## How does the vertical lines tool work?

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Aeriscera
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### How does the vertical lines tool work?

Hello Everyone,

I know the intended use of the VL tool, but sometimes I find it does not work as well as I would want which has left me with some questions about how to *best* use it.

I know that if, for example, the vertical lines in the middle of my image are leaning in towards the centre that one cause of this is that the centre of the image is too low. I've noticed that if I fix this using the VL tool rather than adjusting the centre then AP moves the centre up. This suggests to me that the VL tool works by taking the lines you draw as constraints to the optimisation problem of determining a centre point that makes the lines you draw as vertical as possible. This would also explain why the VL tool "doesn't work" - i.e. there is a problem with the images like you knocked the tripod during the shoot or the camera slipped.

My theory also suggests to me that it is not a good idea to use short lines because any error in drawing the line will be magnified and also that you want as few lines as possible because more lines implies more chance of inconsistency.

So to sum up:

1. Correct vertical lines by adjusting the centre if it is straightforward.
2. If using VL use a few lines as possible (although there must be some minimum - one line will not be enough of a constraint) and make the lines as long as possible.

YesNo?

Aeris

hankkarl
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What if you have 3 or more rows? Is it better to limit the line size to the length of one image, or even restrict it to being only on one row?

Sometimes a bad link will cause one image to tilt (roll) a lot, in this case you probably shouldn't make the line cross two images.
Last edited by hankkarl on Wed Feb 02, 2011 6:52 pm, edited 1 time in total.

Aeriscera
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hankkarl wrote:What if you have 3 or more rows?

I'm not sure if this question is aimed at me or not, but my images are always multi-row so my questions were implicitly being asked in that context.

I would guess that if you limit your lines to going across a single row then it would be easy to draw the line inaccurately. Hmm ... context is important here? I suppose if your image is made of a small number of images then you could zoom right in so that you are viewing the photos at 1:1 - then you could get your line in absolutely the right place. In my case my images are typically 9 cols by 7 rows and it is really tedious to zoom right in, so I aim for large lines that cross say a third of the image. But I like the points you made. I had not considered them before.

A
Last edited by Aeriscera on Wed Feb 02, 2011 7:04 pm, edited 1 time in total.

klausesser
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hankkarl wrote:What if you have 3 or more rows? Is it better to limit the line size to the length of one image, or even restrict it to being only on one row?

Sometimes a bad link will cause one image to tilt (roll) a lot, in this case you probably shouldn't make the line cross two images.

I always draw very long lines. Perfect results.

best, Klaus
Last edited by klausesser on Wed Feb 02, 2011 7:42 pm, edited 1 time in total.
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GURL
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It first should be clear that this tool don't work like Panotools "lines" (actually t1, t2, t3 and t4 control points.)

Autopano line tool is to be used when optimization is fine and the relative orientation of any image pair is OK. As a result there is no reason lines should not cross image boundaries.

The purpose of this tool is to control the global orientation of the panorama.

A single line is not enough because two (opposite by 180°) vertical lines can be found in a panorama while some other "should be vertical" lines are slanted. In some cases (like when they are 90 degrees apart) 2 lines could be enough but in practice placing 3 or more lines is preferable.

Placing lines along well oriented features is as important as placing lines along miss-oriented ones.

Using long lines when possible rather than short ones is preferable as they should be more precise.

(If you are remembering some math, realizing that the aim is to force the zenith-nadir axis of the sphere where source images were stitched having the right orientation could help...)
Georges

Aeriscera
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GURL wrote:It first should be clear that this tool don't work like Panotools "lines" (actually t1, t2, t3 and t4 control points.)

I never used Panotools, but perhaps that's not relevant.

(If you are remembering some math, realizing that the aim is to force the zenith-nadir axis of the sphere where source images were stitched having the right orientation could help...)

Hmm. Good idea but that was 30 years ago. And the course was more about topology that projection iirc. Anyway, may I infer that you are confirming my theory/suggestion that everything you can do with the VL tool can be done by moving the centre point?

A

klausesser
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Aeriscera wrote:Anyway, may I infer that you are confirming my theory/suggestion that everything you can do with the VL tool can be done by moving the centre point?

No. Centerpoint works global - the line-tool works individually. If you have several differently slanted verticals you can correct them individually. Centerpoint canÂ´t do that.

LetÂ´s say two verticals tend to the right, one to the left and the next one right again. Using th linetool on each of them corrects them all. I use it often to correct panos shot with a 45° or 90° slanted camera mounted on a boom.

best, Klaus
Last edited by klausesser on Wed Feb 02, 2011 10:55 pm, edited 1 time in total.
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Aeriscera
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klausesser wrote:
Aeriscera wrote:Anyway, may I infer that you are confirming my theory/suggestion that everything you can do with the VL tool can be done by moving the centre point?

No. Centerpoint works global - the line-tool works individually.

? GURL said:
GURL wrote:The purpose of this tool [VL] is to control the global orientation of the panorama.

klausesser wrote:If you have several differently slanted verticals you can correct them individually. Centerpoint canÂ´t do that.

Neither can VL if what GURL said (and my understanding of it) is correct?

klausesser wrote:LetÂ´s say two verticals tend to the right, one to the left and the next one right again.

I don't think that's enough to make your argument watertight. Or are you assuming the pano in question is not a 360?

klausesser wrote:Using the linetool on each of them corrects them all.

It may do, but does AP move the centre point when it does so?

A

klausesser
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Aeriscera wrote:It may do, but does AP move the centre point when it does so?

Yes. I never care - besides i want it to be on a certain point.

After correcting the verticals you can then move the centerpoint where you like it - the verticals stay proportional correct.
The centerpoint influences the pano as a whole . . which includes the previously corrected verticals

best, Klaus
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GURL
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Aeriscera wrote:Anyway, may I infer that you are confirming my theory/suggestion that everything you can do with the VL tool can be done by moving the centre point?

No.

The VL tool moves the center point AND rotates the panorama at the same time.

In most cases a rotation is to be done to adjust all the verticals in a panorama and the VL tool is by far the easiest way to do that.
Georges

GURL
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klausesser wrote:Centerpoint works global - the line-tool works individually.
[...]
LetÂ´s say two verticals tend to the right, one to the left and the next one right again. Using the linetool on each of them corrects them all. I use it often to correct panos shot with a 45° or 90° slanted camera mounted on a boom.

Both the line-tool and center point work globally. The line-toll can correct all vertical lines in a single step because it compensates for the pano-head not being well-adjusted on the tripod, a non-vertical pole being used or the equivalent when the camera is hand-held.
Georges

Aeriscera
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GURL wrote:In most cases a rotation is to be done to adjust all the verticals in a panorama and the VL tool is by far the easiest way to do that.

Ah. Thanks for the clarification. Clearly I need to take Projective Geometry 101 again :-)

GURL
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Aeriscera wrote:Clearly I need to take Projective Geometry 101 again :-)

If your are used to a 3 knobs tripod, I doubt that.

If one want to photography a building façade "in the right way" (no vanishing point, like in an architect blue print) he must use the 3 knobs.

To adjust the building image in Autopano the three knobs equivalents are yaw (left/right), pitch (up/down) and roll.

The VL tool adjusts yaw, pitch and roll. By moving the center point yaw and pitch only are changed, not roll.

The most interesting point from the above: the changes to the building image you can see in the viewfinder when adjusting the tripod head and the changes to the building image you can see on Autopano screen preview while adjusting yaw, pitch and roll are identical.
Georges

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