P3D Rotational errors and cha cha

[Larry Berlin <lberlin@xxxxxxxxx>]

>Date: Wed, 17 Mar 1999
>From: Michael Kersenbrock <michaelk@xxxxxxxxxxx>
>.........................
>"Gregory J. Wageman" wrote:
>
>> I agree with everything else Peter said, but this.  I find side-step
>> stereos to be very difficult to get right, and I'm not exactly a novice.
>> A little bit of rotational error makes fusion very difficult.  .........
>
>If you have rotational errors, you can just rotate the resulting
>images to make them straight again.  I know it works for digital
>images, but should work for prints the same way.
>

*****  The difficulties with single camera *cha-chas* is much bigger than
simple rotation. However rotation by itself can make viewing difficult.

There are several problems that frequently creep in and some are harder to
nail down and fix.

1. simple rotation 
2. change of elevation of the camera 
3. change of scene framing
4. change of view direction up or down
5. inadvertent toe-in/toe-out

Here are some things to look for with your images. This is easier to do with
digitized images because of the tools available in the computer. Working
with slides all you can do is rotate or move chips, which can leave other
problems untouched or made worse.

Put both pictures in the same viewing window in your graphics program. Use
the ruler and check several types of corresponding points. Particularly
check points in the image near each corner, the center and maybe a couple
other places. Check points that represent foreground, mid distance, and far
distance too. Very often it's distortion or movement of the more distant
view that doesn't get fixed when you make adjustments for the near view and
this creates eye strain.

Carefully note for each set of points whether it is in a higher or lower a
position and where on the image. If you have simple rotation you will be
able to identify the apparent center of rotation as well as how much
rotation is needed to correct things.

If you have distortions other than rotation, there is keystoning and other
distortions taking place by different amounts dependent on the distance from
the camera. Sometimes you can fix this in a digital image. I know of no way
to stretch and manipulate slide chips to fix these complex issues directly
on film. If you are working without access to a computer, shoot the images
again with a better technique.

In a recent image that I edited I had to make significant repairs. I
photographed an old tree with a large hollowed place at ground level. The
ground on which I was standing for the weight shift was not level and
somehow I wasn't as careful as usual. The result was that when I aligned the
view of the nearby tree the horizon lines were out of position.

Due to the angle of the tree, I split the repair between the two images. On
one side I selected the background area and stretched it upwards by a
measured amount. On the other image I selected the same corresponding area
and compressed it downwards so that details in each were perfectly aligned.
I had to do this for backgrounds on each side of the tree. The changes made
the image much easier to look at. It's headed for the web page in the near
future...

Larry Berlin

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


------------------------------