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<title>GAMGI Interfaces: Layer Modify</title>
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<h1>Layer Modify</h1>
<div id="notebook">
<ul>
<li><a href="modify_projection.html">Projection</a></li>
<li><span>Position</span></li>
<li><a href="modify_visibility.html">Visibility</a></li>
<li><a href="modify_view.html">View</a></li>
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<div class="contents">
Change here the volume 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>Eye</h3>
Change the user position, in the layer space. Any position in 3D space
is valid. Initially, the user is at (0, 0, +z), looking down to
(0, 0, 0), and the up direction is (0, 1, 0), so the x axis
points to the right, the y axis points to the top, and the z
axis points to the user.
<h3>Center</h3>
Change the position to where the user is looking. The distance from
<b>Eye</b> to <b>Center</b> must be within the limits of visibility
(from <b>Near</b> to <b>Far</b>).
<h3>Up</h3>
Change the direction in layer space that points up, to the top of the
screen. This direction is converted internally to a unit vector
normal to the direction <b>Eye</b> -> <b>Center</b>, so any non-zero
vector that is not parallel to that direction is valid.
<p/>
The user position and orientation can be defined by a (X,Y,Z)
layer referential, pointing from the middle of the screen to: the
right (X), the top (Y), the user (Z). <b>Eye</b> is the origin,
<b>Eye</b> <- <b>Center</b> defines the Z axis, and <b>Up</b>
the Y axis, permiting to get the X axis by the cross product
of the other two axes. Initially, the axes are orientated along
(1,0,0) for X, (0,1,0) for Y and (0,0,1) for Z, but this is no
longer the case when the layer referential is rotated.
<p/>
To move the user around the objects, <b>Eye</b>, <b>Center</b>
and <b>Up</b> must change. A simple, interactive way to do this
is to press <b>Axes</b> in the window toolbar and then either drag
the mouse or use the ruler, as described in <b>Introduction Mouse</b>
and <b>Introduction Tools</b>.
<p/>
Each layer has its own position 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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