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201 lines
6.7 KiB
201 lines
6.7 KiB
<chapter id="selections">
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<title>Selections</title>
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<para>
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This chapter gives a short introduction on selections.
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</para><para>
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You can select a part of an image masking off the rest. This is handy when
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you want to cut, copy or just modify a part of the image without affecting
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the rest. For processing selected objects &chalk; applies a mask. Each pixel of
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the selection is processed based on a value of its mask, or the <quote>level
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of the selection</quote>, that can range from 0 (unselected) to 255
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(selected). Yes, that is right, you can have fractionally selected pixels.
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And by working on individual pixels you can <quote>paint</quote> your selection.
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</para><para>
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The selection mask is visualized with unselected pixels having a blueish
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tint, and selected pixels looking like normal. Fractionally selected pixels
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are shown as something in between. Additionally a red border is drawn around
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the selected areas. Fractionally selected pixels are inside the border, so
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even inside the red border you can possibly see the blueish tint on some pixels.
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</para>
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<sect1 id="selections-making">
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<title>Making a selection</title>
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<para>
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A whole range of tools exist to make selections. From rectangles, ellipses
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and freehand to the more exotic like color range select. When you make
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several selections they add up. So a rectangle select followed by an
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ellipse select select both areas. Later on, you can subtract areas from
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the selection by using, for example, the <guilabel>Erase Selection</guilabel> tool.
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</para><para>
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To get back to normal (no active selection), choose
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<menuchoice><guimenu>Select</guimenu><guimenuitem>Deselect</guimenuitem>
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</menuchoice>. To select all pixels, choose
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<menuchoice><guimenu>Select</guimenu><guimenuitem>Select All</guimenuitem>
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</menuchoice>.
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</para><para>
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You may think that those two actions give the same result, but it
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is much more efficient to have no active selection than to have selected
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everything.
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</para><para>
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After having deselected you can bring your selection back by choosing
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<menuchoice><guimenu>Select</guimenu><guimenuitem>Reselect</guimenuitem>
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</menuchoice>.
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</para>
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</sect1>
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<sect1 id="selections-painting">
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<title>Painting your selection</title>
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<para>
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As said above you can essentially paint your selection, and just like
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when you paint normally you can choose to paint your selection freehand or
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guided with rectangles, ellipses, &etc;. You also have the choice of different
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paint tools like pen, brush, airbrush, &etc;. Choose the guide tool, and the
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paint tool in the toolbox, and go ahead and <quote>paint</quote> your
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selection.
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</para><para>
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The guide tools work just like you may be used to from other applications. So
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holding down shift while drawing a rectangle or an ellipse still forces them to
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be a square or a circle respectively.
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</para>
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<screenshot>
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<screeninfo>Painting a selection</screeninfo>
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<mediaobject>
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<imageobject>
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<imagedata fileref="using-selections-1.png" format="PNG" />
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</imageobject>
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<textobject>
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<phrase>Painting a selection</phrase>
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</textobject>
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<caption><para>Painting a selection</para></caption>
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</mediaobject>
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</screenshot>
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<screenshot>
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<screeninfo>Painting a selection</screeninfo>
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<mediaobject>
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<imageobject>
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<imagedata fileref="using-selections-2.png" format="PNG" />
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</imageobject>
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<textobject>
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<phrase>Painting a selection</phrase>
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</textobject>
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<caption><para>Painting a selection</para></caption>
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</mediaobject>
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</screenshot>
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</sect1>
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<sect1 id="selections-unselecting">
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<title>Unselecting</title>
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<para>
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All the selection paint tools have an option to add or subtract from the
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selection. This means that you can use all your familiar tools to both select
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and unselect. There is also a true selection eraser among the selection paint
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tools.
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</para>
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<screenshot>
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<screeninfo>Unselecting</screeninfo>
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<mediaobject>
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<imageobject>
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<imagedata fileref="using-selections-3.png" format="PNG" />
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</imageobject>
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<textobject>
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<phrase>Unselecting</phrase>
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</textobject>
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<caption><para>Unselecting</para></caption>
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</mediaobject>
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</screenshot>
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</sect1>
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<sect1 id="selections-making-new">
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<title>Making a new selection</title>
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<para>
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When you want to make a new selection, replacing the currently active one, you
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first need to deselect the active selection. Choose
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<menuchoice><guimenu>Select</guimenu><guimenuitem>Deselect</guimenuitem>
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</menuchoice>.
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</para>
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</sect1>
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<sect1 id="selections-contigious">
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<title>Selecting a contiguous area (magic wand)</title>
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<para>
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To follow the analogy of painting your selection &chalk; also provides an
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equivalent to filling a contiguous area. Some paint applications call this
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selection tool the magic wand tool. What it does is select the nearby
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pixels as long as they have nearly the same color as the pixel you click
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on. The selection floods out from the point you click on. In the fuzziness
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option you can set how different the colors are allowed to be before the
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flooding stops.
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</para>
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<screenshot>
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<screeninfo>Before the magic wand</screeninfo>
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<mediaobject>
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<imageobject>
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<imagedata fileref="using-selections-4.png" format="PNG" />
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</imageobject>
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<textobject>
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<phrase>Before the magic wand</phrase>
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</textobject>
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<caption><para>Before the magic wand</para></caption>
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</mediaobject>
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</screenshot>
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<screenshot>
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<screeninfo>A magic wand selection</screeninfo>
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<mediaobject>
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<imageobject>
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<imagedata fileref="using-selections-5.png" format="PNG" />
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</imageobject>
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<textobject>
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<phrase>A magic wand selection</phrase>
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</textobject>
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<caption><para>A magic wand selection</para></caption>
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</mediaobject>
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</screenshot>
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</sect1>
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<sect1 id="selections-similar">
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<title>Selecting similar colors</title>
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<para>
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The <guilabel>Select Similar</guilabel> tool lets you pick a pixel and then select all pixels that
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have a similar color. Picking a color in one corner of the image may select a
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pixel in another corner if they have similar color.
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With the <guilabel>Fuzziness</guilabel> option you can set how similar the colors must be to become
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selected.
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</para>
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<screenshot>
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<screeninfo>Selecting similar colors</screeninfo>
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<mediaobject>
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<imageobject>
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<imagedata fileref="using-selections-6.png" format="PNG" />
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</imageobject>
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<textobject>
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<phrase>Selecting similar colors</phrase>
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</textobject>
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<caption><para>Selecting similar colors</para></caption>
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</mediaobject>
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</screenshot>
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</sect1>
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<sect1 id="selections-inverting">
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<title>Inverting the selection</title>
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<para>
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In some cases it is easier to specify your selection the other way around. That
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is, first you select the parts that ultimately should not be selected and then
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then you choose
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<menuchoice><guimenu>Select</guimenu><guimenuitem>Invert</guimenuitem>
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</menuchoice>.
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What invert does, is that for every pixel it flips the selection level so to
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speak, by setting it to 256 minus the current selection level. Thus what was
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selected becomes unselected and vice versa.
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</para>
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</sect1>
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</chapter>
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