggressively to
    achieve the same file size."

    'Web' images are usually around 300 pixels wide but your 50% reductions
    are probably plenty big to not matter much.

    --
    Paul Furman Photography
    http://edgehill.net
    Bay Natives Nursery
    http://www.baynatives.com
    Paul Furman, Sep 24, 2007
    #4
  5. Bill Tuthill

    Bill Tuthill Guest

    Paul Furman <> asked:
    >> When 50% downsampling digital camera images with 2x1 chroma subsampling,
    >> (for sending a CD to friends) should I use 1x1 or 2x1 chroma subsampling?
    >> Theoretically it seems like there is enough color information for 1x1,
    >> but I've never done any side-by-side comparisons.

    >
    > Hmm, I'd never heard of that... what program gives you that option?


    GIMP, ImageMagick, and PSP. Irfanview and Photoshop allow 1x1 or 2x2 only.
    In Photoshop you really don't have much control over chroma subsampling;
    at quality 7 and above you get 1x1, otherwise 2x2. Likewise in Photoshop
    Save-for-Web the dividing line is around 50.

    > http://www.wfu.edu/~matthews/misc/jpg_vs_gif/JpgCompTest/JpgChromaSub.html
    > "Chroma subsampling may be a reasonable compression strategy for high
    > resolution images, but for color web images, one is better served by
    > turning off chroma subsampling and compressing more aggressively to
    > achieve the same file size."


    Thanks for the interesting reference, which studies only 1x1 and 2x1
    but neglects 2x1 (used by most digital cameras because they have already
    lost chroma information due to RGBG Bayer-pattern sensors.)

    > 'Web' images are usually around 300 pixels wide but your 50% reductions
    > are probably plenty big to not matter much.


    True, chroma subsampling (especially 2x2) looks worst in small images.
    Bill Tuthill, Sep 25, 2007
    #5
  6. Bill Tuthill <> writes:
    >When 50% downsampling digital camera images with 2x1 chroma subsampling,
    >(for sending a CD to friends) should I use 1x1 or 2x1 chroma subsampling?


    >Theoretically it seems like there is enough color information for 1x1,
    >but I've never done any side-by-side comparisons.


    How large is a pixel in angular terms as seen by the eye? This depends
    on print/display pixels per inch, plus viewing distance. The human
    eye's resolution for chroma is about 10 times worse than its luminance
    resolution, and there's little point providing more resolution than the
    eye can see at the closest normal viewing distance.

    For example, if you're making prints at 600 PPI viewed from 10 inches,
    you can theoretically resolve up to about 50 cycles/degree, and 35
    cycles/degree is practical. With 2:1 downsampling, chroma would be
    limited to 25 cycles/degree - but you can't see more than about 6
    cycles/degree. So go ahead and downsample; you'll never see the
    difference.

    On the other hand, if you're displaying on an old monitor at 72 PPI, the
    resolution limit is only 6 cycles/degree. Chroma downsampling would
    reduce chroma resolution to 3 cycles/degree, and you *can* see that
    difference.

    Dave
    Dave Martindale, Sep 26, 2007
    #6
  7. Bill Tuthill

    Bill Tuthill Guest

    Dave Martindale <> wrote:
    >
    >> When 50% downsampling digital camera images with 2x1 chroma subsampling,
    >> (for sending a CD to friends) should I use 1x1 or 2x1 chroma subsampling?

    >
    > How large is a pixel in angular terms as seen by the eye? This depends
    > on print/display pixels per inch, plus viewing distance.


    Most people I know use a 1280x1024 LCD display that measures ~ 90 ppi.

    > The human eye's resolution for chroma is about 10 times worse than its
    > luminance resolution, and there's little point providing more resolution
    > than the eye can see at the closest normal viewing distance.


    That's the theory behind JPEG, but in practice, chroma subsampling often
    produces blocky artifacts in continuous-tone areas around sharp edges
    of detailed objects. What surprised me is how bad these blocky artifacts
    look in B&W (grayscale) images! You'd think B&W lacks chroma.

    > For example, if you're making prints at 600 PPI viewed from 10 inches,
    > you can theoretically resolve up to about 50 cycles/degree, and 35
    > cycles/degree is practical. With 2:1 downsampling, chroma would be
    > limited to 25 cycles/degree - but you can't see more than about 6
    > cycles/degree. So go ahead and downsample; you'll never see a difference.


    If anyone wants a print, I can go back to the out-of-camera original JPEG.

    > On the other hand, if you're displaying on an old monitor at 72 PPI, the
    > resolution limit is only 6 cycles/degree. Chroma downsampling would
    > reduce chroma resolution to 3 cycles/degree, and you *can* see that
    > difference.


    So at 90 PPI the resolution limit it 7.5 cycles/degree, and 2x1 chroma
    subsampling would reduce (horizontal-only) chroma resolution to 3.75
    cycles/degree? I decided to use 1x1 (no chroma subsampling) but someday
    I should do some comparisons.
    Bill Tuthill, Sep 27, 2007
    #7
    1. Advertising

Want to reply to this thread or ask your own question?

It takes just 2 minutes to sign up (and it's free!). Just click the sign up button to choose a username and then you can ask your own questions on the forum.
Similar Threads
  1. Dan Williams

    Does Sony DVP-NC875V/B have the chroma bug??

    Dan Williams, Nov 23, 2004, in forum: DVD Video
    Replies:
    0
    Views:
    441
    Dan Williams
    Nov 23, 2004
  2. Monk

    Chroma Key Background Image?

    Monk, Apr 9, 2005, in forum: Digital Photography
    Replies:
    4
    Views:
    429
    Mxsmanic
    Apr 10, 2005
  3. Ed Sievers

    Portraiture with Chroma Key

    Ed Sievers, Feb 18, 2006, in forum: Digital Photography
    Replies:
    5
    Views:
    415
    Clyde
    Feb 25, 2006
  4. Scubabix

    Green Screen ( Chroma Key) question

    Scubabix, May 20, 2007, in forum: Digital Photography
    Replies:
    2
    Views:
    591
    Bill Funk
    May 21, 2007
  5. Marc Wossner

    luminance noise and chroma noise: What is it?

    Marc Wossner, Jul 3, 2007, in forum: Digital Photography
    Replies:
    6
    Views:
    1,181
Loading...

Share This Page