Problem with vpython axis, only accepts a vector such as (1,0,0) ie one axis, not something like (2,2,1)

Question

Python 3.7.4 with Vpython 7.5.1. When not a 'simple' vector such as (1,0,0) it displays the scene without the arrow or does not display at all. I want to display a triangle and show an arrow depicting the normal from the surface. The program displays the coordinate axes with arrows and a sphere. When I try any values in the norm arrow the display sometimes fails, and sometimes does display but not with the norm arrow. It works if I put in vectors (1,0,0) or (0,1,0) etc, but not for eg (2,2,0) Tried everything I can think of for several days

###################################
                                 ##
from vpython import *            ##
import numpy as np               ##
                                 ##
###################################

# Python 3.7.4 with Vpython 7.5.1

#=====================================================================================================##
                                                                                                      ##
def axisArrows():                                                                                     ##
#                                                                                                     ##
    al = 10.0 #arrow length                                                                           ##
    Xarrow = arrow(pos=vector(0,0,0), shaftwidth=.2,length=al, axis=vector(1,0,0), color=color.red)   ##
    Yarrow = arrow(pos=vector(0,0,0), shaftwidth=.2,length=al, axis=vector(0,1,0), color=color.green) ##
    Zarrow = arrow(pos=vector(0,0,0), shaftwidth=.2,length=al, axis=vector(0,0,1), color=color.blue)  ##
    sphere(radius=0.5)                                                                                ##
    return                                                                                            ##
#=====================================================================================================##

# Scene
# =====
canvas(title='Triangles', background=vector(.8,.9,.05), x=350, y=0, width=2000, height=1100)

# AxisArrows
# ==========
axisArrows()

# Triangle Coords
# ===============
va=vector(-5,7,-5)
vb=vector(7,-3,2)
vc=vector(5,7,0)
vd=vector(5,7,0)

# Convert coords to numpty array
# ==============================
p1 = np.array([va.x,va.y,va.z])
p2 = np.array([vb.x,vb.y,vb.z])
p3 = np.array([vc.x,vc.y,vc.z])

# Calc average and normal  (vn, normal to triangles plane)
# =======================
#
    # normal (a vector)
N  = np.cross(p2-p1, p3-p1)
print("p1= ", p1, "N = ", N)
N=N/50
vn = vector(N[0],N[1],N[2])  # vector for normal arrow
print("    vn= ", vn)
    # average (a vector)
average = (va+vb+vc+vd)/4
print("   Average = ", average)

sphere(radius=.5,pos=average)     #                           ******************
    # Display the norm arrow                                  ******************
N_arrow = arrow(pos=vector(average), shaftwidth=.2, length=4, axis=vector(0,0,2), color=color.blue)  
#                                                             ******************
#                                                             ******************

#                                                             vector(0,0,2) shows, vector(0,2,2) doesn't
# Create a triangle (quad with two vertices the same)         most value don't work
# =================
a = vertex(pos=va,color=color.red)
b = vertex(pos=vb,color=color.green)
c = vertex(pos=vc,color=color.blue)
d = vertex(pos=vd,color=color.white)

tr1 = quad(v0=a,v1=b,v2=c,v3=d)

I don't have an answer yet, but I discovered that if I comment out the Yarrow statement the program works properly. To be continued. — user1114907
– user1114907, Commented Feb 4, 2020 at 21:21

user1114907 · Accepted Answer · 2020-02-05 02:20:13Z

0

Ok, it's definitely a VPython bug. VERY oddly, it doesn't show up in GlowScript VPython, despite the fact that VPython 7 uses the same GlowScript graphics library. Boiling it down, the second of the following two statements to be executed produces a deformed arrow:

arrow(pos=vec(1,0,0), axis=vector(1,1,0), color=color.red) arrow(pos=vector(0,0,0), axis=vector(-1,1,0), color=color.green)

And if you make the first arrow statement have axis=vec(0,1,0), the second arrow doesn't even appear, as you saw. At the moment I (1) have no idea what's wrong and (2) it's very strange that this bug has not previously been reported.

I'll comment that for VPython questions it's better to post to the VPython forum, where there are many more VPython users who will see your question than if you post to stackoverflow:

https://groups.google.com/forum/?fromgroups&hl=en#!forum/vpython-users

answered Feb 5, 2020 at 2:20

user1114907

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user1114907 Over a year ago

This was fixed long ago.

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