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<TITLE> Description of Objects in VPython</TITLE>
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<H1 CLASS="Heading-1"> <font color="#0000A0">Composite Objects with frame</font></H1>
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<P CLASS="Normal">
You can group objects together to make a composite object that can be moved and rotated as though it were a single object. Create a frame object, and associate objects with that frame:</P>
<DIV>
<H2 CLASS="program0">
f = frame()</H2>
<P CLASS="program">
cylinder(frame=f, pos=(0,0,0), radius=0.1, length=1, color=color.cyan)</P>
<P CLASS="program">
sphere(frame=f, pos=(1,0,0), radius=0.2, color=color.red)</P>
<P CLASS="program">
f.axis = (0,1,0)</P>
<P CLASS="program">
f.pos = (-1,0,0)</P>
<P CLASS="Normal"> By default, frame() has a position of (0,0,0) and axis
in the x direction (1,0,0). The cylinder and sphere are created within the
frame. When any of the frame attributes are changed (<span class="attribute">pos</span>,
<span class="attribute">x</span>, <span class="attribute">y</span>, <span class="attribute">z</span>,
<span class="attribute">axis</span>, or <span class="attribute">up</span>),
the composite object is reoriented and repositioned.</P>
<P CLASS="Normal">Another frame attribute is <span class="attribute">objects</span>,
which is a list of currently visible objects contained in the frame (the
list does not include objects that are currently invisible). If you want
to make all the objects in a frame be red, do the following (assume the
frame is named f):</P>
<div>
<h2 class="program0"> for obj in f.objects:<br>
obj.color = color.red</h2>
</div>
<P CLASS="Normal">If you use this method to make all the objects invisible,
the f.objects list will be empty. If you need a list containing all the
objects, both visible and invisible, you need to maintain your own list
of objects.</P>
<P CLASS="Normal"> If <span class="program0"><font color="#FF0000">ball</font></span>
is an object in a frame, <font color="#FF0000">ball.pos</font> is the position
local to the frame, not the actual position in "world space".
Here is a routine that will calculate the position of a vector such as <font color="#FF0000">ball.pos</font>
in world space:</P>
<div>
<h2 class="program0"> def world_space_pos(frame, local):<br>
"""Returns the position of local in world space."""<br>
x_axis = norm(frame.axis)<br>
z_axis = norm(cross(frame.axis, frame.up))<br>
y_axis = norm(cross(z_axis, x_axis))<br>
return frame.pos+local.x*x_axis+local.y*y_axis+local.z*z_axis<br>
</h2>
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