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Author Topic: Tilt Shift /Pano Question  (Read 1031 times)
Kaypee
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« on: April 23, 2014, 04:30:14 PM »
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Hi. Is there a technique for shooting a pano using stitch technique that can minimise distortion? The images attached are of a theatre where the seating is arranged in an arc. The first shot is the right hand side of the theatre (and the third shot of 3 three shot pano using the 24mm ts-e). 

Its shifted fully and the seats are distorted which I imagine is due to the shift running parallel, but the seats running in a curve. I'm not sure what the correct technique to use here would be? Perhaps turning the the camera on the tripod rather than shifting and then adjusting the tilt to ensure focus. Thanks in advance.

Kaypee.
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elf
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« Reply #1 on: April 24, 2014, 01:11:23 AM »
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It's not really distortion, but perspective.  There are several stitching programs that can manipulate the perspective.  PTAssembler probably has the most options. PTGui also has perspective modifying options.
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Kaypee
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« Reply #2 on: April 24, 2014, 01:53:01 AM »
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Thanks for the software info. Is there a technique for doing this more accurately out of camera?
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BartvanderWolf
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« Reply #3 on: April 24, 2014, 05:06:21 AM »
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Thanks for the software info. Is there a technique for doing this more accurately out of camera?

Hi,

No. You are projecting an extremely wide angle of view on a flat plane, which will stretch the projected image as the image rays get more oblique. The only remedy is to look at the flat plane projection from the same (uncomfortably close) relative perspective as the lens.

So if this was from a 24mm lens, shifted +/-12mm, in landscape orientation, your getting a 102.7 degree horizontal angle of view. The virtual sensor size has become 60 x 24 mm with a correct perspective when viewed from 24mm distance (identical to the lens that projected it). Scaling all dimensions to get a viewing distance of 10 inches (254 mm) dictates an output size of (254/24=10.58x magnification) 635 x 254 mm or 25 x 10 inches. We normally want to view such an image size from a larger distance (e.g. 27 inches), and thus from the wrong perspective point, which will introduce predictable projection distortion (a seemingly stretched, 'wide angle distortion').

You can compensate a bit for this wrong viewing distance, by adding a progressive size compression towards the edges, but then straight lines will be distorted as well, depending a bit on the type of compression (anamorphosis) distortion you introduced.

Cheers,
Bart
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Kaypee
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« Reply #4 on: April 24, 2014, 05:10:58 AM »
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Thanks Bart. So the trade off would be to shoot from further away which would reduce the problems with perspective but ultimately not give the effect I was aiming to achieve?
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BartvanderWolf
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« Reply #5 on: April 24, 2014, 05:39:36 AM »
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Thanks Bart. So the trade off would be to shoot from further away which would reduce the problems with perspective but ultimately not give the effect I was aiming to achieve?

Hi,

Yes, it's a trade-off. Basically only printing very large and viewing from the correct distance will produce an undistorted view for such a flat plane projection. By anticipating that the image will be viewed from the wrong viewing distance for an undistorted view, one can exploit the 'wide-angle effect', but it's nothing else than projection distortion at play.

The same thing happens with a 'Tele-lens effect' when we view images from too close a distance, perspective seems to be compressed when in fact we only view it from the wrong perspective point.

For interior building shots, we often need the angle of view to get an overview of the lay-out and we cannot move back far enough so we need wide angle shots with short focal lengths, but that also introduces the 'Wide-angle effect' apparent distortion.

A good Pano stitching software can mitigate the effect a bit by introducing a progressive compression of the image dimensions, or by using a different output projection method (e.g. Cylindrical), but that will also start to distort (straight line) features in certain directions. When the image only has straight vertical lines, a compression in only the horizontal direction (or a Cylindrical projection) may help a bit.

It's the same thing that the DxO Raw converter uses for its anamorphosis distortion correction, and it was also added to their Viewpoint software. It think that a Pano stitcher uses better resampling algorithms though, and offers a lot of other useful (stitching) features.

Cheers,
Bart

P.S. Another solution for viewing on-line, may be by using a virtual reality 3D viewer which allows to rotate through the view while keeping a more natural looking perspective (like turning one's head and looking straight at surfaces instead of at an angle on a flat plane). One would need to output the image, usually created with a Pano stitcher application, in a suitable projection method other than rectilinear. 'Equirectangular' or 'Panini' methods are popular, and can also be restricted to less than 360 degree panoramic views.

Likewise, one could produce curved surface (printed) output, e.g. with a Cylindrical projection method.
« Last Edit: April 24, 2014, 06:11:38 AM by BartvanderWolf » Logged
hjulenissen
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« Reply #6 on: April 24, 2014, 05:56:49 AM »
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I wonder what a "route panorama" might do for your scene.

Basically: move the (video) camera along a linear path parallel to the seating, snapping frames at regular spatial intervals (using constant speed if you use a video camera).

http://en.wikipedia.org/wiki/Route_panorama


It does seem to look better for "flat" scenes without too much depth:


-h
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