Bits per channel

Discussion in 'Digital Photography' started by Siddhartha Jain, Jan 6, 2005.

  1. Digicams sensors have 8 bits per channel to record represent voltage
    per pixel. So luminescence is represented by a number between 0 and
    255.

    Is there something to gain by moving to 16-bit or 32-bit? If yes, when
    are we moving? Things seem to be at 8-bit for pretty long now.
    - Siddhartha
     
    Siddhartha Jain, Jan 6, 2005
    #1
    1. Advertisements

  2. Siddhartha Jain

    Owamanga Guest

    State your source. Most DSLRs already use 12 bits per channel.
    We already did.
    Then get a better camera.

    But, before you get too fret up, remember that your graphics card
    can't even display 11 bits per channel. So, if you want 32 bits per
    channel you'll never see the difference.
     
    Owamanga, Jan 6, 2005
    #2
    1. Advertisements

  3. You get an analog value from the sensor that can be represented with about
    12 bits, i.e. 4096 grey levels, with a linear scaling of grey level to
    luminance. This is what you get in RAW files. This is the situation
    inside the camera, before conversion to 8-bits takes place.

    In image files, 8-bit JPEG or TIFF files, the digital value does not
    directly represent light level, but a value nearer to the log of the light
    level. A gamma-correction of approximately 0.45 power law is used to
    convert the 0..4095 range of the sensor data into the 0..255 range of the
    8-bit image data. In practice, perhaps only value 0..2047 are converted,
    the remaining values in the RAW file representing the extra "headroom"
    which people mention.

    The effect of the gamma correction is to reduce the number of light levels
    which can be separately represented at the bright end of the range. I.e.
    the eye cannot distinguish between light levels of 2045 and 2046, so they
    are both mapped to "254", for example. Light levels at the low end of the
    0..4095 range (for example 1 or 2) are much more accurately represented in
    the 8-bit JPEG/TIFF image, so shadow detail is preserved.

    The display device typically has a gamma around 2.2, i.e. it is rather
    non-linear between the drive voltage in and the light level out. The
    combination of a 0.45 * 2.2 gamma (camera and display) result in an
    approximately linear net transfer between light into the sensor and light
    out of the display.

    This is all a simplification, but should help you understand why 8-bit
    data is adequate (just) for normal usage. Personally, I would like to see
    rather more than 8-bits, perhaps 10-bit or 12-bit JPEGS, so that all of
    the sensor range and more could be used for subsequent processing steps.

    Cheers,
    David
     
    David J Taylor, Jan 6, 2005
    #3
  4. Ok, correction. 12-bits.

    I don't think that answers the question.

    Ok, so why not?

    What I am trying to understand is that are there no significant
    benefits in moving to a broader bus?
     
    Siddhartha Jain, Jan 6, 2005
    #4
  5. Thanks. Correction, 12-bits.
    I don't think that answers the question.
    What I am trying to understand is are there no significant advantages
    to pushing the number of bits upwards?

    - Siddhartha
     
    Siddhartha Jain, Jan 6, 2005
    #5
  6. Siddhartha Jain wrote:
    []
    "significant" is the operative word.

    Tests have show that the eye has problems in using more than 8-bit data
    when applied to a gamma-corrected monitor as I described, although you can
    set up some special cases which show that for colour slightly more may be
    required. Prior to conversion to 8-bits for display, though, there may be
    a slight advantage working in the linear 12/16-bit domain.

    My guess is that it will be like CDs - for domestic use 16-bit 44KHz audio
    is adequate, for production studios have moved to 24-bit/96/192Khz audio.

    Cheers,
    David
     
    David J Taylor, Jan 6, 2005
    #6
  7. Siddhartha Jain

    paul Guest


    My desktop display properties indicate 32 bit 'color quality' with an
    option for 16 bit. I'm not sure if this is the same terminology. My
    older PC ran a lot slower in 32 bit mode as I recall.
     
    paul, Jan 6, 2005
    #7
  8. Siddhartha Jain

    Owamanga Guest

    That's right - diminishing returns. This whole thing is designed
    around what the human eye can see. There is no point going crazy with
    16 bits, 32 bits, 64 bits, 128 bits per channel when we can't display,
    print or see the added detail.

    Can you see the difference between 24 bit mode (8 per channel) and 32
    bit mode (10.5 per channel) on your graphics card ? I am sure if
    someone switched mine down to 24 bits one morning, I'd probably never
    even notice it had happened.

    The only argument for 48 bit scanners (16 per channel) and the like is
    because it allows slightly more scope for exposure correction (ie, you
    get to choose later which 8 bits per channel you want to keep)

    ...same with digital audio. Take CDs - 44.1Khz at 16 bits per sample is
    plenty good enough for our human ears. Since that invention 20 years
    ago, many subsequent digital audio standards have actually been much
    worse and the best one is only just over double that sampling rate and
    nobody is buying it. We have reached a plateau.
     
