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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. |
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| | [[Image:Barrel-distortion.jpg|frame|none|A photograph showing exaggerated barrel distortion]] | | [[Image:Barrel-distortion.jpg|frame|none|A photograph showing exaggerated barrel distortion]] |
| − | | + | [[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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