flawed megapixel experiment

Discussion in 'Digital Photography' started by Bucky, Nov 22, 2006.

  1. [A complimentary Cc of this posting was sent to
    acl
    You can look at a picture from different distances; moreover,
    different people have MAJORLY different vision. Thus one cannot
    optimize picture for one particular viewing condition.

    If listening to music at different speed would be as frequent (imagine
    people listening to 33rpm LP at 78rpm turntable), there might have
    been similar problems with audio.

    E.g., to compensate for eye MTF at a particular viewing distance, we
    could boost higher frequencies a lot. But a person comes closer, and
    the same "features" are now at larger viewing angle, where eye MTF is
    almost ideal - no compensation is needed, and now the ringing is
    visible.

    [This is just my random musing, probably Scott meant something
    completely different. ;-]

    Hope this helps,
    Ilya

    P.S. Let me recall that the *crucial* difference between AA in audio
    and photo is the "length" of the filter.

    In audio, a (digital) 20-step AAF filter is close to a norm.
    While in photo, the AAF in cameras is not even a 1-step filter!

    It is not even low-pass filter, its MTF is |cos(f/f0)|. It just
    happens to kill *some* frequencies. Since even such a filter is
    not simple to construct, AND it has significant aberrations, it
    is a very questionable tradeoff.

    But we discussed this already...
     
    Ilya Zakharevich, Dec 2, 2006
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  2. [A complimentary Cc of this posting was sent to
    David J. Littleboy
    I would prefer if you would keep a little bit farther from doing
    math-related statements. ;-) In-camera AAF kill about 50% of useful
    and representable info (theoretical value is 4/pi^2).
    I DO agree with this. But in-camera AAFs are as far from "correct
    AAF" as bacterias are from you and me.

    Hope this helps,
    Ilya
     
    Ilya Zakharevich, Dec 2, 2006
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  3. That is not an anti-alias filter. That is just a 100% fill factor.

    This kind of 'filter' is not nearly strong enough to kill all aliasing
    effects.
     
    Philip Homburg, Dec 2, 2006

  4. Low pass filtering *is* anti-aliasing. Nothing more and nothing less.


    rafe b
    www.terrapinphoto.com
     
    Raphael Bustin, Dec 2, 2006
  5. Bucky

    acl Guest

    Ilya,
    I'd appreciate it if you did not also email these posts to me. Thanks.

    Achilleas
     
    acl, Dec 2, 2006
  6. Bucky

    Scott W Guest

    Pretty much by definition.

    Scott
     
    Scott W, Dec 2, 2006
  7. Bucky

    Ray Fischer Guest

    Yes, it is. That is exactly what an anti-aliasing filter is.
    There is no such thing.
     
    Ray Fischer, Dec 2, 2006
  8. Bucky

    Ray Fischer Guest

    Wrong. The differing between a low-pass filter and anti-aliasing is
    that a low-pass filter can be applied to the image after it's been
    captured while anti-aliasing cannot.
     
    Ray Fischer, Dec 2, 2006
  9. Bucky

    acl Guest

    Wow! OK, look at this example
    http://www.dpreview.com/learn/?/key=moire
    The building has no red or blue (on the right photo), but the moire
    pattern has these colours. So it can't be what you say, can it?

    Well, unless you believe that "white is made up of all colours" means
    that if I could sample a small enough patch of white light I'd get eg
    red.
     
    acl, Dec 2, 2006

  10. Let me rephrase, then:

    Anti-aliasing *is* low pass filtering . Nothing more and
    nothing less.

    LPF can be applied in post-processing. AA can not.

    "Blurring" may be a pejorative description of AA, but
    has a basis in fact.

    In fact, it helps to explain why some manufacturers
    choose to forego AA entirely or implement it
    inadequately.


    rafe b
    www.terrapinphoto.com
     
    Raphael Bustin, Dec 2, 2006
  11. Bucky

    Ray Fischer Guest

    Sure can.
    If you knew something about how the camera's sensor worked then the
    explanation would be obvious. Apparently my simplified example has
    only confused you.

    Oh well.
     
    Ray Fischer, Dec 2, 2006
  12. Bucky

    acl Guest

    OK then, please explain what these patches that you showed represent.
    Also, explain why a regularly repeating pattern of black/white lines
    can give rise to a) black and white moire b) colour effects (such as
    shown in the example I linked to earlier). If your explanation involves
    cutoffs in (spatial) frequency space, please explain the link to the
    patches that you showed. If not, I think we have found the problem here.
     
    acl, Dec 2, 2006
  13. If the amount of offset between the two images that the AA filter
    creates in the X direction (ignore Y for the moment) is exactly 1 pixel,
    then the first zero in its response is at exactly 0.5 cycles/pixel. So
    with such a filter you know that any luminance information at exactly
    0.5 cycles/pixel horizontal frequency will simply turn into grey before
    reaching the sensor.

    Meanwhile, when coloured light passes through the Bayer filter, it
    modulates the signal from that scanline at 0.5 cycles/pixel (because
    the filter alternates R/G or G/B in one row or one column). So, if you
    look at the signal from that row of the sensor, you can filter out a
    narrow band of frequencies around 0.5 cycles/pixel and be pretty sure
    that they are due to the colour of light falling on the Bayer filter,
    not luminance detail at the same frequency (because that was already
    suppressed).

    So the AA filter has the important job of filtering out the range of
    luminance frequencies that would be mistaken as colour information, and
    displayed as colour moire, not just the frequencies that would alias to
    produce wrong-frequency luminance information.

    Colour moire is *really* annoying, so perhaps suppressing it is more
    important than just general aliasing reduction. Maybe that's the main
    thing the AA filter does, in which case having a very deep notch at 0.5
    cycles/pixel is exactly what you want.

    Dave
     
    Dave Martindale, Dec 3, 2006
  14. Bucky

    Ray Fischer Guest

    Already done.
    No.
     
    Ray Fischer, Dec 3, 2006
  15. [A complimentary Cc of this posting was sent to
    Dave Martindale
    In theory, this would be fine.

    In practice, the "zero" [*] of the MTF of AAF is not at 0.5
    cycles/pixel, but slightly higher (about 0.65 cycles/px), as the
    (calculated) MTF curves of the sensor+lens show. I do not know what
    makes the camera manufactures choose this particular design; however,
    this shows that *in their opinions* some other factors are at least as
    important as one which you pointed out.

    [*] since MTF=|OTF|, and OTF is complex, real-life MTF does not has
    zeros, but just some small values where OTF becomes close to 0.

    Thanks for a nice argument (I did not hear this put out so nicely before),
    Ilya
     
    Ilya Zakharevich, Dec 3, 2006
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