| Spandex Rutabaga 2007-08-10, 6:22 pm |
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Barry Katcher wrote:
>
> I just used the straightening tool to straighten a photo that was scanned at
> a slight angle, but there's a noticeable blurring of detail. Is there any
> other way to straighten the photo?
Yes, there are other ways to straighten photos. You could use
Image > Rotate or the Deform tool for instance. However, *all*
the ways you use will "blur the photo".
The reason the image changes when you rotate it is that the
original pixels no longer align with the horizontal and vertical
after rotation. Consequently, new pixels that do align have to
be calculated from the old by a process called interpolation.
The resulting image no longer has the exact original content in
it and has undergone some degradation from the interpolation.
There are two things you can do to minimize any degradation or
blurring of the image, whatever means you choose to accomplish
the rotation. First, rotate only *once* by the exact angle you
need and not by a series of successive rotations until you get
the overall rotation right. (Remember that you can always undo
an incorrect rotation.) Successive rotations cause interpolation
errors (which result from the use of integer arithmetic) to
accumulate. Rotating once only minimizes these errors. Second
start with an image that has a lot of pixels. When there are
a lot of pixels the degradation at any given pixel is slight and
not noticeable. When there are few pixels the same overall level
of degradation is spread over fewer pixels so that the effect at
any one of them is large and easy to notice. You have probably
scanned the photo at too low a DPI setting in the scanner.
You have probably exacerbated your problem by scanning at too
low a resolution. Use 200 or 300 dpi for a photo. You can read
more about how to choose the scanner resolution setting at
Wayne Fulton's excellent Scan Tips site http://www.scantips.com.
If you have thousand or more pixels across and down your image
you won't see any problems when you rotate it. If you have a
couple of hundred or fewer you very likely will.
Obviously, as with so much image editing the best solution is to
avoid the problem at the outset by orienting the photo properly
before scanning. Sometimes this is easier if you put a weight
like a book on the photo instead of trying to hold the photo
down with the scanner lid, which can easily result in small
movements of the photo as you lower the lid.
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