John, thanks for the detailed information. The two images I'm using were taken
with my lens set at 24mm (a bit to the wide angle) so the building leans to the
left in the left image and to the right in the right image; that's why I was
using the vertical line control points. The overlap area contains the entire
vertical part of the building in both images so I was able to but exactly the
same t1 points on both images (for example, the vertical tower in the middle of
the building). So was that redundant? Would putting that t1 point on only one
image or the other have been more then enough? (I did have a few other t1
points across the overlap and also outside the overlap to the right and left.)
It would save a lot of time not putting the same vertical line on each image
(and I understand your point about a vertical line going across two images that
are one above the other and the use of two t1 points on different parts of that
vertical feature).
On Apr 21, 2014, at 11:35 AM, panostar j.hough...@ntlworld.com wrote:
On Friday, April 18, 2014 9:10:02 PM UTC+1, Donald Johnston wrote:
I have two overlapping images of a building that I'm creating an
architectural rending from. I auto generated the control pints and then
added vertical and horizontal line control points. In the overlapping area I
put the same vertical line in both images. It did a great job of squaring up
the building.
My question. Did I have to put the same vertical line on each image or would
it have been better to put it on only one and let all the other CPs
straighten the other image to match?
Levelling a panorama is usually accomplished with t1 (vertical line) control
points. Provided the images have been accurately aligned by the optimizer
and the lens parameters are good, only two t1 control point pairs (hereafter
referred to simply as two t1 points) are needed to level the panorama
perfectly. I.e. two horizontally separated vertical features need to be
marked with a t1 point on each. The two vertical features must not be
separated by exactly 180 degrees of yaw otherwise the horizon will not
necessarily be levelled even though the marked verticals will be correctly
brought into the upright position. Assigning three t1 points on three
vertical features is guaranteed to work since they cannot all be separated by
180 degrees of yaw.
Assigning more than the minimum required number of t1 points can be useful to
assist in the evaluation of the lens parameters. For example, to correct
barrel distortion you can apply one t1 point on the top half of a bowed
vertical edge and a second t1 point on the lower half (or the second one on
the full extremities of the bowed edge).
Marking several vertical features can also be useful to obtain an averaging
effect when there is some doubt about which vertical features are actually
vertical (if any). Lamp posts are somewhat variable in this respect. When
doing this, it's probably best to exclude the already optimized lens
parameters from the levelling optimization in order to avoid the lens
parameters being skewed in an unreasonable attempt to get all the not quite
vertical features simultaneously vertical.
As to the question about which image to use when a vertical feature appears
in two overlapping images: it doesn't really matter provided that the images
have already been nicely aligned with normal control points. Also, it's ok
to assign a single t1 point on the top and bottom of a single vertical
feature that extends over two or more images in different rows. I.e. a
single t1 control point pair need not be confined to a single image. You
assign the t1 point to the vertical feature with little regard for which
image(s) it happens to appear in.
John
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