152 lines
3.3 KiB
Plaintext
152 lines
3.3 KiB
Plaintext
.TH QUATERNION 2
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.SH NAME
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qtom, mtoq, qadd, qsub, qneg, qmul, qdiv, qunit, qinv, qlen, slerp, qmid, qsqrt \- Quaternion arithmetic
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.SH SYNOPSIS
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.B
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#include <draw.h>
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.br
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.B
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#include <geometry.h>
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.PP
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.B
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Quaternion qadd(Quaternion q, Quaternion r)
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.PP
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.B
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Quaternion qsub(Quaternion q, Quaternion r)
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.PP
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.B
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Quaternion qneg(Quaternion q)
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.PP
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.B
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Quaternion qmul(Quaternion q, Quaternion r)
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.PP
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.B
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Quaternion qdiv(Quaternion q, Quaternion r)
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.PP
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.B
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Quaternion qinv(Quaternion q)
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.PP
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.B
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double qlen(Quaternion p)
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.PP
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.B
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Quaternion qunit(Quaternion q)
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.PP
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.B
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void qtom(Matrix m, Quaternion q)
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.PP
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.B
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Quaternion mtoq(Matrix mat)
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.PP
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.B
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Quaternion slerp(Quaternion q, Quaternion r, double a)
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.PP
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.B
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Quaternion qmid(Quaternion q, Quaternion r)
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.PP
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.B
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Quaternion qsqrt(Quaternion q)
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.SH DESCRIPTION
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The Quaternions are a non-commutative extension field of the Real numbers, designed
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to do for rotations in 3-space what the complex numbers do for rotations in 2-space.
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Quaternions have a real component
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.I r
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and an imaginary vector component \fIv\fP=(\fIi\fP,\fIj\fP,\fIk\fP).
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Quaternions add componentwise and multiply according to the rule
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(\fIr\fP,\fIv\fP)(\fIs\fP,\fIw\fP)=(\fIrs\fP-\fIv\fP\v'-.3m'.\v'.3m'\fIw\fP, \fIrw\fP+\fIvs\fP+\fIv\fP×\fIw\fP),
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where \v'-.3m'.\v'.3m' and × are the ordinary vector dot and cross products.
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The multiplicative inverse of a non-zero quaternion (\fIr\fP,\fIv\fP)
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is (\fIr\fP,\fI-v\fP)/(\fIr\^\fP\u\s-22\s+2\d-\fIv\fP\v'-.3m'.\v'.3m'\fIv\fP).
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.PP
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The following routines do arithmetic on quaternions, represented as
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.IP
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.EX
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.ta 6n
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typedef struct Quaternion Quaternion;
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struct Quaternion{
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double r, i, j, k;
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};
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.EE
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.TF qunit
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.TP
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Name
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Description
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.TP
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.B qadd
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Add two quaternions.
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.TP
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.B qsub
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Subtract two quaternions.
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.TP
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.B qneg
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Negate a quaternion.
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.TP
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.B qmul
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Multiply two quaternions.
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.TP
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.B qdiv
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Divide two quaternions.
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.TP
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.B qinv
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Return the multiplicative inverse of a quaternion.
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.TP
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.B qlen
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Return
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.BR sqrt(q.r*q.r+q.i*q.i+q.j*q.j+q.k*q.k) ,
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the length of a quaternion.
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.TP
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.B qunit
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Return a unit quaternion
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.RI ( length=1 )
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with components proportional to
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.IR q 's.
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.PD
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.PP
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A rotation by angle \fIθ\fP about axis
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.I A
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(where
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.I A
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is a unit vector) can be represented by
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the unit quaternion \fIq\fP=(cos \fIθ\fP/2, \fIA\fPsin \fIθ\fP/2).
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The same rotation is represented by \(mi\fIq\fP; a rotation by \(mi\fIθ\fP about \(mi\fIA\fP is the same as a rotation by \fIθ\fP about \fIA\fP.
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The quaternion \fIq\fP transforms points by
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(0,\fIx',y',z'\fP) = \%\fIq\fP\u\s-2-1\s+2\d(0,\fIx,y,z\fP)\fIq\fP.
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Quaternion multiplication composes rotations.
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The orientation of an object in 3-space can be represented by a quaternion
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giving its rotation relative to some `standard' orientation.
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.PP
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The following routines operate on rotations or orientations represented as unit quaternions:
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.TF slerp
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.TP
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.B mtoq
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Convert a rotation matrix (see
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.IR matrix (2))
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to a unit quaternion.
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.TP
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.B qtom
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Convert a unit quaternion to a rotation matrix.
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.TP
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.B slerp
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Spherical lerp. Interpolate between two orientations.
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The rotation that carries
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.I q
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to
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.I r
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is \%\fIq\fP\u\s-2-1\s+2\d\fIr\fP, so
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.B slerp(q, r, t)
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is \fIq\fP(\fIq\fP\u\s-2-1\s+2\d\fIr\fP)\u\s-2\fIt\fP\s+2\d.
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.TP
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.B qmid
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.B slerp(q, r, .5)
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.TP
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.B qsqrt
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The square root of
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.IR q .
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This is just a rotation about the same axis by half the angle.
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.PD
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.SH SOURCE
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.B /sys/src/libgeometry/quaternion.c
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.SH SEE ALSO
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.IR matrix (2),
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.IR qball (2)
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