How to prevent shearing in pano?

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jdw6415
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How to prevent shearing in pano?

Post by jdw6415 » Sun Nov 03, 2013 10:33 pm

Hello,

This is my first post on the forum. I've been playing around with PTA 6.2 and cannot seem to figure out a way to get rid of shearing lines in the image (see attachment). I can manually edit in photoshop, but can't imagine doing panos of more than a few input images.

I am using a panosaurus which was calibrated to my lens/camara using sawing needle method. I used manual focus + manual exposure + cloudy WB. I took shots at each +1 increment on the panosaurus (about 10 degrees).

Thanks for any help!

~jdw
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maxlyons
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Re: How to prevent shearing in pano?

Post by maxlyons » Mon Nov 04, 2013 9:56 am

jdw6415 wrote:I've been playing around with PTA 6.2 and cannot seem to figure out a way to get rid of shearing lines in the image (see attachment).
It looks like the images are slightly misaligned and/or one or more project parameters (e.g. FOV, b, etc.) isn't quite right. In theory, it should be possible to achieve perfect alignment where there is absolutely no misalignment visible in the images. In practice, perfect alignment is surprisingly hard to achieve. For example, just because a camera's EXIF data reports that the focal length was 18mm, it might be the case that the camera is only reporting an approximation and the true value was 18.3mm; that 0.3mm difference can be enough to make it impossible for the optimizer to make everything to align. Similarly, if the camera was mounted a small distance from the "perfect" nodal point, then this can also cause problems. And, depending on the camera/lens, it is possible that the nodal point may change at different zoom settings and at different focus distances, so determining the "perfect" nodal point is surprisingly difficult.

With that in mind here are a few thoughts/suggestions:

1. This is a difficult image for stitching: taken at close range (where the visibility of minor parameter errors tends to be magnified), of a scene with lots of obvious straight lines (which makes it easy for the eye to spot misalignments).
2. Are you using a "smart" blender like Enblend or Smartblend? These programs include intelligent algorithms to route the seam lines in such a way as to minimize the visibility of misalignments; they don't make the images better aligned, but they make any misalignments less visible.
3. You could try adding/checking your control points. For a scene like this, I would aim for about 6-8 control points as close to evenly distributed as possible across the length of each overlap region. If any control points are positioned incorrectly, get rid of them.
4. You could try optimizing some of the parameters that PTAssembler's auto-optimizer doesn't optimize by default. For example, perhaps your camera lens has an unusual distortion pattern, which can't be corrected by simply optimizing the "b" parameter. You could try adding in the "a" and "c" parameters. Simlarly, perhaps the center of the distortion doesn't correspond to the exact center of your images, and you could try optimizing the "d" and "e" parameters to counteract this. And so on.
5. You could try optimizing the FOV parameter. Be careful with this one, though, because under some circumstances the optimizer may settle on an unreasonably small value for FOV, which is (from the optimizer's point of view) a mathematical improvement, but obviously incorrect.

If you want to post your images and project file, it might also help identify problems.

Max

jdw6415
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Re: How to prevent shearing in pano?

Post by jdw6415 » Wed Nov 13, 2013 9:47 pm

Thank you Max for informative reply. Are there any guidelines for optimizing acde params?

The 260k attachment limit is very limiting. Downsampled to 512 pixels / quality=8 my 4 images become overly pixellated and don't show shearing.

spamkiller
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Re: How to prevent shearing in pano?

Post by spamkiller » Thu Nov 14, 2013 3:45 am

jdw6415 wrote:The 260k attachment limit is very limiting. Downsampled to 512 pixels / quality=8 my 4 images become overly pixellated and don't show shearing.
You can upload files to a free file sharing site such as http://www.ge.tt (no need to register) and post a link to them here.

John

maxlyons
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Re: How to prevent shearing in pano?

Post by maxlyons » Thu Nov 14, 2013 9:54 pm

jdw6415 wrote:Thank you Max for informative reply. Are there any guidelines for optimizing acde params?
In general, most folks will be able to get a "good enough" result without needing to optimize these. As mentioned earlier, for a variety of reasons, a 100% perfect optimization is unlikely. If you want to experiment with these, I'd start with d/e, and then maybe add a/c to the mix of other variables (typically yaw, pitch and roll for everything except the reference image).

Also...keep in mind that the optimizer tries to calculate a mathematically optimal result, which may not be a visually optimal result if there are others problems (e.g. mis-specified FOV, incorrectly positioned control points, etc.).

Max

jdw6415
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Re: How to prevent shearing in pano?

Post by jdw6415 » Fri Nov 15, 2013 9:44 pm

Thanks!

Here's the link to my source files, project and output: http://ge.tt/3ZX94sz/v/0

johnh
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Re: How to prevent shearing in pano?

Post by johnh » Sat Nov 16, 2013 7:20 am

jdw, Looking at your project file, the main problem is the specification of the crop factor (focal length multiplier). This should be 1.6 for your camera, not 3.17. After updating that, optimizing, and thinning the control points to eliminate poorly positioned ones, and optimizing again, a good result was obtained. You really don't need such a large image overlap. 30-50% is enough. I've put my project file and stitch here: http://ge.tt/7AjKbtz/v/0?c .

John

maxlyons
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Re: How to prevent shearing in pano?

Post by maxlyons » Sat Nov 16, 2013 9:43 am

johnh wrote:jdw, Looking at your project file, the main problem is the specification of the crop factor (focal length multiplier). This should be 1.6 for your camera, not 3.17. After updating that, optimizing, and thinning the control points to eliminate poorly positioned ones, and optimizing again, a good result was obtained. You really don't need such a large image overlap. 30-50% is enough. I've put my project file and stitch here: http://ge.tt/7AjKbtz/v/0?c .

John
John...thanks for solving this!

jdw...the crop factor is used to calculate the lens field of view (FOV). So, an incorrectly specified crop factor will cause an incorrectly specified FOV. I also echo John's comments about not needing more than about 30% overlap. In fact, if you have more than 50% overlap, you'll have some portions of the scene that are present in three component images; this can make any residual misalignment more visible once the blender program has decided which of the three component images to use at each point in the final panorama. I would suggest something like 20-25% overlap is probably plenty.

Max

jdw6415
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Re: How to prevent shearing in pano?

Post by jdw6415 » Sun Nov 17, 2013 1:52 pm

Thank you John and Max. This gives me very good results too!

~jdw

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