Trying to rotate triangles

[Gilles Dubois]

Here is a small program building a triangle and trying to rotate it around the x axis

from vpython import *

A,B,C=vec(-1,0,0),vec(0,sqrt(3),0),vec(1,0,0)
VA,VB,VC=vertex(pos=A),vertex(pos=B),vertex(pos=C)
T=triangle(v0=VA,v1=VB,v2=VC)
print(B)
T.rotate(axis=vec(1,0,0),angle=pi/2, origin=vec(0,0,0)) #no error but does nothing
print(B) # confirmation B still on the same place

OK ! try something else :

from vpython import *

A, B, C = vec(-1, 0, 0), vec(0, sqrt(3), 0), vec(1, 0, 0)
VA, VB, VC = vertex(pos=A), vertex(pos=B), vertex(pos=C)
T = triangle(v0=VA, v1=VB, v2=VC)
print(VB.pos)
# T.rotate(axis=vec(1,0,0),angle=pi/2, origin=vec(0,0,0)) #no error but does nothing

# let's pack T into a compound with only one entry
T1 = compound([T])
# rotate the compound and watch
T1.rotate(axis=vec(1, 0, 0), angle=pi / 2, origin=vec(0, 0, 0)) # according rendering does the job
# but ... only if we comment the T.rotate line
# if we uncomment this line no action at all, does it mean that the system estimates that the first rotate was successful ?
# so we can guess that finally the job is done, let's check
print(VB.pos) # strangely B is still declared at the same place, although visually T1 lies in the xOz plane ???

 This is just the beginning of the story, more to come if somebody interested.

[Bruce Sherwood]

From the documentation for triangles:

The only way to rotate a triangle is to rotate each vertex.

Bruce 

l

[Gilles Dubois]

Actually to pack it into a compound is working too .

[Gilles Dubois]

Ok ! Since we can rotate vertices, let's do it. But we could expect that normals would be rotated the same. But no !

This small program proves that after rotating a vertex, the normal of this vertex is unchanged. Why ?

from vpython import *
i = vec(1, 0, 0)
j = vec(0, 1, 0)
k = vec(0, 0, 1)

# tracé de l'axe x'Ox (abscisses)
xmin = vec(-2, 0, 0)
xmax = vec(2, 0, 0)
axex = curve(pos=[xmin, xmax], color=color.blue) # segment de longueur 20

# tracé de l'axe y'Oy (ordonnées)
ymin = vec(0, -2, 0)
ymax = vec(0, 2, 0)
axey = curve(pos=[ymin, ymax], color=color.green) # même chose

# tracé de l'axe z'Oz (cotes)
zmin = vec(0, 0, -2)
zmax = vec(0, 0, 2)
axez = curve(pos=[zmin, zmax], color=color.yellow) # idem

scene.camera.rotate(angle=pi / 24, axis=k, origin=vec(0, 0, 0))
scene.range = 0.5
A=j
sphere(pos=A,radius=0.1)
VA=vertex(pos=A)
nar=arrow(axis=VA.normal,pos=A)
VA.rotate(axis=i,angle=pi/2,origin=i)
sphere(pos=VA.pos,radius=0.1)
nar2=arrow(axis=VA.normal,pos=VA.pos) #normal unchanged not modified by rotation.

[Bruce Sherwood]

Here is an example of rotating a triangle:

v1 = vertex(pos=vec(0,1,0), normal=vec(0,0,1), color=color.red)
v2 = vertex(pos=vec(-1,0,0), normal=vec(0,0,1), color=color.green)
v3 = vertex(pos=vec(1,0,0), normal=vec(0,0,1), color=color.blue)
t = triangle(vs=[v1,v2, v3])
scene.autoscale = False
distant_light(direction=vec(1,0,0), color=color.white)
scene.pause()
v1.normal = vec(1,0,0)
v2.pos=vec(0,0,1)
v2.normal =v1.normal
v3.pos=vec(0,0,-1)
v3.normal = v1.normal

Bruce

[Gilles Dubois]

Thank you Bruce. However I begin to understand how things work and I can manage almost all situations

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[Bruce Sherwood]

Another approach:

v1 = vertex(pos=vec(0,1,0), normal=vec(0,0,1), color=color.red)
v2 = vertex(pos=vec(-1,0,0), normal=vec(0,0,1), color=color.green)
v3 = vertex(pos=vec(1,0,0), normal=vec(0,0,1), color=color.blue)
t = triangle(vs=[v1,v2, v3])
scene.autoscale = False
distant_light(direction=vec(1,0,0), color=color.white)
scene.pause()

v2.rotate(angle=pi/2, axis=vec(0,1,0))
v3.rotate(angle=pi/2, axis=vec(0,1,0))