what is sharpening?

Discussion in 'Digital Photography' started by chibitul, Oct 19, 2003.

  1. chibitul

    chibitul Guest

    Hi,

    what exactly is "sharpening"? would that be the opposite of gaussian
    blur, for example? remember, there are no stupid question, only stupid
    answers...
    Thanks!
     
    chibitul, Oct 19, 2003
    #1
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  2. It is the opposite of blur, to be very basic about it.

    It does not add actual detail, however. It just adds to the existing pixels
    to make it "appear" more sharp, and it will not have an effect on a photo if
    it is out of focus bad enough, to begin with. You can adjust the levels of
    sharpening depending on the image editor you're using. Too much, and you
    can ruin the image. Just enough, and prints/screen photos will turn out
    better.

    Greg
     
    DigitalCameraBasics, Oct 19, 2003
    #2
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  3. chibitul

    Robertwgross Guest

    I used this example one time, and it worked. Imagine pixels with these
    intensity values:

    +0 +1 +2 +3 +4 +5 +4 +3 +2 +1 +0

    What would that look like? Probably a gradual dark going to light and back.

    Now, try this one:

    +0 +0 +0 +0 +3 +5 +3 +0 +0 +0 +0

    That would look less gradual.

    Now the last one.

    +0 +0 +0 +0 +5 +5 +5 +0 +0 +0 +0

    That would look like a hard transition. Very sharp.

    The processors in digital cameras have different degrees that they shift from
    one to another sharpening technique.

    ---Bob Gross---
     
    Robertwgross, Oct 19, 2003
    #3
  4. chibitul

    Bob Niland Guest

    Several different algorithms may be used.
    Common ones are:
    * increase contrast at apparent edges
    * boost high-frequency detail generally
    If you consider the difficulties in
    "what's an edge?" and "won't that sharpen
    noise also?", you can see that opportunities
    abound for visible unpleasant artifacts.

    I consider sharpening to be the last thing
    to tweak, and usually end up not doing it
    at all.

    --
    Regards, PO Box 248
    Bob Niland Enterprise
    mailto: Kansas USA
    which, due to spam, is: 67441-0248
    email4rjn AT yahoo DOT com
    http://www.access-one.com/rjn

    Unless otherwise specifically stated, expressing
    personal opinions and NOT speaking for any
    employer, client or Internet Service Provider.
     
    Bob Niland, Oct 19, 2003
    #4
  5. chibitul

    Rafe B. Guest



    Unsharp maskig (USM) works by boosting contrast at
    existing edges.

    Consider a "border" between two regions of pixels, say
    one region at 100 and the other at 200. After USM, the
    values of the pixels across the border might look like:


    100 100 100 99 98 97 96 204 203 202 201 200 200 200

    IOW, the darker region is made just a bit darker near
    the transition, and the lighter region is made to be just
    a bit lighter at the edge. Instead of a delta of 100 points
    at the border, the delta (in this example) is now 108 points.
    The extra contrast gives the appearance of sharpness.

    Obviously, if you carry this too far it can seriously degrade
    the image. The trick with unsharp masking is knowing when
    to stop. If you're not sure, err on the conservative side.

    There are many variations on the theme of digital
    sharpening, but they all come down to heightening
    contrast at existing tonal transitions.


    rafe b.
    http://www.terrapinphoto.com
     
    Rafe B., Oct 19, 2003
    #5
  6. chibitul

    Bob Niland Guest

    And keep in mind the ultimate use.

    If the final product is a JPEG, the
    compression causes "ringing" on edges,
    visible as "ripple". These artifacts are
    aggravated on sharpened images and at
    higher JPEG compression (lower quality).

    So be sure to keep a pre-sharpened image
    on file, in case you don't like the final
    results later in the workflow.

    --
    Regards, PO Box 248
    Bob Niland Enterprise
    mailto: Kansas USA
    which, due to spam, is: 67441-0248
    email4rjn AT yahoo DOT com
    http://www.access-one.com/rjn

    Unless otherwise specifically stated, expressing
    personal opinions and NOT speaking for any
    employer, client or Internet Service Provider.
     
    Bob Niland, Oct 19, 2003
    #6
  7. chibitul

    chibitul Guest

    Hi, it is a please to read your replies guys and gals, and thanks
    everybody for not making fun of me for asking such a basic question. I
    didn't know that some (most?) camera do sharpening/unsharpening as the
    image is taken and that it could improve the print if done properly
    later on computer. I thought that besides a basic white balance and jpeg
    compression, the image stays more or less the same as captured by the
    CCD(or CMOS) sensor. But from what I understand there is also sharpeing
    going on, and I wonder what else is going behind the scenes...

    I would love to be able to put a digital sensor that captured RAW files
    on a back of a classic camera. I don't need
    autofocus/autoexposure/autoeverything, but working with digital seems
    so convenient compared to film :)

    If only that silicon film would become true... I know, it's vaporware :(
     
    chibitul, Oct 19, 2003
    #7
  8. chibitul

    Mark Herring Guest

    Here's an attempt at a general definition:

    Sharpening is the selective increase in contrast at higher spatial
    frequencies---the equivalent of applying an inverse MTF curve to the
    image. Different effects are obtained by changing the shape of the
    inverse MTF curve (AKA transfer function)

    -mark

    digital photos, more and better computers,
    and never enough time to do the projects.
    Private e-mail: Just say no to No
     
    Mark Herring, Oct 19, 2003
    #8
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