User talk:Aleric Inglewood
About Coordinate Systems and Rotations
The four coordinate systems
There are four coordinate systems that are related to LSL programming:
- World coordinates
- Region coordinates
- Object coordinates (root prim coordinates)
- Child prim coordinates (coordinates relative to the prim that the script is in, or refers to by link number)
The World coordinates refer to the map, and allow to include the sim in the coordinates, or refer to void water.
Region coordinates are relative to a given sim. The origin is in the South/West corner at height 0. The North/East corner then is 256, 256 and a Z coordinate for the height up to 4096 meter (on opensim you can go even higher).
Object coordinates are relative to the root prim. Hence, if the object is moved or rotated then the orientation of a child prim, when given in object coordinates, doesn't change. In LSL "local position" and "local rotation" refer to this coordinate system. "local" means relative to the root prim.
Prim Coordinates are relative to a given prim. If this prim is the root prim then the Prim Coordinates are the same as the Object Coordinates. For example, if a child prim is a cube with a size of 1,1,1 and one red surface where the center of that surface is at 1,0,0 then it will still be at 1,0,0 no matter how you move or rotate that child prim (relative to the other linked prims).
Positions
A different position of the origin of a coordinate system is easy to understand: You can think of positions as vectors that start in the origin of the coordinate system that they are given in and end in the point that they refer to. While the length of the vector is independent of the rotation of the coordinate system, the three coordinates are not; but a mental picture of an arrow doesn't have little numbers for three coordinates, so that picture works independent of the rotation too.
Since the rotation of the World Coordinate system and the Region Coordinate system is the same (X, Y and Z axis are parallel of both), and since World Coordinates aren't used in many LSL functions to begin with, we will ignore World Coordinates for now and only refer to Region Coordinates, or say "global" when we mean Region Coordinates.
Rotations
An LSL rotation internally stores a vector that is the axis around which to rotate and the angle of the rotation around that axis. Let V = <u, v, w> be the normalized vector (that is, with length 1) around which we rotate and let a be the angle around which we have to rotate. Then the LSL rotation is a quaternion stored as r = <x, y, z, s> = < V * sin(a/2), cos(a/2) >. Thus, r.x = x * sin(a/2), and r.s = cos(a/2) etc. Note that the quaternion is also normalized. Also note that there is a duality here because inverting V (making it point the opposite way) and inverting the angle gives the same rotation; r and -r have different values but are the same rotation. Also note that if you don't rotate at all (a == 0) then it doesn't matter what axis V you pick, which is apparent because V drops out since sin(0) = 0. The quaternion <0, 0, 0, 1> is the ZERO_ROTATION quaternion.
The point of this technical story is to show that for an LSL rotation to make sense in terms of orientation, you need to be able to express a vector in three coordinates (u, v, w above): the axis around which we rotate is expressed relative to the X-, Y- and Z-axes of the coordinate system. Hence, it is the orientation of the X-, Y- and Z-axes that defines the meaning of a rotation in LSL.
In terms of a mental picture the origin with the (orientation of the) three axis, the red X-axis, the green Y-axis and the blue Z-axis is all the reference we need, combined with a vector for position or a quaternion for rotation.
When you edit an object, the viewer shows either 'World' or 'Local' axes, but really the 'World' axes show the wrong origin, shifted to an averaged center of the object, because if the origin was drawn at (0, 0, 0) you'd most likely not see it. The 'Local' ruler shows the correct coordinate system for the selected prim as its Prim Coordinate System. Selecting the root prim with 'Local' ruler on then shows the Object Coordinate System.
The Dimension Of Rotations
As said before, given some coordinate system, any point in space can be represented with a vector. Obviously space is three dimensional, and thus vectors exists of three real values: one needs three distinct floating point numbers, the x coordinate, the y coordinate and the z coordinate to uniquely identify a position in a given coordinate system.
However, if one limits oneself to only normalized vectors, vectors with a length of one, then those represent all points on the surface of a sphere with radius 1. A surface is obviously two dimensional, so it should be possible to uniquely identify any point on the surface of such a sphere with only two floating point numbers.
One might think that selecting just two coordinates of the three of the vector will suffice because the third is fixed by the length requirement, but that only works for half spheres; for example, if x and y are known then z can still be either plus the square root of x squared plus y squared, or minus that value.
