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Thread: Window coordinates - simple question

  1. #1
    Junior Member Newbie
    Join Date
    Nov 2008

    Window coordinates - simple question

    If I have a pair of (x,y) window coordinates (where the mouse clicked in an opengl view) how do I find the corresponding coordinates of that click in terms of the opengl world space? I'm using gluPerspective for a 3d perspective projection matrix.

    I've looked around but I'm not sure what the official name for this is, which makes it hard to search.


  2. #2
    Member Contributor
    Join Date
    Jul 2004

    Re: Window coordinates - simple question

    You can't get 3D coordinates from 2D coordinates because the projection loses information. However if you have the depth as well then you can use:
    glReadPixels(x,y,GL_DEPTH_COMPONENT, GL_FLOAT, &depth);
    gluUnProject(x,y,depth, model_view, projection, viewport, &outX, &outY, &outZ);
    See documentation for explanation of the function.

    Typically this sort of thing is called "picking" because when user clicks somewhere inside a window, you project a ray into 3D and return the intersection with the first object. That's kind of what the code above does - the depth component will contain the z for the closest object.

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