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View Full Version : Matrix transformations

nk
05-24-2004, 07:15 AM
Given a modelview and projection matrix and a viewport(cx, cy), what are the operations OGL performes in order to get a 2D point on the screen
(something like gluProject code)?
I know that the vertices are multiplied with modelview then with projection matrix, scaled ..etc but I want a concrete example...
It will be very usefull also a link where I can find detailed information about functions like gluPerspective, gluLookAt, glOrtho,...etc(to what matrix they must be applied and what is the effect - both modelview and projection are modified)

yooyo
05-24-2004, 11:02 AM
From MESA (glu/project.c):

/*
* Transform a point (column vector) by a 4x4 matrix. I.e. out = m * in
* Input: m - the 4x4 matrix
* in - the 4x1 vector
* Output: out - the resulting 4x1 vector.
*/
static void
transform_point(GLdouble out[4], const GLdouble m[16], const GLdouble in[4])
{
#define M(row,col) m[col*4+row]
out[0] =
M(0, 0) * in[0] + M(0, 1) * in[1] + M(0, 2) * in[2] + M(0, 3) * in[3];
out[1] =
M(1, 0) * in[0] + M(1, 1) * in[1] + M(1, 2) * in[2] + M(1, 3) * in[3];
out[2] =
M(2, 0) * in[0] + M(2, 1) * in[1] + M(2, 2) * in[2] + M(2, 3) * in[3];
out[3] =
M(3, 0) * in[0] + M(3, 1) * in[1] + M(3, 2) * in[2] + M(3, 3) * in[3];
#undef M
}

/* projection du point (objx,objy,obz) sur l'ecran (winx,winy,winz) */
GLint GLAPIENTRY
gluProject(GLdouble objx, GLdouble objy, GLdouble objz,
const GLdouble model[16], const GLdouble proj[16],
const GLint viewport[4],
GLdouble * winx, GLdouble * winy, GLdouble * winz)
{
/* matrice de transformation */
GLdouble in[4], out[4];

/* initilise la matrice et le vecteur a transformer */
in[0] = objx;
in[1] = objy;
in[2] = objz;
in[3] = 1.0;
transform_point(out, model, in);
transform_point(in, proj, out);

/* d'ou le resultat normalise entre -1 et 1 */
if (in[3] == 0.0)
return GL_FALSE;

in[0] /= in[3];
in[1] /= in[3];
in[2] /= in[3];

/* en coordonnees ecran */
*winx = viewport[0] + (1 + in[0]) * viewport[2] / 2;
*winy = viewport[1] + (1 + in[1]) * viewport[3] / 2;
/* entre 0 et 1 suivant z */
*winz = (1 + in[2]) / 2;
return GL_TRUE;
} Take a look

yooyo http://www.mesa3d.org/

nk
05-25-2004, 01:06 AM
Cool. Thank you.