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<TITLE> Description of Objects in VPython</TITLE>
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<b class="navlabel">Previous:</b> <a class="sectref" HREF="graph.html">Graph Plotting</A>
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<b class="navlabel">Next:</b> <a class="sectref" HREF="mouse.html">Mouse Interactions</A>
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</DIV>
<DIV>
<h1 CLASS="Heading-1"> <font color="#0000A0">Controlling One or More Visual
Display Windows</font></h1>
</DIV>
<DIV> Initially, there is one Visual display window named <span class="attribute">scene</span>.
Display objects do not create windows on the screen unless they are used, so
if you immediately create your own display object early in your program you
will not need to worry about scene. If you simply begin creating objects such
as sphere they will go into scene.
<P CLASS="attributes"> <span class="attribute">display()</span> Creates a display
with the specified attributes, makes it the selected display, and returns
it. For example, the following creates another Visual display window 600 by
200, with 'Graph of position' in the title bar, centered on (5,0,0) and with
a background color of cyan filling the window.</P>
<DIV>
<H2 CLASS="program0"> scene2 = display(title='Graph of position',</H2>
<P CLASS="program"> width=600, height=200,</P>
<P CLASS="program"> center=(5,0,0), background=(0,1,1))</P>
<P CLASS="Normal"> <font color="#0000A0">General-purpose options</font></P>
<P CLASS="attributes"> <span class="attribute">select()</span> Makes the specified
display the "selected display", so that objects will be drawn
into this display by default; e.g. <span class="attribute">scene.select()
</span></P>
</DIV>
</DIV>
<blockquote>
<blockquote>
<blockquote>
<DIV class="attributes">
<DIV>The function <font color="#FF0000">display._selected()</font> returns
a reference to the display in which objects are currently being created
(which will be None if no display has been created yet).</DIV>
</DIV>
</blockquote>
</blockquote>
</blockquote>
<DIV>
<DIV>
<P CLASS="attributes"> <span class="attribute">foreground</span> Set color
to be used by default in creating new objects such as sphere; default is
white. Example: <span class="attribute">scene.foreground = (1,0,0)</span>
</P>
<P CLASS="attributes"> <span class="attribute">background</span> Set color
to be used to fill the display window; default is black.</P>
<P CLASS="attributes"><span class="attribute">stereo</span> Stereoscopic option;
<span class="attribute">scene.stereo = 'redcyan'</span> will generate a
scene for the left eye and a scene for the right eye, to be viewed with
red-cyan glasses, with the red lens over the left eye. (There are also <span class="attribute">'redblue'</span>
and <span class="attribute">'yellowblue'</span> options; note that objects
that were not originally white may be somewhat dim.) <br>
<br>
Setting <span class="attribute">scene.stereo = 'crosseyed'</span> produces
side-by-side images which if small enough can be seen in 3D by crossing
your eyes but focusing on the screen (this takes some practice). Setting
<span class="attribute">scene.stereo = 'passive'</span> produces side-by-side
images which if small enough can be seen in 3D by looking "wall-eyed",
looking into the far distance but focusing on the screen (this too takes
some practice). <span class="attribute"><br>
<br>
scene.stereo = 'active'</span> will render alternating left eye/right eye
images for viewing through shutter glasses if the graphics system supports
quad buffered stereo. If stereo equipment is not available, setting the
option has no effect, and <span class="attribute">scene.stereo</span> will
have the value <span class="attribute">'nostereo'</span>. You can also use
<span class="attribute">scene.stereo = 'passive'</span> with quad buffered
stereo for display using two polarized projectors (for stereo viewing using
simple passive polarized glasses). (Quad buffered 'active' stereo is only
available on specialised graphics systems that have the necessary hardware
and shutter glass connector, such as PCs with nVidia Quadro graphics cards.
