Erik Krause: 1 revision(s)

2007-03-21T18:09:03Z

1 revision(s)

Bruno: Tutorial on mending broken lines

2007-03-21T18:07:00Z

Tutorial on mending broken lines

Bruno: Tutorial on mending broken lines

2006-11-06T21:19:26Z

Tutorial on mending broken lines

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Shooting panoramas hand-held is a useful technique, but a major drawback is [[parallax]] error when assembling later. This tutorial shows how to use the ''shear'' tool in your image manipulation program to retouch these errors invisibly.

Here is a portion of a stitched [[equirectangular]] panorama, you can see that there are extreme parallax problems with the overhead wires.

[[Image:shear-tool-1.jpg]]

A common retouching technique is to select the wires and rotate them until they line up, but this is problematic for two reasons:

* Rotation has to be done by eye using trial and error.
* Parallax errors are actually closer to a shear transformation than a rotation.

By using the ''shear'' tool instead of the ''rotate'' tool, it is possible to correct the error exactly, first time every time.

Start by measuring the horizontal or vertical difference of the ''fault'', in this case it is ten pixels.

[[Image:shear-tool-2.jpg]]

Now select as much as you can on one side of the fault with the ''rectangular selection'' tool.

[[Image:shear-tool-3.jpg]]

Using the ''shear'' tool, shear the selection by ten pixels vertically, to be precise you can enter this number using the keyboard.

[[Image:shear-tool-4.jpg]]

Finally, use the ''move'' tool to move the selection vertically by half of the shear amount (ie. five pixels). In the [[Gimp]] you can nudge the selection five pixels by hitting an arrow key five times. Now the selected area lines-up perfectly without any trial and error.

[[Image:shear-tool-5.jpg]]

Here is the image after cleaning up with the ''resynthesizer'' tool (cleanup could also be done manually with the ''clone'' tool for similar results).

[[Image:shear-tool-6.jpg]]

== Notes ==

The ''move'' tool only works in pixel increments, this limits the shear value to even numbers (2, 4, 6, 8, etc...). So if you find that the error you need to fix is an odd number such as eleven, then simply scale the image 200% and work on this instead.

For better results, perform the operation twice, once on each side of the fault. For example the final image above was actually sheared by four pixels on the left side of the fault and by six pixels on the right.

This tutorial was assembled using the [[Gimp]], but probably something similar can be achieved with other software such as [[Photoshop]].

[[Category:Tutorial]][[Category:Tutorial:Nice_to_know]]

← Older revision		Revision as of 18:07, 21 March 2007
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	This tutorial was assembled using the [[Gimp]], but probably something similar can be achieved with other software such as [[Photoshop]].		This tutorial was assembled using the [[Gimp]], but probably something similar can be achieved with other software such as [[Photoshop]].

−	~~[[Category:Tutorial]]~~[[Category:Tutorial:Nice_to_know]]	+	[[Category:Tutorial:Nice_to_know]]

Mending parallax errors with the shear tool - Revision history

Erik Krause: 1 revision(s)

Bruno: Tutorial on mending broken lines

Bruno: Tutorial on mending broken lines