Lerping a rotation

Math and Physics Programming

Started by rgibson11 November 11, 2010 12:04 PM

10 comments, last by rgibson11 13 years, 5 months ago

130

Author

November 11, 2010 12:04 PM

I have a small problem.
My multiplayer game currently has a packet being delivered every 100ms.
The packet contains position and a heading angle which is -pi to pi.

I am actually rendering 100ms behind real server time in order to be able to lerp the positions (which work fine). I tried to lerp the heading angle this way and it works but theres a sudden snapback at the end of every rotation. I just cant figure out what it could be

//lerp:
//return a + (b - a) * t;

float h1 = (*ito).headings.front(); //Heading 100ms agofloat h2 = (*ito).headings.back(); //Current server headingfloat currh = 0.0f; //Lerped headingcurrh = Math::lerp(h1, h2, zlerp);//zlerp = elapsed time since last update / 100.0 (ms) This works perfect for pos

Anyone could know what im doing wrong?

snake5

1,589

November 11, 2010 12:33 PM

You can't lerp between angles without problems. You can either use SLERP or normalize the angles, find the fastest route from one angle to another (remember - angle is equal to the remainder of a division between that angle and the "full circle angle" in the same units, 2 pi radians or 360 degrees), modify the 2nd angle to make it fit the fastest route and only then do a lerp between those angles.

http://en.wikipedia.org/wiki/Modulo_operation

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skytiger

294

November 11, 2010 01:29 PM

I calculate the shortest difference between two angles like this:

d = b - a;a = abs(d);s = sign(d);if (a > pi) { d = s * (2 * pi - a) * -1;}//if

now you can:

lerp = a + d * i;

you can map back into (-pi,pi) range using the same code again:

a = abs(d);s = sign(d);if (a > pi) { d = s * (2 * pi - a) * -1;}//if

this uses less cycles than modulo

rgibson11

130

Author

November 11, 2010 01:40 PM

Thanks for reply.
That seems to help but my maths isnt that great?
Do you have a quick example of what you said? or should I go ahead learning slerp.
Also why is your user rating so low? :S

-edit thanks skytiger. I got an example now. cheers :D

bubu LV

1,436

November 11, 2010 02:00 PM

Quote:Original post by snake5
You can either use SLERP or normalize the angles

Isn't slerp only for 3d rotation, not 1d angle?

Emergent

982

November 11, 2010 02:45 PM

Quote:Original post by bubu LV
Quote:Original post by snake5
You can either use SLERP or normalize the angles

Isn't slerp only for 3d rotation, not 1d angle?

SLERP gets you between points on a sphere.

What do points on a sphere have to do with rotations?

In 3d, you can represent a rotation as a unit quaternion; the unit quaternions lie on a sphere in four dimensions, so you can use SLERP to get between them.

In 2d, you can represent a rotation as a unit vector. The unit vectors lie on a sphere in 2 dimensions (i.e., a circle), so you can use SLERP to get between them.

In both cases, this does what you want because the notion of distance on the sphere agrees with what you'd want to call the distance between two rotations.

rgibson11

130

Author

November 11, 2010 03:46 PM

Quote:Original post by skytiger
I calculate the shortest difference between two angles like this:

*** Source Snippet Removed ***

now you can:

*** Source Snippet Removed ***

you can map back into (-pi,pi) range using the same code again:

*** Source Snippet Removed ***

this uses less cycles than modulo

What is sign() ? Is that a function you made yourself? cant find it anywhere in standard libraries

rubicondev

296

November 11, 2010 05:39 PM

try sgn()

------------------------------Great Little War Game

rgibson11

130

Author

November 11, 2010 07:52 PM

Ok I tried using slerp and it seems to be working very well.
Except for one slight error.
The rotations are smooth as hell except sometimes it stops rotation and quickly rotates the other way.

Please can someone who is familiar with slerps check this code. I would be very greatful. Im sure its some simple error im making

float h1 = (*ito).headings.front(); //100ms behind server timefloat h2 = (*ito).headings.back(); //Current server timefloat hcurr = 0.0f;Quaternion hh1, hh2, ff;float uu, uu2, uu3;hh1.fromHeadPitchRoll(h1, 0.0f, 0.0f);hh2.fromHeadPitchRoll(h2, 0.0f, 0.0f);ff = Quaternion::slerp(hh1, hh2, zlerp); //zlerp = time since last update / 100.0ff.toHeadPitchRoll(uu, uu2, uu3); //uu should be the slerped heading angle

the actual slerp function is from a well known maths library so im pretty sure its correct

Zakwayda

2,389

November 12, 2010 01:34 AM

Quote:Original post by rgibson11
the actual slerp function is from a well known maths library so im pretty sure its correct

What math library is it? Does the SLERP function in question correct for quaternion aliasing?

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