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Index › Programming Questions & Help › OpenGL and 3D Libraries › OCD: arc only arcs 180 degrees
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OCD: arc only arcs 180 degrees? (Read 599 times)
Dan_Olner
OCD: arc only arcs 180 degrees?
Dec 2nd, 2008, 10:33am  
Hello

I'm using the awesome OCD library in:

http://www.personal.leeds.ac.uk/~gy06do/processing/ces_function/

Question: 'circle' allows the user to circle a full 2PI, and OCD kindly resets the value to 0 and carries on round.

For 'arc', however (holding down space and dragging, in the above example), the user can only go 180 degrees - 90 degrees up or down.

Now, clearly, I could find some workaround for this. I was just wondering if anyone knew a reason why this has to be so? I've been scratching my head and can't think of one. 'circle' just moves position relative to the y axis; 'arc' does the same for the x axis. (I use that fact to compensate in the above so the text faces the viewer.)

Is this something a later version of OCD might change? That is, allow full 2PI arcing?
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julapy
Re: OCD: arc only arcs 180 degrees?
Reply #1 - Dec 8th, 2008, 1:54pm  
think this has something to do with polar coordinates, with two angles ( theta and phi ) you can move to any point on a sphere, with a 2PI rotation around the Y axis and a PI rotation around the Z axis.  when you think about it, you only need to go up and down 180 degrees if you are going around 360 degrees, otherwise you'd be doubling up.

there are other ways of rotating around a sphere, such as quarternions... big topic... just getting my head around it myself. will post if i figure it out.
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K. Damkjer
Re: OCD: arc only arcs 180 degrees?
Reply #2 - Dec 10th, 2008, 7:18pm  
This issue has been raised before. The limitation is imposed because OCD represents rotations with Euler angles. As julapy mentions, quaternions would allow for the type of behavior you describe, but would require a rewriting of the OCD library.
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