Barrel distortion

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{{Glossary|A distortion produced by the lens itself which causes straight lines to be bent outwards.}}
 
'''Barrel distortion''' is the opposite of [[pincushion distortion]], it is a radial image distortion
 
'''Barrel distortion''' is the opposite of [[pincushion distortion]], it is a radial image distortion
 
introduced by the lens itself.
 
introduced by the lens itself.
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It is a common [[Lens distortion]] that can be corrected by the [[lens correction model]] of panotools.
  
 
[[Image:Barrel-distortion.jpg|frame|none|A photograph showing exaggerated barrel distortion]]
 
[[Image:Barrel-distortion.jpg|frame|none|A photograph showing exaggerated barrel distortion]]
 
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[[Category:Glossary]]
== Panorama Tools a, b & c parameters ==
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The [[Panorama Tools]] a, b & c parameters correspond to a third-order
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radial polynomial describing radial lens distortion. It is never exact,
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but can give a pretty good approximation to the real behaviour of a
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given lens.  The parameters are used by various tools to correct both
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'''barrel distortion''' and [[pincushion distortion]].
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There are a number of ways to determine the a, b, c and [[field of view|fov]]
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parameters for a particular lens/camera combination:
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* Taking a single photograph of a rectangular object, selecting lots of horizontal and vertical control points, then optimising [[roll]], [[pitch]], [[yaw]], [[field of view|fov]], a, b & c. You need to set the output format to [[Rectilinear Projection]] for this technique to work.  The process is similar to this [http://hugin.sourceforge.net/tutorials/architectural/ hugin architectural tutorial]:
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* Taking two or more overlapping photographs and selecting lots of normal control points, then optimising [[roll]], [[pitch]], [[yaw]], [[field of view|fov]], a, b & c. This technique works with any output [[Projections|projection format]]  (note that to get a really accurate measure of the [[field of view]], you have to take a full 360 degree panorama).
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* Using a tool such as [[PTLens]] or [[clens]] to read the [[JPEG]] [[EXIF]] data and correct the image automatically by looking up the lens in an existing database.
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The fov, a, b & c parameters are fairly consistent between shots
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with the same camera/lens combination so you can use them again and
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again.  Though they ''can'' vary slightly depending on the
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focus distance, so many panographers tend to recalculate
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them for every stitch as part of their normal workflow.
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The a & c parameters control distortion at the centre and edges of
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the picture so are not very important, you can normally just zero
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them and use the b parameter by itself.
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See also [http://www.path.unimelb.edu.au/~dersch/barrel/barrel.html Helmut Dersch's barrel distortion page].
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== Tools to correct barrel distortion ==
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* The original [[PTStitcher]] can be scripted to batch process images with known a, b & c parameters.  It can also be operated with one of the [[GUI front-ends]].
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* [[nona]] or [[nona_gui]] (both part of the [[hugin]] distribution) can be used identically to [[PTStitcher]].
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* The Correct filter in the [[Panorama Tools Plugins]] for the [[gimp]] or [[photoshop]] uses the same a, b & c parameters as [[PTStitcher]].  Note that it doesn't know about d & e parameters and uses 'd' as an overall scaling factor instead.
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* [[PTLens]] is a [[Photoshop]] plugin and a stand-alone Windows tool that uses the same a, b & c parameters and comes with a database of popular lenses.
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* [[Clens]] is a command line version of [[PTLens]].
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* [[Gimp wideangle plugin]] uses a different formula altogether to correct distortion.
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* [[Gimp phfluuh plugin]] is another tool that corrects lens distortion using yet another formula.
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* [[CamChecker]] is a tool for automatically determining lens distortion and generates a different set of parameters from everything else.
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* [[zhang_undistort]] is a tool distributed with [[hugin]] that uses [[CamChecker]] parameters to actually correct distortion.
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Latest revision as of 23:07, 17 November 2007


Barrel distortion is the opposite of pincushion distortion, it is a radial image distortion introduced by the lens itself.

It is a common Lens distortion that can be corrected by the lens correction model of panotools.

A photograph showing exaggerated barrel distortion
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