> I haven't quite got the hang of the "straight line" feature of
> (patched) PanoTools and PTGui. I've read the Erik Krause tutorial on 
> correcting lens distortion, and I've read the PTGui help. But when I 
> create control points of this type, my "average control point 
> distance" goes bananas. Control points that used to be within 1 pixel 
> (before adding the "line" points) go into double and triple digits.

Just ignore the control point distance for control points other than 
'normal' ones. The 'real' distance is shown, while you are only 
interested in the distance between the control point and the ideal 
straight line between the 4 points.

So probably the results are just fine.

> Here's what I do: in the Control Points window of PTGui, I open two
> images which both contain parts of a line that was straight in 
> nature. In the Control Point Type selection, I choose "New line 
> (t3)." 
> 
> Now, I know the next four clicks are crucial. But where do I click?
> Any four random points along the same line, alternating between the 
> left and right images?

Yes, this should work.

Be aware that Panorama Tools tries to make it into a straight line in 
the *final panorama*. Unless the panorama is rectilinear, or the line 
is vertical or on the horizon, a straight line in the actual scene is 
represented by a bent line in the panorama. You can approximate that 
bent line by a straight line only if the 4 points are close to each 
other.

IMO you should use the line-type control points only:
- for things like overhead electricity wires, i.e. lines that should 
be aligned but there are no clearly visible features on which to place
control points, or
- for rectilinear panoramas (where straight lines really are straight)

Joost