<html><head><title>Gimp Wideangle Filter</title></head>
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<h1>Gimp Wideangle Filter</h1>
<p>The Wideangle plugin is licensed under the GPL.<br/>
The source code is available <a href="wideangle.c">here</a>. Latest version is
1.0.10.<br/>
<b>gimptool --install wideangle.c</b> will compile the plugin and install
it in your local Gimp plugins directory.<br/>
The plugin appears in the <b>Filters/Distorts</b> menu as
<b>Wideangle...</b>.</p>
<p>The Wideangle filter is used to correct (or simulate) the distortion
typically seen on photographs taken with a wideangle lens. There are six
controls.
<dl>
<dt>X Shift</dt>
<dd>Adjusts the centre of the effect, from the left edge of the image (at -100.0),
through the centre (at 0.0), to the right edge (at 100.0).</dd>
<dt>Y Shift</dt>
<dd>Adjusts the centre of the effect, from the top edge of the image (at -100.0),
through the centre (at 0.0), to the bottom edge (at 100.0).</dd>
<dt>Main</dt>
<dd>Controls the amount of distortion. Negative values correct wideangle (barrel)
distortion, while positive values create it (or correct pincushion distortion).</dd>
<dt>Edge</dt>
<dd>Like the <b>Main</b> control, but the <b>Edge</b> control has more effect
at the edges of the image than at the centre.</dd>
<dt>Zoom</dt>
<dd>Zooms the entire image in or out.</dd>
<dt>Brighten</dt>
<dd>Adjusts the image brightness up or down with distance from the centre.
Can be used to remove (or create) vignetting, the darkening of the corners
of an image.</dd>
</dl>
</p>
<p>The <b>Do Preview</b> toggle enables and disables the preview window.
The preview can show either an overview of the whole image, or a detail view
which shows a 5x5 array of full resolution closeups. The overview is useful for
casual use, while the detail view
allows you to create a precisely calibrated preset for a particular lens. To create
a lens preset, use
a photo of a grid pattern. Start by adjusting the <b>Main</b> control until the
lines of the grid are as straight as possible. Use the <b>Shift</b> controls, if
necessary, to correct for any asymmetry. Finally, if the best setting of the
<b>Main</b> control leaves a "wiggle" in the lines, use the <b>Edge</b>
control to remove it. (You may need to readjust the <b>Main</b> control a little.)
Once you have the correct settings for a particular lens, you should be able
to save the settings as a new preset and use them for all images taken with that
lens.</p>
<p>Or just play around.</p>
<p>The preset controls allow you to reset everything, select a previously defined
preset, or save the current settings (if they've been altered) under a new name.
Preset names can contain (fairly) arbitrary characters. If you mess up the presets,
they are stored in a text file under your Gimp user
directory in the <code>wideangle</code> subdirectory.</p>
<p>Regardless of the Gimp's settings, the Wideangle filter always uses cubic
interpolation. This is not a bug, it's a feature.</p>
<p><b>What's with the pop up menu on the "select" button?</b></p>
<p>I originally had a regular option menu to select presets. However this had
a drawback, namely that the displayed option was only correct until a control
was adjusted. A pop up menu, although not the normal choice, provides exactly
the correct model to the user.</p>
<p>Written by David Hodson. Many thanks for suggestions and improvements
to Lars Clausen. (But bugs are still my fault.)</p>
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