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Why do straight lines get turned into...

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Anonymous
Posted on Tuesday, December 07, 1999 - 9:00 pm:   Edit PostDelete Post

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John Strait
Posted on Tuesday, December 07, 1999 - 9:11 pm:   Edit PostDelete Post

<Edited by John Strait on February 24, 2006>

Most panoramic stitching applications, including The Panorama Factory, simulate the effect produced by rotational and swing-lens panoramic cameras. In images produced by these cameras straight lines also appear to curve.

Planar panoramic cameras (6x17 etc) do not produce curved lines. However, planar panoramic cameras cannot capture as wide a view as rotational cameras. According to A SHORT COURSE IN DIGITAL PHOTOGRAPHY:

"Specially designed cameras with super wide-angle lenses can capture wide slices of a scene but no more than 180-degrees. There are many models of these cameras including the Hasselblad Xpan, Linhof Technorama 617 (115) and the Fugifilm Panorama GX617."

V4 of The Panorama Factory introduced a new feature that permits you to simulate the result of using one of these special, super wide angle cameras. As long as your image is less than about 140 degrees, you can produce an image with conventional perspective (straight lines) by choosing the Perspective projection option on the Select panorama type step of the Wizard or selecting the Perspective cropping option on the Crop image dialog box.

However, if your image is wider than about 140 degrees, you are stuck with the curvature because the perspective projection becomes exceedingly stretched as the field of view increases. In fact, to reproduce a 180 degree field of view, the perspective projection option would require an infinitely large image !

The only way for the image of straight lines to remain straight is if they are projected onto a plane (as in a conventional camera or a planar panoramic camera). When projected onto a cylinder (as in a rotational or swing-lens camers) or onto a sphere, the images of straight lines become curved. Both types of projections are used in digital panoramic photography -- cylindrical to duplicate the effect of rotational or swing lens cameras and spherical to produce images suitable for VR.

When conventional (planar projected) photographs are taken at different angles (as required for panoramic stitching), the images of straight lines are produced at different angles on the individual photos and cannot be made to match correctly. If you have ever tried to match a pair of standard photos, you will have experienced this effect. To reduce the mismatch, you have to take many photos and only use the center section of each. As the sections get smaller and smaller, the individual straight line segments become shorter and shorter at varying angles. The resulting line approaches a smooth curve.

See also What is Stitching at The Guide to Panoramic Photography for a nice explanation of this effect.

An example of this curved-line effect is shown in the picture Fresno from 1000 ft. elevation, part of the Library of Congress Panoramic Photographs collection. The city streets appear to be curved in this photo even though they are laid out on a strict grid system. The picture Blackwell Street, Dover, N.J shows an extreme example of curvature in a street that comes close to the camera.

See also the top section of this image at Introduction to Panoramic, Peripheral and Scanning Photography. This image was produced with a rotational "cirkut" type camera on conventional film. The Panorama Factory produces this type of panoramic image.

For many other examples of this curved-line effect (in digital panoramic photos), see the gallery at The Guide to Panoramic Photography.
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Jim Willmore
Unregistered guest
Posted on Tuesday, October 07, 2003 - 9:59 pm:   Edit PostDelete Post

I have read and understand the article on cylindrical panoramas. I have one stitcher that has a "perspective" option. That works nicely when I want to simulate a planar panorama camera.
The rub with that stitcher is that is is very quirkey when it comes to photos with exposures that don't exactly match. I have found that Panorama factory does a nice job of stitching difficult scenes, and would like to produce the planar type images on occasion. Is there any way to produce a planar style projection from a cylindrical projection. These panoramas would be less than 180 degrees wide.
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John Strait
Moderator
Username: Jstrait

Post Number: 2
Registered: 5-2001
Posted on Thursday, October 09, 2003 - 12:15 pm:   Edit PostDelete Post

Jim--Please refer to the FAQ page I want an unwarped image.
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car322
Unregistered guest
Posted on Saturday, October 11, 2003 - 8:03 pm:   Edit PostDelete Post

John:

I've been playing around with a pair of shots I took at the National Museum of Transportation in Kirkwood, MO. There was a Chicago elevated car that I was particularly interested in. I could only back away enough to get the car in two shots.

I used an HP 612 hand held camera. I used Photoshop to correct for a tilt error on one of the shots, and then used the perspective transformation on both shots to bring the horizontal lines to approximately the same vanishing point.

I then used PF to stitch the photos together, and with a little hand tweaking, I got the results I
was looking for.

I have attached the original shots, the Photoshop adjusted shots, and the final panorama.

I think you'll agree that the panorama looks pretty good considering I tricked the PF software into stitching to a flat or unwarped image.

Jim Willmore
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Jim Willmore
New member
Username: Jim

Post Number: 1
Registered: 10-2003
Posted on Sunday, October 12, 2003 - 2:37 pm:   Edit PostDelete Post

John:

Thanks for the e-mail. Here goes:

Jim
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Jim Willmore
New member
Username: Jim

Post Number: 2
Registered: 10-2003
Posted on Sunday, October 12, 2003 - 4:17 pm:   Edit PostDelete Post

John:

Here is the smaller zip file.

images 488r and 489r are the like the original photos that I took. Images 488ar and 489ar are the photos with the perspective changed in Photoshop. I also rotated 488ar to compensate for the camera not being held level.

Spam can is the finished panorama. The image is as stitched with Panorama Factory with a few tiles adjusted due mostly to the fact that there isn't much detail in some areas.

I hope you find this interesting.

Jim
application/x-zip-compressedZip file of photos used to produce "flat" pan.
spam can.zip (101.1 k)
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Jim Willmore
New member
Username: Jim

Post Number: 5
Registered: 10-2003
Posted on Monday, October 13, 2003 - 5:20 pm:   Edit PostDelete Post

John:

I am learning how to post pix. I am re-posting my post of Sat. Oct. 11 with the pictures inserted where they should go.:

-----------------------------------------------

John:

I've been playing around with a pair of shots I took at the National Museum of Transportation in Kirkwood, MO. There was a Chicago elevated car that I was particularly interested in. I could only back away enough to get the car in two shots.

I used an HP 612 hand held camera. These are the
original photos.

Original photo1
Original photo 2

I used Photoshop to correct for a tilt error on one of the shots, and then used the perspective transformation on both shots to bring the horizontal lines to approximately the same vanishing point.:

Transformed photo 1
Transformed photo 2

I then used PF to stitch the photos together, and with a little hand tweaking, I got the results I
was looking for.

Finished result.

I think you'll agree that the panorama looks pretty good considering I tricked the PF software into stitching to a flat or unwarped image.

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