It generates the illusion of depth by rendering each frame twice from slightly
different viewpoints corresponding to the left and right eyes. Special shutter
glasses are synchronised with the alternating images so that each eye sees
only the matching frame, and your brain does the rest. It's called 'quad
buffered' because there is an OpenGL buffer per eye, both double-buffered
for smooth updating. 'Passive' stereo requires a video card that can drive
two monitors, or two projectors.) </P>
<P CLASS="attributes"><span class="attribute">stereodepth</span> By default,
the front of the scene is located at the location of the physical screen,
which reduces eye strain. Setting <span class="attribute">scene.stereodepth
= 1</span> moves the center of the scene to the location of the physical
screen, with the front half of the scene seeming to stick dramatically out
of the screen. <span class="attribute">scene.stereodepth = 2</span> moves
the scene fully in front of the physical screen, for maximally dramatic
stereo effect.</P>
<P CLASS="attributes"><span class="attribute">ambient</span> Amount of nondirectional
("ambient") lighting. Default is 0.2. Also see the following <span class="attribute">lights</span>
attribute.</P>
<P CLASS="attributes"><span class="attribute">lights</span> List of vectors
representing directions from the origin to the lights. The magnitude of
the vector is the intensity. For example, <span class="attribute">scene.lights
= [vector(1,0,0)]</span> with <span class="attribute">scene.ambient = 0</span>
will light the scene from the right side, with no ambient lighting on the
left. By default there are two lights in the list: (0.17, 0.35, 0.70), magnitude
0.8, and (-0.26, -0.07, -0.13), magnitude 0.3. The attributes <font color="#FF0000">lights</font>
and <font color="#FF0000">ambient</font> must be used with some care, because
if the total lighting intensity exceeds 1 anywhere in the scene the results
are unpredictable. </P>
<P CLASS="attributes"><span class="attribute">cursor.visible</span> By setting
<span class="attribute">scene.cursor.visible = 0</span>, the mouse cursor
becomes invisible. This is often appropriate while dragging an object using
the mouse. Restore the cursor with <span class="attribute">scene.cursor.visible
= 1</span>.</P>
<P CLASS="attributes"><span class="attribute">objects</span> A list of all
the visible objects in the display; invisible objects are not listed. For
example, this makes all boxes in the scene red: </P>
<P CLASS="program"> for obj in scene2.objects: <br>
if obj.__class__ == box # can say either box or
'box'<br>
obj.color = color.red</P>
</DIV>
</DIV>
<p class="attributes"><span class="attribute"><font color="#000000">To obtain
</font>camera position</span>, see <a href="mouse.html">Mouse Interactions</a>.</p>
<p class="attributes"> </p>
<DIV>
<DIV> </DIV>
</DIV>
<DIV>
<DIV>
<P CLASS="Normal"><font color="#0000A0">Controlling the window</font></P>
<P CLASS="attributes"> <span class="attribute">x</span>, <span class="attribute">y</span>
Position of the window on the screen (pixels from upper left)</P>
<P CLASS="attributes"> <span class="attribute">width</span>, <span class="attribute">height</span>
Width and height of the display area in pixels: scene.height = 200</P>
<P CLASS="attributes"> <span class="attribute">title</span> Text in the window's
title bar: scene.title = 'Planetary Orbit'</P>
<P CLASS="attributes"> <span class="attribute">visible</span> Make sure the
display is visible; <span class="attribute">scene2.visible = 1</span><EM CLASS="program">
</EM> makes the display named <span class="attribute">scene2</span> visible.
This is automatically called when new primitives are added to the display,
or the mouse is referenced. Setting <span class="attribute">visible</span>
to 0 hides the display.</P>
<P CLASS="attributes"><span class="attribute">fullscreen</span> Full screen
option; <span class="attribute">scene2.fullscreen = 1</span><EM CLASS="program">
</EM> makes the display named <span class="attribute">scene2</span> take
up the entire screen. In this case there is no close box visible; press
Escape to exit. <br>
There is currently a bug in the fullscreen option for Linux/Unix/Mac OSX.
