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<?xml version="1.0" encoding="UTF-8"?>
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<title>GAMGI Interfaces: Layer Modify</title>
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<h1>Layer Modify</h1>
<div id="notebook">
<ul>
<li><span>Projection</span></li>
<li><a href="modify_position.html">Position</a></li>
<li><a href="modify_visibility.html">Visibility</a></li>
<li><a href="modify_view.html">View</a></li>
</ul>
</div>
<div class="contents">
Change here the projection parameters for a single layer or a list of layers.
<p/>
To modify a layer, click over the graphic area to open a menu and choose a layer,
or write its identification (name and number) on the <b>Layer</b> entry. To modify
a list of layers, press the button <b>List</b> (after creating the list of layers
with <b>Layer->Select</b>). Parameters for empty entries or <b>Local</b> choices
remain unchanged.
<p/>
To change a layer name write the new name in the <b>Layer</b> entry,
followed by the layer number (GAMGI needs the number to identify the layer).
To change the name for a list of layers, press <b>List</b> first
and then write the new common name in the <b>Name</b> entry.
<h3>Perspective, Orthographic</h3>
Change the type of projection of the layer.
GAMGI projects the 3D space on the 2D screen using <b>Perspective</b>
or <b>Orthographic</b> projections. The <b>Perspective</b> projection
is better to simulate realistic views and gives users a depth feedback,
but the <b>Orthographic</b> projection preserves object dimensions,
as these are not distorted by perspective.
<h3>Near, Far, Top</h3>
Change the dimensions of the space volume that is visible to the observer,
after being projected into the window graphic area.
This volume has six faces, and the front and back faces are always perpendicular
to the viewing direction. <b>Near</b> and <b>Far</b> parameters set
the distance from the view point to these front and back faces, respectively,
corresponding to the minimum and maximum distances that the observer can see.
<b>Near</b> must be smaller than <b>Far</b>.
<p/>
In a <b>Orthographic</b> projection, the projection volume is a rectangular
parallelepiped, so the 4 lateral faces are paralell and perpendicular to
each other. In a <b>Perspective</b> projection, the projection volume is
a truncated quandrangular pyramid, with the apex at the view point.
<p/>
<b>Top</b> defines the half-height of the front face of the projection
volume. The width is automatically determined from the ratio height / width
of the window graphic area, so everytime users resize this window, the
projection volume changes accordingly. Increasing <b>Top</b> increases
the visible volume and decreases the size of the objects.
<p/>
Each layer has its own projection parameters, so in a window with
multiple visible layers, the image rendered in the graphic area
is the superposition of the various, totally independent, layer
views.
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