A strange outcome !

[陳冠榮]

Hi:

I use the following code to show the Biot-Savart law and the B-field of a long-straight current-carrying wire (thanks to Ruth Chabay).

https://www.glowscript.org/#/user/cokojo/folder/MyPrograms/program/Bfield-long-wire/edit

It works correctly but the last drawn green arrow !

If I set the last rateval to 20, everything is fine (the left part of the picture), but if it is 200, something goes wrong (the right part of the picture).

I cannot figure out what's going on. Please help me.

[Bruce Sherwood]

Replace line 67, 

Bframe1, Bframe2, dtheta = compound(Barrows[0:80]), compound(Barrows[80:]), 0.5*pi/N

with this:

Bframe1 = compound(Barrows[0:80])

sleep(0.1)

Bframe2 = compound(Barrows[80:])

dtheta = 0.5*pi/N

You have uncovered a previously unknown problem with compound. The Bframe2 statement should of course not be executed before the Bframe1 statement has been fully processed, including freeing up the vertex objects used to create the Bframe1 compound, to be re-used in creating the Bframe2 object, but that's what I think is happening, and somehow sleeping for a short time fixes the problem.

A very minor point is that "display" (the old VPython 5 name) should be replaced with "canvas". 

Bruce

[陳冠榮]

Yes, the problem is solved !

Thanks a lot !

Bruce Sherwood 在 2021年10月15日 星期五上午4:10:56 [UTC+8] 的信中寫道：

[Bruce Sherwood]

After a lot of thrashing around, I made a very simple version of the program which fails:

GlowScript 3.1 VPython

scene.background = color.white

scene.range = .2

    

Barrows = []

r = 0.1

for theta in arange(0, 2*pi, pi/8):

    Barrows.append(arrow(pos=vec(0,-r*sin(theta),r*cos(theta)), color=color.green, axis=vec(0,0,0)))

for B in Barrows:

    B.axis = vec(0,0.02,0)

console.log('before Bframe1')

scene.pause()

Bframe1 = compound(Barrows[0:80])

console.log('after Bframe1')

Bframe1.pos = vec(0.1,0.03,0)

If one comments out the scene.pause() statement, and turns on debugging (shift-ctrl-j), you see the failure. Including scene.pause or sleep(0.1) makes the program work. I currently do not understand what's going on.

[Bruce Sherwood]

A further minor simplification is to replace Barrows[0:80] with Barrows.

Bruce

[Bruce Sherwood]

Your original program now works properly. Thanks for reporting the problem.

Bruce

[陳冠榮]

Thanks again.

 

CKJ

 

從 Windows 的郵件傳送

 

寄件者: Bruce Sherwood
傳送時間: 2021年10月19日 7:42
收件者: Glowscript Users
主旨: [glowscript-users] Re: A strange outcome !

 

Your original program now works properly. Thanks for reporting the problem.

 

Bruce

 

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