Instead, a better choice would be the spherical coordinate system and express the unit vectors with the two the angular coordinates φ (phi) and θ (theta), where φ is the angle between the vector and the positive Z axis, and θ the angle that the projection of the vector on to the X,Y plane makes with the positive X axis. In other words, starting with a unit vector along the positive Z-axis, one can obtain the required point on the surface of the sphere by first rotating this vector around the Y axis (towards the positive X axis) by an angle of φ and then rotating the result around the Z-axis by an angle of θ. Note that in both cases the rotations are counter-clockwise when one looks at it from the positive side of the axis that one rotates around (towards the origin).
What we just did was expressing a unit vector in terms of two rotations: one around the Z axis and one around the Y axis; and indeed every rotation can be expressed as two rotations around those two axis (or really, any two arbitrary axes, as long as they are independent (not parallel)).
It is therefore possible to represent unit vectors (aka, a direction) with a rotation, as both are two dimensional. The mental picture here is that any unit vector can be expressed as some fixed unit vector (for example the one pointing along the positive Z axis, but any other would do as well) and the rotation needed to turn that 'starting vector' into the unit vector that one wants to represent. The other way around almost works as well, with the exception of the starting vector itself, as that can be expressed by any arbitrary rotation around itself, so that information was lost and it is not possible to know which of those rotations was used. In fact, any vector close to the starting vector would give inaccurate results in the light of floating point round off errors and is not a good way to represent a rotation. Of course, the two dimensional vector (θ, φ) would be excellent to represent both: the unit vector, as well as any rotation; but LSL stores vectors in x, y, z coordinates, not in spherical coordinates.
Converting Between Coordinate Systems
Consider a child prim of an object that is not the root prim and express its position with the vector pos_oc, and its orientation with the rotation rot_oc, both relative to the Object Coordinates System.
A script inside that prim can easily obtain those values:
vector pos_oc = llGetLocalPos(); rotation rot_oc = llGetLocalRot();
The postfix '_oc' stands for Object Coordinates and is used to make clear relative to which coordinate system the x, y and z values of both variables are. Remember that also the rotation contains a vector (the axis around which we rotate), so from now on we'll talk about 'coordinates' relative to a coordinate system for both, the position as well as the rotation, where the coordinates refer to the x, y and z components of the position and the rotation. For example, the first sentence of this paragraph will read "...express its position with the vector pos_oc, and its orientation with the rotation rot_oc, both in Object Coordinates."
Note that one has to be careful here, because a script in the root prim of the object will not obtain Object Coordinates but instead Region Coordinates! The reason is that the 'Local' functions return coordinates relative to the 'parent' coordinate system, where the parent of a non-root prim is the root prim, but the parent of the root prim is the region (or avatar in case of an attachment). The position and rotation of the root prim in Object Coordinates is trivial (ZERO_VECTOR and ZERO_ROTATION respectively).
Back to the script in the child prim. It can also easily obtain its position and rotation in Region Coordinates:
vector pos_rc = llGetPos(); rotation rot_rc = llGetRot();
Both sets, (pos_oc, rot_oc) and (pos_rc, rot_rc) refer to the same thing, so it should be possible to convert them into each other.
One can do this by looking at the orientation (position and rotation) of one coordinate system relative to the other and expressed in the coordinates of that other. The position and rotation of the Object Coordinate System relative to the Region Coordinate System, and in Region Coordinates, could be obtained as follows:
vector oc_pos_rc = llGetRootPosition(); rotation oc_rot_rc = llGetRootRotation();
As indicated by the '_rc' postfix, these are again in Region Coordinates.
Having the relative orientation of the Object Coordinate System in Region Coordinates, it is possible to convert Object Coordinates into Region Coordinates. More abstractly put, if you have two coordinate systems A and B, and you have their relative orientation expressed in A then you can convert orientations expressed in B into A.
The formula is as follows:
vector pos_A = pos_B * B_rot_A + B_pos_A; rotation rot_A = rot_B * B_rot_A;
The order of the multiplications is important; swapping the B_rot_A to the front will not work!
Note that the B_*_A are the position and rotation of the coordinate system B expressed in terms of the coordinate system A. The reason that I use the prefix and postfix in the order is directly related to the order enforced for rotation multiplications. Written this way, the left-hand postfix of a multiplication must match the right-hand prefix and conviently short-circuits the notation:
something_A = something_B * B_rot_A
the two B's 'connect' and drop out (along with the '_rot_') to give something_ ... A. This only works when the postfix refers to the coordinate system that the variable is in, and the prefix is whatever is described by the variable, possibly another coordinate system.
