glunproject cheaper alternative

Question

I'm building a game in openframeworks where a camera moves over a stationary 2d plane in 3d space. I need to pick 5 coordinates for every frame as the camera is always moving (1 for the mouse and 4 for the corners of the viewport to workout what to draw). However I've found the gluUnproject function too slow. As I'm only picking coordinates on a stationary plane, currently on Z = 0, I figure I should be able to work out my coordinates pretty cheaply by using the modelview and projection matrices from my camera class but I just can't work out how to do the math.

To summarise, I have

Camera - MODELVIEW and PROJECTION matrices, VIEWPORT Plane aligned to z-axis with dimensions world_dims.x, world_dims.y

I want to transform screen coordinates into unprojected coordinates on the plane without using gluUnproject.

Incase I'm being dumb with my gluUnproject heres the code for that bit

ofVec3f ofxGrabCam::pickCoordinate(ofVec2f t_mouseP) {
//read z value from depth buffer at mouse coords

ofVec3f t_mouseW(t_mouseP.x, t_mouseP.y, 0);

glReadPixels(t_mouseW.x, ofGetScreenHeight()-1-t_mouseP.y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &t_mouseW.z);

if (t_mouseW.z == 1.0f){
    return ofVec3f(-99,-99,-99); //represents point not found
}

GLdouble c[3];

gluUnProject(t_mouseW.x, ofGetScreenHeight()-1-t_mouseW.y, t_mouseW.z, matM, matP, viewport, c, c+1, c+2);

ofVec3f mouseW(c[0],c[1],c[2]);

return mouseW;

}

datenwolf datenwolf · Accepted Answer · 2012-03-09T14:14:43

However I've found the gluUnproject function too slow.

Backtransforming just 5 coordinates should not be a performance bottleneck.

I want to transform screen coordinates into unprojected coordinates on the plane without using gluUnproject.

You have to determine the inverse transformation path. That is: inverting the matrices describing the way from model space to view space, (and at some point inverting the perspective divide) then passing into your coordinates into it. However, this is exactly what gluUnproject does.

glunproject cheaper alternative

1 Answers