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Thread: Mouse X/Y ray to Sphere Intersection to world space point

  1. #1
    Newbie Newbie
    Join Date
    Jan 2013

    Question Mouse X/Y ray to Sphere Intersection to world space point

    Hi all,

    I would like to do precise picking on a sphere surface, converting mouse X,Y to world space XYZ point so that I can draw object at that coord. Sounds simple enough and it should be, but I need some help
    I have built this simple test snippet that would simply create a some geo at the point on a globe where the mouse click ray intersection occurs and its nearly working! Its taking my xy casting that along the projection angle, taking into account the modelView inv (camera pos) etc. The only thing that I have an issue is that intersection with the Y coordinate is upside-down, also strangely I have to swap the Z coord of the mouse ( so ray_start = [x,y,1] and ray_dest = [x,y,0] otherwise its rejected as pointing backwards .
    So, that done, clicking around the equator of the sphere it plots the geo's nicely at the x and y moving orbit the camera left/right it works fine, but plotting above of the center screen creates geo points south or downward and clicking below center screen viceversa plots mirrored north . Simply swapping the final vector does not work as its the intersection thats matters. I know I am so close.. I looking at the computeWindow Matrix calculation as a potential culprit. I Just need a little more light I really appreciate some help if possible.

    Many thanks for any help!

    (This test code is in webGL and javascript)

    // window matrix function (where I think the problem could be)
    this.computeWindowMatrix = function(xstart, ystart, width, height) {
    var translate = osg.Matrix.makeTranslate(1.0, 1.0, 1.0);
    var scale = osg.Matrix.makeScale(0.5*width, 0.5*height, 0.5);
    var offset = osg.Matrix.makeTranslate(xstart,ystart,0.0);
    return osg.Matrix.preMult(offset, osg.Matrix.preMult(scale, translate));

    var ray_start = [x,y,1]; //had to swap the Z so its cast forward

    var ray_dest = [x,y,0];

    var matrix = osg.Matrix.makeIdentity();

    var w = osg.Matrix.copy(viewer.view.viewport.computeWindow Matrix(), []);

    var p = viewer.view.getProjectionMatrix();

    var m = viewer.view.getViewMatrix();

    osg.Matrix.preMult(matrix, w);

    osg.Matrix.preMult(matrix, p);

    osg.Matrix.preMult(matrix, m);

    var inv = [];

    var valid = osg.Matrix.inverse(matrix, inv);

    var ns = osg.Matrix.transformVec3(inv, ray_start, new Array(3));

    var ne = osg.Matrix.transformVec3(inv, ray_dest, new Array(3));

    //======Ray-Sphere intersection========

    var t0, t1; // parametric solutions for t if the ray intersects

    var radius2 = (6371* 6371);

    var ray_dir = osg.Vec3.normalize(osg.Vec3.sub(ns,ne, []) ,[]);

    var L = osg.Vec3.sub([0,0,0],ns,[]);

    var tca =,ray_dir);

    if (tca < 0) return false; //wrong direction

    var d2 =,L) - (tca * tca); //o2=h2-a2

    if (d2 > radius2) return false; //overshoots

    var thc = Math.sqrt(radius2 - d2); //find the exit point

    t0 = tca - thc;

    t1 = tca + thc;

    //Draw a box at the world point

    var hitPoint = osg.Vec3.add(ns, osg.Vec3.mult(ray_dir,t0,[]),[]);

    var geometry = osg.createTexturedBox(0, 0, 0, 1000,1000,1000);


    var xform = new osg.MatrixTransform()



    Last edited by handsfellof; 01-26-2013 at 02:29 AM.

  2. #2
    Newbie Newbie
    Join Date
    Jan 2013
    Fixed it For any interested please read.
    Yes as I thought it was the the viewport transformation to NDC (normalized device coordinate ) coordinates

    Code changed to this fixed it.

    ray_start[0] = (x / viewer.view.viewport.width()) * 2 - 1;

    ray_start[1] = -(y / viewer.view.viewport.height()) * 2 + 1;

    ray_start[2] = 1;

    ray_dest[0] = (x / viewer.view.viewport.width()) * 2 - 1;

    ray_dest[1] = -(y / viewer.view.viewport.height()) * 2 + 1;

    ray_dest[2] = 0;


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