Looking for help fixing distortion in panoramic photos

Discussion forum for techniques and issues relating to the creation of panoramic and/or "mosaic" images

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satsierra
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Joined: Wed Jul 28, 2010 8:45 pm

Looking for help fixing distortion in panoramic photos

Post by satsierra » Wed Jul 28, 2010 8:48 pm

Hello,

I am currently trying to put together a 360 degree pano for a virtual
tour of a house. I am running into some troubles with strange and
excessive distortions.

I am using hugin, and I can seem to get my photos to line up in a
straight fashion so that edges and lines are straight as they should
be, and not very bowed. As you can see here: http://tinypic.com/r/66jdc3/3 I would expect the top and bottom edges to be more concave than convex as they are rendering now.

I have tried playing with the focal length of the lens both on the
camera and as a setting in hugin. I have tried all of the many
projection modes including rectilinear equirectilinear panini and
everything in between. can anyone help me out and suggest how to
prevent my shots from looking so distorted?

thanks!

dsjtecserv
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Post by dsjtecserv » Thu Jul 29, 2010 5:42 pm

I won't attempt to address all of the potential factors in the results you got, or others of the many topics in panorama production that you should probably become familiar with. However, I can easily point out that the curving of horizontal lines (which results in the "bulging" look of building relatively close to the camera) is an unavoidable result of mapping a three-dimensional world onto a flat image plane. You are not doubt familiar with the effect of lines appearing to converge in the distance as the subject get further from the point of observation. In a normal, single-image picture the receding lines can be kept straight when a normal rectilinear lens (most lenses) is used and the film or sensor is kept in one plane. But in a panorama the camera (and therefore the sensor) must be rotated in order to capture the scene. The combined effect of a rotated sensor is that the sensor plane is (in effect, not actually) curved; if you go all the way around it is akin to a cylinder. Since the receding lines of each separate image are projected onto the sensor at a different angle, the panorama stitching software has to warp the images to make them fit together, and horizontal receding lines are unavoidably curved in the process.

Panoramas can be made with a "rectilinear projection" which maps the curved images back onto a flat plane, thus recreating the straight lines. However, this can only be done when the view encompasses no more than about 90 to 100 degrees of horizontal angle. Beyond that the image get's unacceptably stretched out.

The software provided by the host of this forum, PTAssembler, includes several other projections that can be used successfully to counteract this stretching at greater horizontal angles of view, so you may want to look into that as an option. But even then, a 360 degree panorama cannot be made without the curving effect you are noting.

Dave

johnh
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Post by johnh » Fri Jul 30, 2010 7:50 am

360 degree virtual tours on the web usually make use of a viewer that shows a restricted field of view in a rectlinear projection, and you explore the complete view by panning the image. See this example. Straight line features are then shown as straight rather than curved. The fact that they may be curved in the original equirectangular or cylindrical panorama is of no consequence.

John

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