"Digital ICE" without Digital ICE

Discussion in 'Digital Photography' started by Lorenzo J. Lucchini, Oct 26, 2004.

  1. Maybe. Of course it will depend on how precise the stepping motor of
    every individual scanner is.

    Anyway, although this is drifting from the original topic... I was
    wondering, can this now not-very-useful feature of VueScan be made
    better?
    Assume the misalignment between any two scans is not sub-pixel. In
    this case, it would be easy, although time consuming, to manually
    align the scans.

    But can you manually align two images that have sub-pixel
    misalignment? My intuition says you can, although most image editing
    software probably doesn't allow it. Should be a matter of somehow
    "moving" a fraction of every pixel value to one neighboring pixel.

    Even if this can be done, it remains to be seen if this alignment
    process can be automated; otherwise, it would definitely be too
    complicated to be practical.

    Now, I know there exist programs that can automatically align two
    partially or fully overlapping images. I don't think these programs
    can do sub-pixel alignment, though.

    But I understand that there are ways of measuring the 'sharpness' of
    an image: for example, the pamsharpness tool inclued in the latest
    version of NetPBM seems to accomplish such a goal.

    So, unless something of what I said can't be done, the following
    procedure should work to improve the sharpness of a multi-scan:
    1) Measure the sharpness of the image formed by combining the two
    scans
    2) If it is considered satisfactory, then terminate
    3) Choose an (x,y), where x and y are fractions of a pixel
    4) Shift one of the two scanned images of an (x,y) sub-pixel amount
    5) Go back to step 1

    This procedure doesn't find the best possible alignment between the
    two images, but it at least assures it will make the alignment better
    for a sufficient number of iterations.
    What's a sufficient number of iterations and what's "satisfactory", of
    course, remain to be defined, but you could just say "when sharpness
    has improved more than n% from the sharpness measured from the two
    unaligned images, then terminate".
    by LjL
     
    Lorenzo J. Lucchini, Nov 3, 2004
    #21
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  2. Lorenzo J. Lucchini

    Don Guest

    Unfortunately, no stepper motor is precise enough to achieve this
    amount of accuracy. At this level of precision a lot is going on when
    the assembly moves back and forth.

    As I mentioned before this can be confirmed by running a little test.
    Even at the lowest resolution (in my case 50 dpi) there is movement
    between two subsequent scans. So if a stepper motor is incapable of
    perfect registration at 50 dpi you can image the inaccuracy at 1200
    dpi!
    Yes, but it takes clever programming so don't expect to see it in
    VueScan any time soon... ;o)

    But seriously, although some intelligence is needed to determine the
    offset (misalignment) the actual process is just very time consuming
    (see below for more).
    Absolutely. The trouble is this happens very rarely and the odds of it
    are very small. I've been playing with this for about a year now and
    only *once* did I have two subsequent scans happen to be off on
    exactly a full pixel boundary!
    Yes, you can definitely sub-pixel align. I don't know how automatic
    software does it, although I suspect they run various mathematical
    functions and models along the lines you mentioned.

    I simply figured out a way to do it manually in Photoshop without
    resorting to any fancy math. (Actually, it can be done with any image
    editing software.)
    There is software out there to do everything automatically but I
    myself prefer to sub-pixel align manually. It's not really that
    complicated, just time consuming. There are two steps: Determine the
    offset (that's the part I prefer to do manually rather than rely on
    software) and then perform the actual shift (that's the part which is
    time-consuming so I usually do something else while Photoshop
    "thinks"). You could use Photoshop Actions for the second step but I
    just don't bother.

    Don.
     
    Don, Nov 4, 2004
    #22
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