Pick Matrix Internals

Hi there,

Given the parameters to gluPickMatrix

void gluPickMatrix( GLdouble x, GLdouble y, GLdouble delX, GLdouble delY, GLint *viewport )

does anyone know what the pick matrix actually looks like? I’m implementing a picking system using FBOs and since I don’t want to allocate an offscreen buffer as big as the whole window and care for resizing issues, I need a way to manually restrict rendering to the pixel that’s located under the mouse pointer.

Any help is greatly appreciated.

Never used glu but IIRC pickmatrix will crop your projection? All you need to do is scale and offset x and y, this matrix should do the trick:

m = identity;
m[0,0] = vw / (r-l);
m[1,1] = vh / (b-t);
m[3,0] = (vw-l-r) / (r-l);
m[3,1] = (vh-b-t) / (t-b);

From MESA:

gluPickMatrix(GLdouble x, GLdouble y, GLdouble deltax, GLdouble deltay, GLint viewport[4])
    if (deltax <= 0 || deltay <= 0) { 

    /* Translate and scale the picked region to the entire window */
    glTranslatef((viewport[2] - 2 * (x - viewport[0])) / deltax,
	    (viewport[3] - 2 * (y - viewport[1])) / deltay, 0);
    glScalef(viewport[2] / deltax, viewport[3] / deltay, 1.0);

Good riddance those matrix functions were! That’s like turning a house rather than the lightbulb to screw it in. How long does that take? 50 ms? Do you then follow it up with a nice GetMatrix? I’ve always thought simple little addition and multiplication functions had no place in the api.

Apologies for the OT rant.