Pro OpenGL ES for iOS

CHAPTER 8: Putting It All Together (^265)
void gluGetScreenLocation(GLfloat xa,GLfloat ya,GLfloat za,GLfloat sx, GLfloat
sy,GLfloat sz)
{
GLfloat mvmatrix[16];
GLfloat projmatrix[16];
GLfloat x,y,z;
GLint viewport[4];
glGetIntegerv(GL_VIEWPORT,viewport); //4
glGetFloatv(GL_MODELVIEW_MATRIX,mvmatrix);
glGetFloatv(GL_PROJECTION_MATRIX,projmatrix);
gluProject(xa,ya,za,mvmatrix,projmatrix,viewport,&x,&y,&z);
y=viewport[3]-y; //5
sx=x;
sy=y;
if(sz!=NULL)
sz=z;
float scale=[[UIScreen mainScreen] scale]; //6
sx/=scale;
sy/=scale;
}
void gluMultMatrixVector3f(const GLfloat matrix[16], const GLfloat in[4],GLfloat out[4])
{
int i;
for (i=0; i<4; i++)
{
out[i] =
in[0] matrix[04+i] +
in[1] matrix[14+i] +
in[2] matrix[24+i] +
in[3] matrix[34+i];
}
}
In gluProject() we supply the needed matrices along with the desired xyz coordinates
we’re investigating, and it returns the screen xyz (yes, z) of the point’s projected
location.
„ Lines 1ff map the object coordinates to an array that will then be
multiplied by the modelMatrix (supplied as one of the arguments).