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2<!DOCTYPE sect2 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
3                       "http://www.docbook.org/xml/4.3/docbookx.dtd">
4<!-- section history:
5  2017-10-22 j.h: update
6-->
7<sect2 id="filters-blur-introduction">
8  <title>Introduction</title>
9
10  <indexterm>
11    <primary>Filters</primary>
12    <secondary>Blur</secondary>
13  </indexterm>
14
15  <figure>
16    <title>Original for demo</title>
17    <mediaobject>
18      <imageobject>
19        <imagedata format="PNG"
20          fileref="images/filters/examples/blur-demo-orig.png"/>
21      </imageobject>
22    </mediaobject>
23  </figure>
24  <para>
25    This is a set of filters that blur images, or parts of them,
26    in various ways.  If there is a selection, only the selected parts
27    of an image will be blurred. There may, however, be some leakage
28    of colors from the unblurred area into the blurred area.  To help
29    you pick the one you want, we will illustrate what each does when
30    applied to the image shown at right.  These are, of course, only
31    examples:  most of the filters have parameter settings that allow
32    you to vary the magnitude or type of blurring.
33  </para>
34
35  <figure>
36    <title>Gaussian blur (radius 10)</title>
37    <mediaobject>
38      <imageobject>
39        <imagedata format="PNG"
40          fileref="images/filters/examples/blur-demo-gauss10.png"/>
41      </imageobject>
42    </mediaobject>
43  </figure>
44  <para>
45    The most broadly useful of these is the Gaussian blur. (Don't let
46    the word "Gaussian" throw you: this filter makes an image blurry
47    in the most basic way.)  It has an efficient implementation that
48    allows it to create a very blurry blur in a relatively short
49    time.
50  </para>
51
52  <figure>
53    <title>Selective blur</title>
54    <mediaobject>
55      <imageobject>
56        <imagedata format="PNG"
57          fileref="images/filters/examples/blur-demo-selective.png"/>
58      </imageobject>
59    </mediaobject>
60  </figure>
61  <para>
62    The Selective Blur filter allows you to set a threshold so that
63    only pixels that are similar to each other are blurred together.
64    It is often useful as a tool for reducing graininess in photos
65    without blurring sharp edges.  (In the example, note that the
66    graininess of the background has been reduced.)  The
67    implementation is much slower
68    than a Gaussian blur, though, so you should not use it unless you
69    really need the selectivity.
70  </para>
71
72  <figure>
73    <title>Pixelize</title>
74    <mediaobject>
75      <imageobject>
76        <imagedata format="PNG"
77          fileref="images/filters/examples/blur-demo-pixelize.png"/>
78      </imageobject>
79    </mediaobject>
80  </figure>
81  <para>
82    The Pixelize filter produces the well-known "Abraham Lincoln"
83    effect by turning the image into a set of large square pixels.
84    (The <link linkend="gimp-filter-oilify">Oilify</link> filter, in
85    the Artistic Filters group, has a similar effect, but with
86    irregular blobs instead of perfectly square pixels.)
87  </para>
88  <note>
89    <para>
90      You can find a nice explanation of the Abraham Lincoln effect
91      at <xref linkend="bibliography-online-bach"/>. You will see the
92      Salvador Dali's painting <quote>Gala Contemplating the
93      Mediterranean Sea</quote> turning to an Abraham Lincoln's portrait
94      when looking at it from a distance.
95    </para>
96  </note>
97
98  <para>
99    With GIMP-2.10, the Motion Blur filter has been divided in Circular,
100    Linear, and Zoom.
101  </para>
102
103  <figure>
104    <title>Circular Motion Blur filter</title>
105    <mediaobject>
106      <imageobject>
107        <imagedata format="PNG"
108          fileref="images/filters/examples/blur-demo-circular.png"/>
109      </imageobject>
110    </mediaobject>
111  </figure>
112  <para>
113    The Circular Motion Blur filter blurs in a rotational direction
114    around a center that you can set.
115  </para>
116
117  <figure>
118    <title>Linear Motion Blur filter</title>
119    <mediaobject>
120      <imageobject>
121        <imagedata format="PNG"
122          fileref="images/filters/examples/blur-demo-linear.png"/>
123      </imageobject>
124    </mediaobject>
125  </figure>
126  <para>
127    The Linear Motion Blur filter blurs in a direction that you can set.
128  </para>
129
130  <figure>
131    <title>Zoom Motion Blur filter</title>
132    <mediaobject>
133      <imageobject>
134        <imagedata format="PNG"
135          fileref="images/filters/examples/blur-demo-zoom.png"/>
136      </imageobject>
137    </mediaobject>
138  </figure>
139  <para>
140    The Zoom Motion Blur filter blurs in a radial direction around a
141    center that you can set.
142  </para>
143
144  <para>
145    Finally, the Tileable Blur filter is really the same thing as a Gaussian
146    blur, except that it wraps around the edges of an image to help you reduce
147    edge effects when you create a pattern by tiling multiple copies of the
148    image side by side.
149  </para>
150  <note>
151    <para>
152      Tileable Blur is actually implemented by a Script-Fu script that invokes
153      the Gaussian blur plug-in.
154    </para>
155  </note>
156</sect2>
157