Question

OpenGL - Rotating around a point

Asked by: SuperMario

Hi everyone,
I have a cube defined in GL_QUADS that I want to rotate at a distance of nRadius from the origin (0, 0, 0) about the y-axis.

I don't want it to rotate around it's local y-axis, I actually want it to "orbit" the origin at a distance of nRadius units.

How can I accomplish this? I have tried using glRotatef but I must be missing something!

Thanks so much,
-Dan

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Asked On
2002-11-03 at 19:45:27ID20389434
Tags

around

,

opengl

,

point

,

rotate

Topic

3D Game Programming

Participating Experts
4
Points
100
Comments
8

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Answers

 

by: SuperMarioPosted on 2002-11-03 at 20:01:13ID: 7404306

Fixed it using trig... Never thought I'd use that.

Is there a faster way to do it? I just used translate with sines and cosines, but it's probably a performance hit.

 

by: fl0ydPosted on 2002-11-04 at 00:02:34ID: 7404659

There are faster ways to calculate sin/cos. The easiest would be a lookup-table with a fixed number of values. Use this together with linear interpolation and you should be fine. Also, if you are using msvc to write your code you can put

#pragma intrinsic( sin, cos )

in your source code to instruct the compiler to use built-in FPU-support for those functions. It's still somewhat slow. To speed things up even more you could use MMX/SSE-code to calculate sin/cos. Intel provides a library that uses this approach. Take a look at the "Approximate Math Library" here: http://cedar.intel.com/cgi-bin/ids.dll/content/content.jsp?cntKey=Generic%20Editorial::pentium4_stoner_mathlibs&cntType=IDS_EDITORIAL&catCode=0

.f

 

by: nomad516Posted on 2002-11-04 at 20:31:39ID: 7408838

you can call a glTranslate and then glRotate (the order matters though)

 

by: ozoPosted on 2002-11-04 at 23:51:34ID: 7409121

In what plane do you want it to orbit the origin?

 

by: SuperMarioPosted on 2002-11-05 at 00:27:46ID: 7409178

Again, I've already done it using translations (sines and cosines to determine the x and y components of the object's position vector at runtime)

but it's rotating in the ZY plane. :)

 

by: UncleSquirrelPosted on 2002-11-05 at 20:06:57ID: 7413033


Nomad516 is right: you probably don't want to bother doing your own trig in this case.  Take advantage of the fact that OpenGL has a very nice suite of transformation functions built in.

To rotate an object 45 degrees counter-clockwise in place around the Y axis, do the following:

glRotatef( 45.0f, 0.0f, 1.0f, 0.0f );

...where (0, 1, 0) is the +Y vector about which we are rotating.

However, this will rotate the object in place.  If you want it to orbit this point by <degrees> at a distance of <nRadius>, you should do:

glRotatef( degrees, 0.0f, 1.0f, 0.0f );
glTranslatef( nRadius, 0.0f, 0.0f );

This (1) Rotates the world, then (2) moves out <nRadius> units along the new, rotated, "relative" x-axis.

Note that, as nomad516 also points out, the order of these calls DOES matter; if you were to reverse them (i.e. translate THEN rotate), you would have a box rotating in place but in a fixed, remote location.

Lastly, make sure you preserve your transformation matricies so these rotations and translations don't keep adding up on top of each other.  You can do this by using glPushMatrix() to store the previous transformation state before you jack with it and glPopMatrix() to restore it when you're finished.  The final code should therefore look more like this:

glPushMatrix();
glRotatef( degrees, 0.0f, 1.0f, 0.0f );
glTranslatef( nRadius, 0.0f, 0.0f );

// Draw box here, axis-aligned and origin-centered
...

glPopMatrix();

Hope this helps,
-sq

 

by: SuperMarioPosted on 2002-11-06 at 15:40:15ID: 7417193

Sorry Nomad, I knew you got this first, but UncleSquirrel's answer was helpful plus the fact that he gave me a trick to help performance.

Thanks a lot sq! Much appreciated.

-Dan

 

by: nomad516Posted on 2002-11-09 at 06:53:30ID: 7428410

nah, that's ok, unclesquirrel made the better effort...

congratulations unclesquirrel!...=)

20120131-EE-VQP-002

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