    Owamanga, Jan 6, 2005
    #8
  9. Well, 12 bits is 50% more than 8 bits, last I checked; so the benefit
    of going all the way to 16 from 12 is less than the benefit of going
    to 12 from 8.
    Because your eyes can't distinguish that many colors.
    One limitation is the human visual system. Now, it's useful to
    capture more than that in the initial shot -- but it has to be reduced
    to what humans can see to work as a print for humans. Like a negative
    being printed.

    And, as people said, we *are* moving to broader buses. When the web
    was new, it was rare to see pictures in more than 256 colors. Now
    24-bit color is pretty much the baseline; and better cameras and
    scanners produce 12 bits or more per channel. That sounds like
    exactly the movement you're asking about, to me.
     
    David Dyer-Bennet, Jan 6, 2005
    #9
  10. Siddhartha Jain

    Owamanga Guest

    32 bits per pixel. Split this into the three color components of Red,
    Green and Blue and you've got theoretical 10.6 bits per channel. In
    fact, most (if not all) are using 32 bits just to pad 24 actual bits
    into something that fits neatly into 4 bytes - this is for performance
    and design simplicity reasons. So, these modes are actually only
    displaying 8 bits per channel. 16,776,215 discrete colors.

    24/32 bit modes will be slower because they use 4 bytes of card memory
    per pixel instead of 2 bytes in 16 bit mode (65,536 colors) or 1 byte
    in 8 bit (256 color) mode.

    Someone might correct me and tell me there is now available a true
    10.6 bit, 12 or 16 bit per channel graphics card out there - anything
    is possible I am sure.
     
    Owamanga, Jan 6, 2005
    #10
  11. Siddhartha Jain

    paul Guest


    So I guess the real advantage is (maybe in printing?) and with post
    processing it is really helpful to have more information so if you apply
    curves & adjust contrast, there is info in there which can be made
    visible with adjustments. This includes sharpening. unprocessed DSLR
    images look quite bland and soft but there is a heck of a lot more info
    in there to fiddle with and bring out.
     
    paul, Jan 6, 2005
    #11
  12. Owamanga wrote:
    []
    There is no difference in the colour displayed - in each case it's 8 bits
    of red, 8 of green, and 8 of blue. The extra 8 bits are for alpha masks.
    It can be faster to move 32-bit data to the card, not that you could
    perceive the difference with today's PCs.

    David
     
    David J Taylor, Jan 6, 2005
    #12
  13. Siddhartha Jain

    Steve Wolfe Guest

    Digicams sensors have 8 bits per channel to record represent voltage
    I think that you would see a much greater improvement by moving to another
    color space instead of increasing the bits - the RGB color space doesn't
    even come close to covering the gamut that the human eye can see.

    steve
     
    Steve Wolfe, Jan 6, 2005
    #13
  14. Siddhartha Jain

    Steve Wolfe Guest

    My desktop display properties indicate 32 bit 'color quality' with an
    Actually, 32-bit mode gives you 24 bits of color (8 bits per channel),
    and an additional 8 bits of alpha (transparency).

    steve
     
    Steve Wolfe, Jan 6, 2005
    #14
  15. Siddhartha Jain

    Owamanga Guest

    Makes sense, but it is an alpha transparency to what ? - ie what's
    underneath?
     
    Owamanga, Jan 6, 2005
    #15
  16. Apart from RGB variants what other colour-spaces are feasible?
    - Siddhartha
     
    Siddhartha Jain, Jan 6, 2005
    #16
  17. Siddhartha Jain

    RSD99 Guest

    Sounds Good ... but just what "Color Space" would that be?
     
    RSD99, Jan 6, 2005
    #17
  18. There are video cards that can composite the computer-generated image
    onto a background image that comes from somewhere else (e.g. a video
    source). In that case, the alpha channel determines the opacity of the
    upper layer.

    But mostly it's not used in PCs. It may still be advantageous to use 32
    bit instead of 24 bit representation, wasting 1/4 of the memory, because
    most processors can't address 24 bit packed data except by using byte
    operations, while 32-bit access is faster.

    Dave
     
    Dave Martindale, Jan 6, 2005
    #18
  19. Siddhartha Jain

    JPS Guest

    In message <>,
    Actually, it is 12 bits per channel for most current digitals.

    Either marketing concerns, cost concerns, storage concerns, technical
    problems, or any combination are keeping the current data at 12-bit.
    Current sensors could easily warrant 16 bits, as their lower 12 bits at
    ISO 100 would have the same quality as ISO 1600 currently has on the
    same sensor. Maybe the problem is in the analog-to-digital converter.
    Maybe converters that can do 16 bits are expensive or impractical, but
    current sensors could certainly warrant it.
    --
     
    JPS, Jan 6, 2005
    #19
  20. Siddhartha Jain

    JPS Guest

    In message <>,
    It's not about seeing it directly on the screen, raw. It's about
    boosting shadows, compressing highights globally while expanding their
    local detail, etc, etc.
    --
     
    JPS, Jan 6, 2005
    #20
    1. Advertisements

Ask a Question

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

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.