VPython is a programming environment that enables even novices to write programs that produce navigable real-time 3D animations. One to two hours of carefully crafted instruction is sufficient to bring novice students to the point of being able to do serious computer modeling (http://www.matterandinteractions.org). VPython is based on the Python programming language (http://python.org) which has a large user community. Like Python, VPython is open source freeware available for Windows, Linux, and Macintosh (http://vpython.org). Workshop participants will learn to write 3D programs

Chabay, R.

Sherwood, B.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2010-10612-3Colloquia: MPTL14IL NUOVO CIMENTO Vol. 33 C, N. 3 Maggio-Giugno 2010VPython: 3D programming for ordinary mortalsB. Sherwood and R. ChabayNorth Carolina State University - Raleigh, NC, USA(ricevuto il 30 Novembre 2009; pubblicato online il 20 Luglio 2010)Summary. — VPython is a programming environment that enables even novices towrite programs that produce navigable real-time 3D animations. One to two hours ofcarefully crafted instruction is sufficient to bring novice students to the point of beingable to do serious computer modeling (http://www.matterandinteractions.org).VPython is based on the Python programming language (http://python.org)which has a large user community. Like Python, VPython is open source freewareavailable for Windows, Linux, and Macintosh (http://vpython.org). Workshopparticipants will learn to write 3D programs.PACS 01.50.-i – Educational aids.PACS 01.50.F- – Audio and visual aids.1. – Python and VPythonVPython is a programming environment that enables even novices to write programsthat produce navigable real-time 3D animations. The new version 5 supports trans-parency, local as well as distant lights, and material displays such as wood or marble onall platforms, and for the first time runs as a native application on the Mac. Students inintroductory physics courses use VPython to write programs to model physical systemsand to visualize electric and magnetic fields (http://www.matterandinteractions.org;Chabay 2007; Chabay 2008). Educators use VPython to create lecture-demonstrationprograms. Researchers use VPython to model systems and to visualize data in 3D.One to two hours of carefully crafted instruction is sufficient to bring novice studentsto the point of being able to do serious computer modeling. VPython is based on thePython programming language (http://python.org) which has a large user community.Like Python, VPython is open source freeware available for Windows, Linux, and Mac-intosh (http://vpython.org). A set of VPython computational modeling activities forthe introductory physics course taken by engineering and science students is availableat http://www.compadre.org/psrc/items/detail.cfm?ID=5692. Figure 1 shows anexample of a VPython program that models the 3D motion of a mass hanging from aspring, written by a student in an introductory mechanics course.c© Societa` Italiana di Fisica 5960 B. SHERWOOD and R. CHABAYFig. 1. – A student VPython program written in an introductory mechanics course.Python is a clean language in which it is easy to do simple things right from the start,with little excess overhead. For example, the complete Python version of the classic“Hello World” program in Python looks like this, with no extraneous distracters:print “Hello World”Nor is there the distraction of extraneous setup or make procedures. After installingPython, one can invoke the built-in IDLE integrated development environment, type theone-line program shown above, and press F5 to see it run. The simplicity and immediacyof this environment has led to some introductory programming courses using Python asa first language.While it is very easy to do simple things in Python, it also supports sophisticatedobject-oriented programming, and there is an unusually gentle slope leading to creatingand using classes. There is a sizable community using Python for scientific program-ming (http://scipy.org), and a significant fraction of Google programming is done inPython.VPython is the combination of Python plus an extension module called “visual”.VPython builds on the Python tradition of cleanliness and simplicity by making it pos-sible to do simple things in 3D very easily. To display a sphere on the screen, all thatone needs to do is to invoke the visual module and create a sphere:from visual import *sphere()Enter this text into an IDLE edit window and press F5. You will see a 3D sphere,and using the mouse you can rotate the camera around the scene and zoom in and out.There are many useful defaults built into VPython. The simple statement “sphere()”triggers the creation of a graphics window containing an OpenGL context, and the state-ment produces by default a white sphere centered at the origin, of radius 1, with the“camera” automatically positioned to look toward the origin from a distance which makesthe sphere fill the window. Defaults can be overridden. For example, the following state-ment creates an orange sphere with the appearance of wood, with radius 0.5 and 30%VPYTHON: 3D PROGRAMMING FOR ORDINARY MORTALS 61opacity, with its center located at x=2, y=−1, z=1:sphere(pos=(2,-1,1), color=color.orange, radius=0.5, opacity=0.3,material=materials.wood)The default autoscaling means that a program showing the orbit of the Earth aroundthe Sun, with a scale of 1e11m, and a program showing alpha scattering off a gold nucleus(Rutherford scattering) with a scale of 1e-13m both make displays in which the sceneautomatically fills the graphics window.The inventory of graphics objects currently supported by VPython includes sphere,arrow, box, cone, convex, curve (an