A complete example script follows: <lsl> default {
touch(integer num_detected)
{
llSay(0, "---------");
// Get the pos and rot in Object Coordinates.
vector pos_oc = llGetLocalPos();
rotation rot_oc = llGetLocalRot();
// Fix these values when this script is in the root prim!
if (llGetLinkNumber() == 0)
{
pos_oc = ZERO_VECTOR;
rot_oc = ZERO_ROTATION;
}
// Get the pos and rot in Region Coordinates.
vector pos_rc = llGetPos();
rotation rot_rc = llGetRot();
// Get the pos and rot of the Object Coordinate System in Region Coordinates.
vector oc_pos_rc = llGetRootPosition();
rotation oc_rot_rc = llGetRootRotation();
// Print the values of the positions (the rotations are too hard to understand).
llSay(0, "oc_pos_rc = " + (string)oc_pos_rc);
llSay(0, "pos_os = " + (string)pos_os);
llSay(0, "pos_rc = " + (string)pos_rc);
// Calculate pos_rc and rot_rc from pos_oc and rot_oc : converting between _oc and _rc.
vector pos2_rc = pos_oc * oc_rot_rc + oc_pos_rc;
rotation rot2_rc = rot_oc * oc_rot_rc;
// Print the result for the position, and show the difference with what llGetPos() and llGetRot() returned.
llSay(0, "pos2_rc = " + (string)pos2_rc);
// These values should be ZERO_VECTOR and ZERO_ROTATION (or -ZERO_ROTATION).
llSay(0, "pos_rc - pos2_rc = " + (string)(pos_rc - pos2_rc));
llSay(0, "rot_rc / rot2_rc = " + (string)(rot_rc / rot2_rc));
}
} </lsl>
Obviously there is no preference for one coordinate system over the other for this conversion; one can equally as well convert orientations in A into B:
vector pos_B = pos_A * A_rot_B + A_pos_B; rotation rot_B = rot_A * A_rot_B;
The question arises then how to obtain the A_*_B values. For example, there is no LSL function that returns directly the position or rotation of the region relative to a prim (aka, rc_*_oc).
Intuitively one would say that it has to be possible to express A_*_B in B_*_A on grounds of symmetry, and that is indeed the case.
To show how this works, lets just take the formula for pos_A and rot_A (see above) and rework them to get pos_B and rot_B on the left-hand side. This results first in
pos_B * B_rot_A = pos_A - B_pos_A rot_B * B_rot_A = rot_A
Then divide both sides by B_rot_A to get:
pos_B = pos_A / B_rot_A - B_pos_A / B_rot_A rot_B = rot_A / B_rot_A
from which we can conclude that
A_pos_B = - B_pos_A / B_rot_A A_rot_B = ZERO_ROTATION / B_rot_A
Note how A_rot_B is the inverse of B_rot_A: A_rot_B * B_rot_A = ZERO_ROTATION. As per our notation convention you'd expect A_rot_B * B_rot_A to be A_rot_A, and that is actually the case as A_rot_A means the relative orientation of coordinate system A expressed in A's coordinates, which is the trivial ZERO_VECTOR and ZERO_ROTATION as we saw before for the root prim orientation expressed in Object Coordinates.
And for fun, note that if we do this inversion again we get:
B_pos_A = - A_pos_B / A_rot_B = - (- B_pos_A / B_rot_A) / (ZERO_ROTATION / B_rot_A) = B_pos_A / B_rot_A * B_rot_A = B_pos_A B_rot_A = ZERO_ROTATION / A_rot_B = ZERO_ROTATION / (ZERO_ROTATION / B_rot_A) = B_rot_A
as expected.
We can now also convert Region Coordinates to Object Coordinates.
One could first calculate,
vector rc_pos_oc = - llGetRootPosition() / llGetRootRotation(); rotation rc_rot_oc = ZERO_ROTATION / llGetRootRotation();
and then convert as usual:
vector pos_oc = pos_rc * rc_rot_oc + rc_pos_oc; rotation rot_oc = rot_rc * rc_rot_oc;
or if you don't want to calculate the intermediate rc_*_oc values, you could do immediately:
vector pos_oc = (pos_rc - llGetRootPosition()) / llGetRootRotation(); rotation rot_oc = rot_rc / llGetRootRotation();
To verify this, you could extend the above script with <lsl>
// Calculate pos_oc and rot_oc from pos_rc and rot_rc.
vector pos2_oc = (pos_rc - llGetRootPosition()) / llGetRootRotation();
rotation rot2_oc = rot_rc / llGetRootRotation();
llSay(0, "pos2_oc = " + (string)pos2_oc);
// These values should again be ZERO_VECTOR and +/- ZERO_ROTATION.
llSay(0, "pos_oc - pos2_oc = " + (string)(pos_oc - pos2_oc));
llSay(0, "rot_oc / rot2_oc = " + (string)(rot_oc / rot2_oc));
</lsl>
Cascading Coordinate Systems
Imagine you have a castle object where the floor is the root prim. Everything is linked because you want it to be easy to move the castle around. One of the child prims is a sculpty representing the hinges around which you want to rotate a large gate. The gate has a rotating wheel on it.
