Re: Digital De-twisting?

[P3D Larry Berlin <lberlin@xxxxxxxxx>]

>Date: Thu, 2 Oct 1997
>From: P3D Bob Wier  writes:
>..................
>The full frame stereo pairs on my home page were corrected for any
>rotational errors by using Photoshop (kind of expensive to use
>Photoshop purely for that, though). At any rate, I just threw across
>a horizontal guide and aligned a given point(s) in both images
>on the line, doing rotations as necessary. 
>
>It's best to avoid doing this if possible, though since unless you
>are lucky and need an integer number of pixels displacement, you'll
>get re-sampling, which may fuzz the image a bit at extreme magnifications.
>On web pages, though, it's not noticible.
>

This information is very true. Therefore the obvious extension is that the
best method to repair a digital image that needs rotation is to go back to
the scanner and rescan it so it is correctly aligned. Then you won't lose
image quality from the anti-aliasing involved in making a small rotation.

If the problem is that the camera rotated slightly during the taking
process, you can't use the picture edges as a reliable guide. Use the ruler
in a good photo program to help identify which image needs fixed and about
how much to rotate it. In the scanner, examine the prescan to make sure that
the reference points line up better. Use the rectangular function in the
scanner software as square edges to compare vertical or horizontal lines to.
When you are sure it's staight, then make the scan.

FYI, The program 3D Stereo Image Factory(tm) supports scan acquisition so
you wouldn't need to use PhotoShop as long as you line things up carefully
before scanning the image. It also supports the process by providing several
ways to preview the stereo image and check for proper alignment. 

Larry Berlin

Email: lberlin@xxxxxxxxx
http://www.sonic.net/~lberlin/
http://3dzine.simplenet.com/


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