
MATRIX(9.2) MATRIX(9.2)
NAME
ident, matmul, matmulr, determinant, adjoint, invertmat, xformpoint, xformpointd, xform
plane, pushmat, popmat, rot, qrot, scale, move, xform, ixform, persp, look, viewport 
Geometric transformations
SYNOPSIS
#include <libg.h>
#include <geometry.h>
void ident(Matrix m)
void matmul(Matrix a, Matrix b)
void matmulr(Matrix a, Matrix b)
double determinant(Matrix m)
void adjoint(Matrix m, Matrix madj)
double invertmat(Matrix m, Matrix inv)
Point3 xformpoint(Point3 p, Space *to, Space *from)
Point3 xformpointd(Point3 p, Space *to, Space *from)
Point3 xformplane(Point3 p, Space *to, Space *from)
Space *pushmat(Space *t)
Space *popmat(Space *t)
void rot(Space *t, double theta, int axis)
void qrot(Space *t, Quaternion q)
void scale(Space *t, double x, double y, double z)
void move(Space *t, double x, double y, double z)
void xform(Space *t, Matrix m)
void ixform(Space *t, Matrix m, Matrix inv)
int persp(Space *t, double fov, double n, double f)
void look(Space *t, Point3 eye, Point3 look, Point3 up)
void viewport(Space *t, Rectangle r, double aspect)
DESCRIPTION
These routines manipulate 3space affine and projective transformations, represented as
4x4 matrices, thus:
typedef double Matrix[4][4];
Ident stores an identity matrix in its argument. Matmul stores axb in a. Matmulr stores
bxa in b. Determinant returns the determinant of matrix m. Adjoint stores the adjoint
(matrix of cofactors) of m in madj. Invertmat stores the inverse of matrix m in minv,
returning m's determinant. Should m be singular (determinant zero), invertmat stores its
adjoint in minv.
The rest of the routines described here manipulate Spaces and transform Point3s. A Point3
is a point in threespace, represented by its homogeneous coordinates:
typedef struct Point3 Point3;
struct Point3{
double x, y, z, w;
};
The homogeneous coordinates (x, y, z, w) represent the Euclidean point (x/w, y/w, z/w) if
w!=0, and a ``point at infinity'' if w=0.
A Space is just a data structure describing a coordinate system:
typedef struct Space Space;
struct Space{
Matrix t;
Matrix tinv;
Space *next;
};
It contains a pair of transformation matrices and a pointer to the Space's parent. The
matrices transform points to and from the ``root coordinate system,'' which is represented
by a null Space pointer.
Pushmat creates a new Space. Its argument is a pointer to the parent space. Its result
is a newly allocated copy of the parent, but with its next pointer pointing at the parent.
Popmat discards the Space that is its argument, returning a pointer to the stack. Nomi
nally, these two functions define a stack of transformations, but pushmat can be called
multiple times on the same Space multiple times, creating a transformation tree.
Xformpoint and Xformpointd both transform points from the Space pointed to by from to the
space pointed to by to. Either pointer may be null, indicating the root coordinate sys
tem. The difference between the two functions is that xformpointd divides x, y, z, and w
by w, if w!=0, making (x, y, z) the Euclidean coordinates of the point.
Xformplane transforms planes or normal vectors. A plane is specified by the coefficients
(a, b, c, d) of its implicit equation ax+by+cz+d=0. Since this representation is dual to
the homogeneous representation of points, libgeometry represents planes by Point3 struc
tures, with (a, b, c, d) stored in (x, y, z, w).
The remaining functions transform the coordinate system represented by a Space. Their
Space * argument must be nonnull  you can't modify the root Space. Rot rotates by
angle theta (in radians) about the given axis, which must be one of XAXIS, YAXIS or ZAXIS.
Qrot transforms by a rotation about an arbitrary axis, specified by Quaternion q.
Scale scales the coordinate system by the given scale factors in the directions of the
three axes. Move translates by the given displacement in the three axial directions.
Xform transforms the coordinate system by the given Matrix. If the matrix's inverse is
known a priori, calling ixform will save the work of recomputing it.
Persp does a perspective transformation. The transformation maps the frustum with apex at
the origin, central axis down the positive y axis, and apex angle fov and clipping planes
y=n and y=f into the doubleunit cube. The plane y=n maps to y'=1, y=f maps to y'=1.
Look does a viewpointing transformation. The eye point is moved to the origin. The line
through the eye and look points is aligned with the y axis, and the plane containing the
eye, look and up points is rotated into the xy plane.
Viewport maps the unitcube window into the given screen viewport. The viewport rectangle
r has r.min at the top lefthand corner, and r.max just outside the lower righthand cor
ner. Argument aspect is the aspect ratio (dx/dy) of the viewport's pixels (not of the
whole viewport). The whole window is transformed to fit centered inside the viewport with
equal slop on either top and bottom or left and right, depending on the viewport's aspect
ratio. The window is viewed down the y axis, with x to the left and z up. The viewport
has x increasing to the right and y increasing down. The window's y coordinates are
mapped, unchanged, into the viewport's z coordinates.
SOURCE
/sys/src/libgeometry/matrix.c
MATRIX(9.2) 