Keyset input is not recognized, including the Escape key. If you use the
fullscreen option on these systems, be sure to program a mouse input for
quitting the program.</P>
<P CLASS="attributes"> <span class="attribute">exit</span> If <span class="attribute">sceneb.exit
= 0</span>, the program does not quit when the close box of the <span class="attribute">sceneb</span>
display is clicked. The default is <span class="attribute">sceneb.exit =
1</span>, in which case clicking the close box does make the program quit.</P>
<P CLASS="attributes"> </P>
<P CLASS="attributes"> </P>
<P CLASS="Normal"> <font color="#0000A0">Controlling the view</font></P>
<P CLASS="attributes"> <span class="attribute">center</span> Location at which
the camera continually looks, even as the user rotates the position of the
camera. If you change <span class="attribute">center</span>, the camera
moves to continue to look in the same "compass" direction toward
the new center, unless you also change <span class="attribute">forward</span>
(see next attribute). Default (0,0,0).</P>
<P CLASS="attributes"><span class="attribute">autocenter</span> scene.center
is continuously updated to be the center of the smallest axis-aligned box
containing the scene. This means that if your program moves the entire scene,
the center of that scene will continue to be centered in the window.</P>
<P CLASS="attributes"> <span class="attribute">forward</span> Vector pointing
in the same direction as the camera looks (that is, from the current camera
location, given by scene.mouse.camera, toward scene.center). The user rotation
controls, when active, will change this vector continuously. When <span class="attribute">forward</span>
is changed, the camera position changes to continue looking at <span class="attribute">center</span>.
Default (0,0,-1).</P>
<P CLASS="attributes"> <span class="attribute">fov</span> Field of view of
the camera in radians. This is defined as the maximum of the horizontal
and vertical fields of view. You can think of it as the angular size of
an object of size range, or as the angular size of the longer axis of the
window as seen by the user. Default pi/3.0 radians (60 degrees).</P>
<P CLASS="attributes"> <span class="attribute">range</span> The extent of
the region of interest away from <span class="attribute">center</span> along
each axis. This is always 1.0/scale, so use either <span class="attribute">range</span>
or <span class="attribute">scale</span> depending on which makes the most
sense in your program. Default (10,10,10) or set by <span class="attribute">autoscale</span>.</P>
<P CLASS="attributes"> <span class="attribute">scale</span> A scaling factor
which scales the region of interest into the sphere with unit radius. This
is always 1.0/range, so use either <span class="attribute">range</span>
or <span class="attribute">scale</span> depending on which makes the most
sense in your program. Default (0.1,0.1,0.1) or set by autoscale.</P>
<P CLASS="attributes"> <span class="attribute">uniform = 0</span> each axis
has different units and scales</P>
<P CLASS="attributes"> autoscale will
scale axes independently</P>
<P CLASS="attributes"> the x and y
axes will be scaled by the aspect ratio of the window</P>
<P CLASS="attributes"> <span class="attribute">uniform = 1</span> each axis
has the same scale</P>
<P CLASS="attributes"> autoscale scales
axes together</P>
<P CLASS="attributes"> the aspect
ratio of the window does not affect scaling</P>
<P CLASS="attributes"> <span class="attribute">up</span> A vector representing
world-space up. This vector will always project to a vertical line on the
screen (think of the camera as having a "plumb bob" that keeps
the top of the screen oriented toward up). The camera also rotates around
this axis when the user rotates "horizontally". By default the
y axis is the <span class="attribute">up</span> vector.</P>
<P CLASS="attributes"> There is an interaction between <span class="attribute">up</span>
and <span class="attribute">forward</span>, the direction that the camera
is pointing. By default, the camera points in the -z direction (0,0,-1).
In this case, you can make the x or y axes (or anything between) be the
<span class="attribute">up</span> vector, but you cannot make the z axis
be the <span class="attribute">up</span> vector, because this is the axis
about which the camera rotates when you set the <span class="attribute">up</span>
attribute. If you want the z axis to point up, first set <span class="attribute">forward</span>
to something other than the -z axis, for example (1,0,0).</P>
<P CLASS="attributes"> <span class="attribute">autoscale = 0</span> no automatic
scaling (set range or scale explicitly)</P>
<P CLASS="attributes"> <span class="attribute"> autoscale
= 1</span> automatic scaling (default)</P>
<P CLASS="attributes"> It is often useful to let Visual
make an initial display with autoscaling, then turn autoscaling off to prevent
further automated changes.</P>
<P CLASS="attributes"> <span class="attribute">userzoom = 0</span> user cannot
zoom in and out of the scene</P>
<P CLASS="attributes"> <span class="attribute">userzoom
=<span class="attribute"> </span>1</span> user can zoom (default)</P>
<P CLASS="attributes"> <span class="attribute">userspin = 0</span> user cannot
rotate the scene</P>
<P CLASS="attributes"> <span class="attribute">userspin
= 1</span> user can rotate (default)</P>
</DIV>
</DIV>
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