[Visualpython-users] Re: VPython arrow object

[Bruce S. <bas...@un...>]

Currently when an arrow head length is greater than half the total length 
of the arrow, the head length (and shaft and head widths) are scaled down 
proportionally, preserving the correct total length but shrinking the 
lateral dimensions so that the arrow still looks like a reasonable arrow.

Two users have asked for an option on the arrow object to not scale down 
the widths of the arrow shaft and arrow head when the head size is larger 
than half the total length of the arrow. A possible interpretation of this 
request is that when the head length is greater than half the total length, 
the head length should be made to be half the total length, and the lateral 
dimensions not be affected. In the extreme case of arrows whose total 
length is very small compared to the widths, the object will look like a 
box with a plane attached at one end. For some purposes this may be 
preferable to the shrinking currently done by Visual.

I append a little program which displays standard arrows (cyan) and 
new-style arrows (red). The new-style arrows are mocked up by using box and 
convex. The shortest new-style arrow looks a bit like a thumbtack. If 
people think this is a good option to have, it would be possible to add a 
"fixedwidth" attribute to arrow which would give this effect if set to true 
(1).

Suggestions are also welcome for other possible rules for the fixedwidth 
case. Note that there does have to be a rule for shortening the head length 
when necessary, because it is not acceptable for the head length to be 
longer than the total length of the arrow.

Bruce Sherwood

from visual import *

# Display standard cyan arrow's, whose shaft and head widths shrink
# for short lengths, next to proposed red arrows with fixed widths.

def newarrow(pos=(0,0,0), axis=(1,0,0), color=color.white,
                shaftwidth=0.1, headwidth=2.*0.1, headlength=3.*0.1):
     pos = vector(pos)
     axis = vector(axis)
     L = mag(axis)
     sl = L-headlength
     hl = headlength
     if headlength > 0.5*L:
         sl = hl = 0.5*L
     fr = frame(pos=pos)
     box (frame=fr, pos=vector(sl/2.,0,0),
         size=(sl, shaftwidth, shaftwidth), color=color)
     p = vector(L,0,0)
     dx = -hl
     dy = headwidth/2.
     convex(frame=fr, pos=[p, p+vector(dx,dy,dy),
                 p+vector(dx,dy,-dy), p+vector(dx,-dy,-dy),
                 p+vector(dx,-dy,dy)], color=color)
     fr.axis = axis

for y in arange(1, 10, 1):
     newarrow(pos=(0,y-5,0), axis=(y/4.,0,0), headlength=1, headwidth=1,
              shaftwidth=0.4, color=color.red)
     arrow(pos=(-2.6,y-5,0), axis=(y/4.,0,0), headlength=1, headwidth=1,
              shaftwidth=0.4, color=color.cyan)