array of connected points), cylinder, ellipsoid, faces(an array of triangles), helix, label (to display text), points (an array of disconnectedpoints), pyramid, ring, and frame (for grouping objects together). One can create distantand local lights. There are statements for creating autoscaling graphs, controls (buttons,toggles, sliders, menus), and a platform-independent file dialog box. VPython supportshandling of mouse and keyboard events. A single statement suffices to make the 3Dscene truly three-dimensional, to be viewed with red-cyan glasses or, with two projectorsaimed at a metallic screen, with polarized 3D glasses.Of great importance for scientific work is support for vectors and vector operationssuch as sum, difference, dot product, cross product, magnitude, etc. Students writ-ing VPython programs to model physical systems such as gravitational orbits writecoordinate-free vector statements to calculate forces, update momenta, and update po-sitions. This is the first environment within which students have had an opportunityto use and experience vectors as powerful unitary entities as opposed to mere sines andcosines.2. – AnimationsHere is a complete program that calculates and displays the orbit of the Earth orbitingthe Sun (the radii of Sun and Earth have been exaggerated for display purposes):from visual import *G = 6.7e-11Sun = sphere(pos=(0,0,0), color=color.yellow, radius=2e10)Earth = sphere(pos=(1.5e11,0,0), color=color.cyan, radius=6e9)Sun.m = 2e30Earth.m = 6e24Earth.p = Earth.m*vector(0,3e4,0)dt = 100while True:r = Earth.pos - Sun.posrmag = mag(r)rhat = r/rmagFmag = G*Sun.m*Earth.m/rmag**2Fnet = -Fmag*rhatEarth.p = Earth.p + Fnet*dtEarth.pos = Earth.pos + (Earth.p/Earth.m)*dtAfter entering these statements into an IDLE editor window, press F5 and an ani-mated orbit appears with the Earth circling the Sun. With the mouse one can view thescene from different angles and different distances.62 B. SHERWOOD and R. CHABAYThe program starts by importing all of the visual capabilities, including the standardmath library. The Sun and Earth are represented by out-size spheres so as to be visiblein the nearly empty Solar System, and they are positioned at an appropriate distanceapart. The Sun’s mass is specified, as are the Earth’s mass and initial momentum. Atime step for the numerical integration is chosen to be 100 seconds. The statement “whileTrue:” means that the indented code under the while statement will run forever, untilthe user closes the graphics window.The statement “r = Earth.pos - Sun.pos” is a vector subtraction of the current positionof the Earth minus the position of the Sun, so that r represents a vector pointing fromthe Sun to the Earth. The magnitude of r and the corresponding unit vector (“rhat”)are calculated and used to calculate the magnitude of the gravitational force and thevector force, which is used to update the momentum. Then the position of the Earth isupdated.Pedagogically it is beneficial that a student who writes such a program is working ina coordinate-free way (except for setting initial conditions), thinking of and using vectorsas powerful objects rather than as mere sines and cosines. It appears that students gain amuch more sophisticated concept of “vector” as a result of such computational modelingexperiences.Simple additions to this program include appending points to a curve object to displaythe trajectory, and updating arrow objects to display force and momentum vectors.3. – How does it work?It is surprising that in the while loop of the orbit program shown above there seemto be no graphics output statements that would produce an animation, only computa-tional statements to predict the motion. The key is that every time through the loopthe pos (position) attribute of the sphere object representing the Earth is updated.Approximately 30 times per second, these computations are interrupted by a separate“rendering” thread which creates a new image based on the current attributes of theexisting objects. The rendering thread clears a region of memory and for each existingobject (in this case, two spheres) uses OpenGL to add that object to the image, usingthe current attributes of those objects. The effect is that each time the rendering threadcreates an image of the Sun and Earth, the Earth is in a new position, and as long as therendering thread can run about 30 times every second, the animation will look smoothto the human viewer. When completely rendered, the scene is handed to the graphicscard to display the image on the screen, and the rendering thread returns control to thecomputational thread. The rendering process also takes into account mouse moves forpositioning the camera, which makes the scene navigable.The effect is to make navigable 3D animations a side effect of computations, therebymaking feasible the creation of sophisticated displays by programming novices. This wasa key invention of David Scherer in the spring of 2000, when he created VPython whilean undergraduate student at Carnegie Mellon University.∗ ∗ ∗These developments were supported in part by the National Science Foundationthrough Grant Nos. DUE-0320608, DUE-0237132, and DUE-0618504.VPYTHON: 3D PROGRAMMING FOR ORDINARY MORTALS 63REFERENCESChabay R and Sherwood B (2007) Matter & Interactions, John Wiley & Sons, HobokenNJ, USAChabay R and Sherwood B (2008) Computational physics in the introductory calculus-based course, American Journal of Physics, 76(4&5), 307-313

VPython: 3D programming for ordinary mortals

http://eprints.bice.rm.cnr.it/16866/1/ncc9618.pdf

VPython: 3D programming for ordinary mortals

Abstract

Similar works

Full text

Available Versions

Scientific Open-access Literature Archive and Repository