VPython Object

Claimed by Ddebord3 (2015) / Edited by Vnistala3 (2017)

A VPython Object is a representation of data in a specific in VPython both visually and numerically. Each object represents data through its attributes, specific characteristics assigned to each individual object. Objects play an essential role in the necessary knowledge for the coding portion of PHYS 2211 and 2212, as most of your programs will require the extensive usage and manipulation of objects. This page is intended to provide you a background on these objects, especially if you have no prior knowledge of coding or VPython.

The Main Idea

Objects in VPython are intended to represent data in ways that can be easily visualized and understood by both the user and the computer. Each object has a type it belongs to, which determines its default attributes, as well as how the computer will choose to display it when the program runs. Objects must be uniquely named if you wish to refer to them again later in your program, and any graphical object you create (spheres, boxes, curves, arrows, etc.) continue to exist for as long as your program remains running - VPython will continue to display them regardless of where they are positioned. If you change an attribute of an object, such as its position or color, VPython will automatically display the object in its new location and/or with its new color.

For example, let's say you create a sphere called 'ball' - by default, ball has attributes pre-assigned to it, such as ball.pos (the .pos attribute signifies the position of the sphere) or ball.color (the .color attribute allows you to change the color of the sphere). Also, in addition to the preset attributes, you can also create new ones. Besides the position and radius, you can also create attributes for mass (ball.mass), velocity (ball.vel), momentum (ball.momentum), or anything else you see fit. Of course, not all attributes need to be added manually. They can be left blank and filled in with a default value which the program automatically assigns them

The general syntax for creating an object is as follows.

objectName = objectType([insert attributes here])  

Attributes

Many objects in VPython carry common attributes regardless of their type. This section will cover the attributes you will be utilizing the most in this class.

Position

The position attribute is common to almost every single object, and is probably the most important of the attributes. It determines the position of the object's center (in the case of spheres, boxes, etc.) or the object's endpoint (arrows, curves, etc.). It is of the vector type. To access and/or edit this attribute, it can be accessed using objectName.pos. Returning to the example from the above section, we can access the position of our sphere called 'ball' by typing

ball.pos

This would return the position of the ball in vector form, or allow you to refer to said position in calculations.

In addition, the position attribute has three components of its own, which can be accessed by typing

ball.pos.x
ball.pos.y
ball.pos.z

depending on the desired component of the position. The same rules apply as to ball.pos, however they are returned as single numbers instead of as a vector.

To set the position of an object, one must first set the type of the object and then type pos([vector coordinates]). For instance, if we wanted to create an object called ball centered at (4,2,0), we would type

ball = objectType(pos(4,2,0))

Color

The color attribute defines the object's color when it appears in the display window. This attribute is not important for data but more for display - it is simply to distinguish objects from one another, which is especially useful if you are working with a multitude of objects at once.

Color can be assigned in one of two ways. The first way it can be assigned is through a preset color. The preset colors in VPython are blue, orange, green, red, purple, brown, pink, gray, olive, and cyan - any of these can be inserted to create the desired color. For instance, the syntax for creating a red color is:

color = color.red

Another way of assigning color is through an RGB vector, an additive form of assigning color. Red, green and blue layers are added together in various saturations and opacities to reproduce a broad array of colors. The vector contains three numbers from 0 to 255 - The first number represents the red, the second represents the green, and the third represents the blue. The higher the value of the number, the more it resembles that particular color. For instance, if we wanted to assign cyan to color, we would type

color = (0, 255, 255)

To set the color of an object, one must first set the type of the object and then set the color within the parentheses. For instance, if we wanted to create an object called ball which was cyan, we would type either

ball = objectType(color = color(cyan))
ball = objectType(color = (0,255,255))

Other Attributes

There are other attributes which are also important but not as essential (or common) as position and color, such as radius and make_trail, among others. Most of these attributes are self-explanatory or will not be used in the course, so if you would like full descriptions of these attributes, refer to the VPython Documentation.

Common Objects

In order to display and understand your code effectively, there are a certain few objects which will be essential, especially for this class.

Sphere

A sphere object is a representation of a sphere with a set radius, position, and color. A magenta sphere center centered at the origin with radius one can be created through the syntax:

nameSphere = sphere(radius = 1, pos = vector(0,0,0), color=color.magenta)

The example sphere

Arrow

An arrow object creates an arrow with a position, an axis, and a color. The axis is a vector representing the arrow's direction and length. Position is the location of its tail, not its center. A yellow arrow centered at (0,7,2) in the direction of (3,-9,1) can be created through:

nameArrow = arrow(pos=(0,7,2), axis=(3,-9,1), color=color.yellow)

The example arrow

Box

The example box

Curve

A curve is series of connections between points. This can be used to show the path of an object through the append function. This function adds an additional point to the curve allowing the curve to follow each position added. To create a red curve and add the point (4,4,4) use the syntax:

nameCurve = curve(color.color = red)

nameCurve.append(4,4,4)

Error creating thumbnail: sh: /usr/bin/convert: No such file or directory Error code: 127

The example curve

Cylinder

The example cylinder

Examples

Simple

Create a green sphere named "answer" with a radius of .7 located at the origin.

Use the following to create the sphere:

answer = sphere(radius=.7, pos=vector(0,0,0), color=color.green)

Middling

Create two objects. One, a magenta sphere of radius 3.5 named sphereOne at the position (9,0,8). Two, a cyan arrow named arrowTwo that points from the origin to the center of sphereOne.

sphereOne = sphere(radius=3.5, pos=vector(9,0,8), color=color.magenta)
position = vector(0,0,0)
arrowTwo = arrow(pos=position, axis=(sphereOne.pos - position), color = color.cyan)

Note: the position vector is used here because arrowTwo.pos cannot be called yet. The arrow has yet to be created in line 3.

Difficult

Create a two spheres and a red curve. Make the first sphere have a radius of 2 and locate it at (-4,-3,8). Make the second sphere have a radius of 1, and locate it at (6,11,0). Start the curve at the origin, then bring it to the first sphere, and then the second sphere.

sphereOne = sphere(radius = 2, pos=vector(-4,-3,8))
sphereTwo = sphere(radius = 1, pos=vector(6,11,0))
curveOne = curve(color=color.red, pos = (0,0,0))
curveOne.append(sphereOne.pos)
curveOne.append(sphereTwo.pos)

Connectedness

This topic is important because objects are the primary way to visually make sense of data managed by the program. Understanding this process will allow the user to implement physics concepts such as moving charges and electric fields into computer code form.

See Also

VPython

VPython Basics

VPython Functions

References

http://vpython.org/

http://vpython.org/contents/docs/index.html

http://vpython.org/contents/docs/color.html

http://vpython.org/contents/docs/primitives.html

http://guigui.developpez.com/cours/python/vpython/en/?page=object