3MP from a 6MP?

Discussion in 'Digital Cameras' started by Ken Murphy, May 8, 2005.

  1. Ken Murphy

    Ken Murphy Guest

    When set to 3MP, how does a 6MP camera generate the image? Logically,
    I would expect it to throw away every other pixel, but is that really
    what happens?
     
    Ken Murphy, May 8, 2005
    #1
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  2. Ken Murphy

    Mark² Guest

    That's nice, but what has that to do with his question?
    :)
     
    Mark², May 8, 2005
    #2
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  3. Ken Murphy

    Ed Mullikin Guest

    Again, all very true, but I don't understand either. Is the light striking
    the photosensitive focused on 3 megs instead of 6 megs? You are over this
    individual's head, too. What are the mechanics of it? It it all done by
    software?
     
    Ed Mullikin, May 9, 2005
    #3
  4. No. In the first place, throwing away every second pixel in a 6 MP
    image would give you a 1.5 MP image, because it would be reduced in size
    by a factor of 2 in both directions. If you only did the
    every-second-pixel thing in one direction, you'd end up with a very
    distorted image.

    In the second place, even if you did want a 1.5 MP image, discarding
    every second pixel would be the wrong way to do it. You'd get
    unpleasant moire effects. One method that *would* work, and which is
    pretty cheap, is to average together blocks of 2x2 input pixels to give
    one output pixel. But that only works when the reduction in size is an
    integer, and doesn't give the best quality either.

    Instead, the camera uses a mathematical technique called resampling.
    To change from 6 MP to 3 MP, the camera reduces the number of pixels by
    sqrt(2) in each direction. For every 100 original pixels, it reduces
    that to about 71. Essentially the camera takes the image data it does
    have and builds a mathematical model of what the image probably looks
    like *between* the existing pixels. Then it removes any detail that
    would cause problems at the smaller image size. Then it calculates the
    intensity of the image at the new pixel locations for the smaller image.

    It sounds complex, and the math is somewhat complex, but it turns out to
    be not *that* expensive to compute. It works for any ratio of input
    to output size (not just integer ratios), and it avoids the moire errors
    produced by simply discarding pixels.

    Dave
     
    Dave Martindale, May 9, 2005
    #4
  5. Ken Murphy

    ASAAR Guest

    No. Ken's question lacks some precision, but many cameras allow
    the selection of reduced resolutions that are half of the maximum,
    and I think that's what he really meant. Not "When set to 1.5MP,
    how does a 6MP camera . . .". It *could* be done by throwing away
    1/2 of the pixels, but as you said, it definitely wouldn't be done
    in a manner that would result in extremely warped images. Your
    explanation of resampling may be what he was looking for, but if
    not, it was probably useful for others. I started to provide a
    similar reply several hours ago but wisely cancelled it, as my reply
    would not have been nearly as clear.
     
    ASAAR, May 9, 2005
    #5
  6. Ken Murphy

    Mark² Guest

    I hope you noted the smiley...
    :)
     
    Mark², May 9, 2005
    #6
  7. Ken Murphy

    Mark² Guest

    It captures the scene on the full sensor...at full resolution.
    Then...
    ....via software (called firmware in the camera) it re-samples the image much
    in the way a photo editor does when you reduce file size and dimensions of
    an image.
     
    Mark², May 9, 2005
    #7
  8. Ken Murphy

    JPS Guest

    In message <>,
    If it did it that way, the image could become *extremely* jaggy; in this
    particular case (halving the megapixels), it would have to drop about 4
    out of every 14 pixels in any row or column, and detail would be pushed
    up, left, down, and right, to distort the image. Instead, what should
    happen is that the values for new pixels are mathematically interpolated
    from the existing pixels, by temporarily creating small sections of the
    image upscaled, and then calculating weighted averages centered around
    the centers of the new pixels. That would be a good *fast* way; there
    are better ways, but they take too long to do in-camera.
    --
     
    JPS, May 13, 2005
    #8
  9. Ken Murphy

    JPS Guest

    In message <>,
    Not exactly; a smaller image is often sharper. The larger image can be
    more *detailed*; a different thing than sharpness. You get the sharpest
    images by dropping pixels (nearest neighbor algorithm), but they are
    sharp with artifacts, not detail.
    --
     
    JPS, May 13, 2005
    #9
  10. Ken Murphy

    JPS Guest

    In message <d5mavj$fdq$>,
    Unless, of course, the image was very soft to begin with.
    --
     
    JPS, May 13, 2005
    #10
  11. Ken Murphy

    JPS Guest

    In message <>,
    Maybe, but I'd have to know what you mean by "pixel ratio".
    The Sigma SD9 does the equivalent of dropping pixels; it never records
    2/3 of the focal plane to begin with, so the image is just like a higher
    MP image with a majority of pixels dropped.
    --
     
    JPS, May 13, 2005
    #11
  12. Ken Murphy

    Zed Pobre Guest

    Actually, if you have a camera with an in-camera jpg sharpening mode,
    that's almost exactly what it's doing. It's just a matter of
    carefully choosing which information to discard, to replace with
    another value chosen specifically to enhance contrast at sharp
    transitions.

    You might want to hunt down a guide on exactly what is happening when
    you use an unsharp mask in an image editor, for instance.
     
    Zed Pobre, May 13, 2005
    #12
  13. No, I mean that if you only drop every second column in the horizontal
    direction, but not drop every second row in the vertical direction,
    you'll shrink the image scale by a factor of 2 in only one direction.
    Circles will now become ellipses that are twice as tall as they are
    wide. That's the kind of distortion I meant.

    There are also aliasing artifacts, but I mentioned that elsewhere.

    Dave
     
    Dave Martindale, May 13, 2005
    #13
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