You want that the object (castle) to keep working when it is moved and/or rotated, but also when it is edited and the hinges are moved and/or rotated. This means that the state of the gate prims must be stored relative to the hinges, which is by far the easiest thing to do to begin with, after all, the gate rotates around the hinges and has no relation with the rest of the castle.
Hence, we have the Region Coordinate System, the Object Coordinate System (the castle) and the Hinges Coordinate System. The latter is our first example of a Prim Coordinates System, but I'll use the postfix '_hc' for "Hinges Coordinates" instead of '_pc'. Finally we have the Spill Coordinate System (or Gate Coordinates) where the spill is a prim around which we want to rotate the wheel, and I'll used '_sc' as postfix for that.
The variables used then would be an orientation for the wheel in *_sc coordinates, the orientation of the spill, and other gate prims, in *_hc coordinates and the orientation of the hinges in *_oc coordinates. Finally we could have variables for the orientation of the castle (root prim) in *_rc coordinates of course.
vector castle_pos_rc; // The position of the castle in Region Coordinates. rotation castle_rot_rc; // The rotation of the castle in Region Coordinates. vector hinges_pos_oc; // The position of the hinges in Object Coordinates. rotation hinges_rot_oc; // The rotation of the hinges in Object Coordinates. vector spill_pos_hc; // The position of the spill in Hinges Coordinates. rotation spill_rot_hc; // The rotation of the spill in Hinges Coordinates. vector wheel_pos_sc; // The position of the wheel in Spill Coordinates. rotation wheel_rot_sc; // The rotation of the wheel in Spill Coordinates.
Now because we used existing prims orientations as coordinate system for the next prim, the position and rotation of those prims are the position and rotation of that coordinate system. Just like before we saw that llGetPos/llGetRot returns the global orientation of the root prim when used in a script in the root prim, while llGetRootPos/llGetRootRot gave us the relative orientation of the Object Coordinate System in Region Coordinates, but is in fact the same as what llGetPos/llGetRot in the root prim returns: the position/rotation of the root prim is the orientation of the Object Coordinate System relative to the Region.
Likewise, in this case, castle_*_rc is the relative orientation of the Object Coordinate System in Region Coordinates as well as the orientation of the root prim in Region Coordinates; they are the same thing.
The same then applies to the *_oc variables, which we defined as the orientation of the hinges in Object Coordinates, but at the same time are the relative orientation of the Hinges Coordinate System in Object Coordinates. The *_hc variables are the orientation of spill in Hinges Coordinates, but also the relative orientation of the Spill Coordinates in Hinges Coordinates.
Lets write it out using the same notation convention as used above, then we have:
vector oc_pos_rc = castle_pos_rc; // The Object Coordinate System orientation in Region Coordinates. rotation oc_rot_rc = castle_rot_rc; vector hc_pos_oc = hinges_pos_oc; // The Hinges Coordinate System orientation in Object Coordinates. rotation hc_rot_oc = hinges_rot_oc;
vector sc_pos_hc = spill_pos_hc; // The Spill Coordinate System orientation in Hinges Coordinates. rotation sc_rot_hc = spill_rot_hc;
It will therefore probably not surprise you that
vector hc_pos_rc = hc_pos_oc * oc_rot_rc + oc_pos_rc; // The Hinges Coordinate System orientation in Region Coordinates. rotation hc_rot_rc = hc_rot_oc * oc_rot_rc; vector sc_pos_rc = sc_pos_hc * hc_rot_rc + hc_pos_rc; // The Spill Coordinate System orientation in Region Coordinates. rotation sc_rot_rc = sc_rot_hc * hc_rot_rc;
And finally, using the latter, we can express the wheel orientation in region coordinates:
vector wheel_pos_rc = wheel_pos_sc * sc_rot_rc + sc_pos_rc; rotation wheel_rot_rc = wheel_rot_sc * sc_rot_rc;
Although this was nothing else but applying the coordinate system conversion formula three times, it might give some insight to get rid of the intermediate variables and express the wheel orientation in region coordinates without them, giving:
wheel_pos_rc = ((wheel_pos_sc * sc_rot_hc + sc_pos_hc) * hc_rot_oc + hc_pos_oc) * oc_rot_rc + oc_pos_rc wheel_rot_rc = wheel_rot_sc * sc_rot_hc * hc_rot_oc * oc_rot_rc