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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

<title>faces</title>

<!--

.style1 {font-size: x-large}

.style2 {font-size: xx-large}

-->

</style>

<!--

function MM_jumpMenu(targ,selObj,restore){ //v3.0

eval(targ+".location='"+selObj.options[selObj.selectedIndex].value+"'");

if (restore) selObj.selectedIndex=0;

}

//-->

</script>

</head>

<body>

<tr>

If you're new to Python

and VPython: <a href="VisualIntro.html">Introduction</a>

A VPython <a href="VPython_Intro.pdf" target="_blank">tutorial</a>

<a href="primitives.html">Pictures</a> of 3D objects

Choose an object:

<option>Choose an object</option>

<option value="cylinder.html">Overview</option>

<option value="arrow.html">arrow</option>

<option value="convex.html">convex</option>

<option value="curve.html">curve</option>

<option value="cylinder.html">cylinder</option>

<option value="ellipsoid.html">ellipsoid</option>

<option value="extrusion.html">extrusion</option>

<option value="faces.html">faces</option>

<option value="frame.html">frame</option>

<option value="helix.html">helix</option>

<option value="label.html">label</option>

<option value="lights.html">lights</option>

<option value="points.html">points</option>

<option value="pyramid.html">pyramid</option>

<option value="sphere.html">sphere</option>

</select>

Work with objects:

<option>Choose an option</option>

<option value="color.html">Color/Opacity</option>

<option value="materials.html">Materials/Textures</option>

<option value="shapes.html">Shapes Library</option>

<option value="paths.html">Paths Library</option>

<option value="defaults.html">Defaults</option>

<option value="rate.html">Animation Speed</option>

<option value="rotation.html">Rotations</option>

<option value="options.html">Additional Options</option>

<option value="delete.html">Delete an Object</option>

<option value="controls.html">Buttons/Sliders</option>

<option value="graph.html">Graphs</option>

<option value="lights.html">Lighting</option>

<option value="files.html">Read/Write Files</option>

<option value="trail.html">Leaving a Trail</option>

<option value="vector.html">Vector Operations </option>

<option value="factorial.html">factorial/combin</option>

</select>

Windows &amp Events:

<option>Choose a topic</option>

<option value="display.html">Windows</option>

<option value="mouse.html">Mouse Events</option>

<option value="mouse_click.html">   Mouse Click</option>

<option value="mouse_drag.html">   Mouse Drag</option>

<option value="keyboard.html">Keyboard Events</option>

</select>

What's new in <a href="new_features.html">Visual 5</a>

<a href="http://vpython.org" target="_blank">VPython web site</a>

<a href="license.txt" target="_blank">Visual license</a>

<a href="http://www.python.org" target="_blank">Python web site</a>

<a href="http://www.python.org/doc/2.5.2/lib/module-math.html" target="_blank">Math module</a> (sqrt etc.)

<a href="http://www.scipy.org/Documentation" target="_blank">Numpy module</a> (arrays) </td>

<tr>

<td width="77%"><div align="center">faces</div></td>

</tr>

</table>

<div>

The "faces" primitive takes a list

of triangles (position, color, and normal for each vertex). This

is useful for writing routines in Python to import 3D models made

with other 3D modeling tools. You would still need to do lots of

calculations of normals and so on, but you would not need to do C

coding to import an arbitrary model file.

The faces object is an array primitive (like curve, convex, etc), so you

have to use a frame to move it around. It consists of a set of one-sided triangles

with user-specified vertices, colors, and normals. The pos, color, and normal attributes look like this:

pos = [ t0v0, t0v1, t0v2,

t1v0, t1v1, t1v2,

        t2v0, t2v1,

t2v2, ... ]

where t0v0 is the position vector (x,y,z) of vertex

0 of triangle 0, t0v1 is vertex 1 of triangle 0, etc. Other faces

attributes are red, green,

blue, visible, material, frame, and display.

Creating a faces object in terms of triangles and normals can be very tedious. A convenient way to create a faces object is to create an <a href="extrusion.html">extrusion</a> or <a href="text.html">text</a> object and then use the create_faces() function, as described in the documentation for extrusion and text.

Each face is a one-sided surface. Which side is illuminated is determined

by the "winding" order of the face. When you are looking at a face,

it is illuminated if the order of the vertices in the pos list goes counter-clockwise. If you need the triangle to be visible from both

sides, you must create another triangle with the opposite winding order. If you have made a one-sided faces object named myfaces, you can make all faces be two-sided with myfaces.make_twosided(). This function became available with VPython 5.3.

If you don't specify normals at the vertices, the face is illuminated only

by "ambient" light. In order for the main lighting to affect the

appearance, you must specify normals to the surface at the vertices. In the

simplest case, a normal at a vertex is perpendicular to the face, and adjoining

faces have a hard edge where they join. A soft edge can be produced by averaging

the normals to the two faces at their common vertices. The brightness of a

face is proportional to the cosine of the angle between the normal and the

light. If you have created a faces object named myfaces without specifying normals, you can create normals perpendicular to all the faces with myfaces.make_normals().

After creating a faces object named myfaces, myfaces.smooth() will

average similar normals at a vertex and make the object look smoother.

The smoothing criterion is that normals will be averaged whose directions

differ by an angle whose cosine is greater than or equal to 0.95, which

means angles less than 18 degrees. You must make sure that all vertices

have nonzero normals before using the smooth function; a simple scheme

is to use myfaces.make_normals() to

make normals perpendicular to all the faces and then use the smooth

function to adjust them for improved appearance. You can specify

the smoothing criterion. For example, myfaces.smooth(0.7) will

average normals whose directions differ by an angle whose cosine is

greater than or equal to 0.7 (angles less than 45 degrees).

If you specify different colors at the vertices of one triangular face, VPython

interpolates across the face, in which case the face is not all one color.

There is a similar interpolation for normals if there are different normals

at the vertices, in which case the face is not all one brightness.

You can append additional vertices to an existing

faces object with one of the following four forms (assuming the object

is named f):

f.append(pos=(x,y,z))

f.append(pos=(x,y,z), normal=(nx,ny,nz))

f.append(pos=(x,y,z), normal=(nx,ny,nz), color=(r,g,b))

f.append(pos=(x,y,z), normal=(nx,ny,nz),

red=r, green=g, blue=b)

Another convenient way to construct a faces object

is to create a list or numpy array of positions, then create the faces

object with the pos attribute set to the

list or array. Then use make_normals(), make_twosided(), and smooth() to

complete the task.

The faces object is intended to help with writing model importers and other

new primitives in Python, not for direct manipulation by normal programs.

It is considerably lower-level than any of the other objects in Visual (although

it is not necessarily any faster, at least right now). It is flexible enough

to implement smooth or facet shading, per-vertex coloration, two-sided or

one-sided lighting, etc, but all of these calculations must be made by the

programmer (when setting up pos, color, normal), although much of this can be done using the make_normals, make_twosided, and smooth functions.

You can specify a <a href="materials.html">material</a>, but

currently you can not specify opacity for faces.

See description of <a href="options.html">Additional

Attributes</a> available for all 3D display objects.

For examples of the use of the faces object, see the faces demo programs.

</div>

</td>

</tr>

<tr>

</tr>

</table>

</body>

</html>

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