Ray picking with gluUnProject()

Question

I have a quad on the y = -50 plane. At the moment, all I want to do is obtain the coordinates of a mouse click on the quad. I've managed to do this to a limited extent. The problem is that the transformations I applied when drawing the quad aren't accounted for. I can add in some constants and make it work, but I let the user rotate the scene about the x and y axes with glRotatef(), so the coordinates get messed up as soon as a rotation happens.

Here's what I'm doing now:

I call gluUnProject() twice, once with z = 0, and once with z = 1.

gluUnProject( mouseX, WINDOW_HEIGHT - mouseY, 0, modelView, projection, viewport, &x1, &y1, &z1);
gluUnProject( mouseX, WINDOW_HEIGHT - mouseY, 1, modelView, projection, viewport, &x2, &y2, &z2);

Normalized ray vector:

x = x2 - x1;
y = y2 - y1;
z = z2 - z1;
mag = sqrt(x*x + y*y + z*z);
x /= mag;
y /= mag;
z /= mag;

Parametric equation:

float t = -(camY) / y;
planeX = camX + t*x;
planeY = camY + t*y;
planeZ = camZ + t*z;

where (camX, camY, camZ) is the camera position passed to gluLookAt().

I want planeX, planeY, and planeZ to be the coordinates of the click on the quad, in the same coordinate system I used to draw the quad. How can I achieve this?

When do you grab the modelView matrix? Before or after the `glRotatef()`? — genpfault, Dec 02 '10 at 16:52

score 1 · Accepted Answer · answered Feb 04 '11 at 17:35

You are not supposed to pass in an explicit z-depth of your choosing. In order to find the world coordinate, you need to pass in the depth buffer value at that particular mouse coordinate.

GLfloat depth;
glReadPixels(x, y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &depth);

Passing that into your gluUnProject should yield the values you are looking for. Plus, as genpfault said in his comment, make sure you are grabbing the model view matrix data at the right moment. Otherwise, you have the wrong matrix.

Ray picking with gluUnProject()

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