Difference between revisions of "LlRotBetween"
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if (aabb) | if (aabb) | ||
{ | { | ||
float ab = (a * b) / aabb; // normalized dotproduct of the arguments | float ab = (a * b) / aabb; // normalized dotproduct of the arguments (cosine) | ||
vector c = (a % b) / aabb; // normalized crossproduct of the arguments | vector c = (a % b) / aabb; // normalized crossproduct of the arguments | ||
float cc = c * c; // squared length of the crossproduct | float cc = c * c; // squared length of the normalized crossproduct (sine) | ||
if (cc) // test if the arguments are (anti)parallel | if (cc) // test if the arguments are (anti)parallel | ||
{ | { | ||
float s | float s; | ||
if (ab | if (ab > -0.707107) // test if the angle is smaller than 3/4 PI | ||
s = 1.0 + ab; // use the cosine to adjust the s-element | |||
else | |||
s | s = 1.0 - llSqrt(1.0 - cc); // use the sine to adjust the s-element | ||
else | |||
s - | |||
float m = llSqrt(cc + s * s); // the magnitude of the quaternion | float m = llSqrt(cc + s * s); // the magnitude of the quaternion | ||
return <c.x / m, c.y / m, c.z / m, s / m>; // return the normalized quaternion | return <c.x / m, c.y / m, c.z / m, s / m>; // return the normalized quaternion | ||
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return <1.0, 0.0, 0.0, 0.0>; // the arguments are anti-parallel | return <1.0, 0.0, 0.0, 0.0>; // the arguments are anti-parallel | ||
} | } | ||
// the arguments are too small or parallel, return zero rotation | return ZERO_ROTATION; // the arguments are too small or parallel, return zero rotation | ||
}//Written by Moon Metty, optimized by Strife Onizuka</lsl> | |||
}//Written by Moon Metty, | |||
===Bad Reference Implementation=== | ===Bad Reference Implementation=== | ||
Revision as of 08:30, 27 September 2012
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Summary
Function: rotation llRotBetween( vector start, vector end );| 21 | Function ID |
| 0.0 | Forced Delay |
| 10.0 | Energy |
Returns a rotation that is the shortest rotation between the direction start and the direction end
| • vector | start | |||
| • vector | end |
Specification
start and end are directions and are relative to the origin <0.0, 0.0, 0.0>. If you have coordinates relative to a different origin, subtract that origin from the input vectors.
Caveats
start * llRotBetween(start, end) == endis only true if start and end have the same magnitude and neither have a magnitude of zero (see #Useful Snippets for a workaround).- This of course is ignoring floating point precision errors.
- Rotations are from -PI to +PI around each axis.
Important Issues
~ All Issues ~ Search JIRA for related Bugs
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SCR-309 | A | llRotBetween has issues, give us something better |
Examples
<lsl>llRotBetween(<1.0, 0.0, 0.0>, <0.0, -1.0, 0.0>) // will return <0.00000, 0.00000, -0.70711, 0.70711> (which represents -90 degrees on the z axis)
llRotBetween(<0.0, 0.0, 0.0>, <0.0, -1.0, 0.0>) // will return <0.00000, 0.00000, 0.00000, 1.00000> (which represents a zero angle on all axis)
// because <0.0, 0.0, 0.0> does not convey a direction.</lsl>Useful Snippets
This function adjusts the magnitude of the quaternion so start * llRotBetween(start, end) == end is true as long as neither have a magnitude really close to zero. They do not have to have the same magnitude. (If either is too close to zero than this will return an unadjusted quaternion). While this is mathematically correct, it won't help with floating point rounding errors, so it's more accurate to say start * return ≈ end.
<lsl>rotation RotBetween(vector start, vector end) //adjusts quaternion magnitude so (start * return == end)
{//Authors note: I have never had a use for this but it's good to know how to do it if I did.
rotation rot = llRotBetween(start, end);
float d = start * start;
if(d)//is 'start' zero?
if((d = llPow(end * end / d, 0.25)))//is 'end' zero?
return <rot.x * d, rot.y * d, rot.z * d, rot.s * d>;
return rot;
}//Strife Onizuka</lsl>
Notes
Vectors that are near opposite each other in direction may lead to erroneous results. <lsl> // First Vector is due north second vector is ALMOST due south. rotation lRotation = llRotBetween( <0., 1., 0.>, <-0.001, -.1, 0.> ); llSay(0, lRotation ); // Provides a result of <1.00000, 0.00000, 0.00000, 0.00000>. </lsl>
See Also
Functions
| • | llAngleBetween |
Deep Notes
Replacement
Due to the annoying quirks of this function Moon Metty wrote a drop in replacement. - SCR-309. The maximum error is reported to be 2.7e-7 @ 2.2 radians. <lsl>rotation RotBetween(vector a, vector b) {
float aabb = llSqrt((a * a) * (b * b)); // product of the lengths of the arguments
if (aabb)
{
float ab = (a * b) / aabb; // normalized dotproduct of the arguments (cosine)
vector c = (a % b) / aabb; // normalized crossproduct of the arguments
float cc = c * c; // squared length of the normalized crossproduct (sine)
if (cc) // test if the arguments are (anti)parallel
{
float s;
if (ab > -0.707107) // test if the angle is smaller than 3/4 PI
s = 1.0 + ab; // use the cosine to adjust the s-element
else
s = 1.0 - llSqrt(1.0 - cc); // use the sine to adjust the s-element
float m = llSqrt(cc + s * s); // the magnitude of the quaternion
return <c.x / m, c.y / m, c.z / m, s / m>; // return the normalized quaternion
}
else if (ab < 0.0) // test if the angle is smaller than PI/2
return <1.0, 0.0, 0.0, 0.0>; // the arguments are anti-parallel
}
return ZERO_ROTATION; // the arguments are too small or parallel, return zero rotation
}//Written by Moon Metty, optimized by Strife Onizuka</lsl>
Bad Reference Implementation
<lsl>//Loosely based on SL source code, like SL's version, it's not very accurate. You want to use the above version. rotation llRotBetween(vector start, vector end) {
vector v1 = llVecNorm(start);
vector v2 = llVecNorm(end);
float dot = v1 * v2;
vector axis = v1 % v2;
if (dot < -0.9999999) {
// 180 degrees or there abouts
vector ortho_axis = llVecNorm(<1.f, 0.f, 0.f> - (sn * (sn.x / (sn * sn))));
if (ortho_axis)
return < ortho_axis.x, ortho_axis.y, ortho_axis.z, 0.f>;
return <0.0, 0.0, 1.0, 0.0>;
}
else if(dot > 0.9999999) {
//parallel
return ZERO_ROTATION;
}
dot = dot + 1.0;
float m = llPow((axis * axis) + (dot * dot), -0.5);
return <axis.x * m, axis.y * m, axis.z * m, dot * m>;
}</lsl>
All Issues
~ Search JIRA for related Issues
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SVC-4415 | A | llRotBetween sometimes gives erroneous results. | |
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SCR-309 | A | llRotBetween has issues, give us something better |
Source
Signature |
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function rotation llRotBetween( vector start, vector end ); |