Part of the Khronos Group

The Industry's Foundation for High Performance Graphics

from games to virtual reality, mobile phones to supercomputers

Results 1 to 4 of 4

Thread: Raycast selection/hit -- About 90% there with one small glitch.

  1. #1
    Junior Member Newbie
    Join Date
    Jun 2011

    Raycast selection/hit -- About 90% there with one small glitch (videos included)

    Moderators... If you are reading this, could you please delete this older post of mine? I am asking another question that is similar, but worded better with better examples of my problem and I don't want to confuse anyone who may be able to help. Thanks...

    I have spent all day trying to figure this out and I feel like I am about 90% there. I have made a screencast video of my problem. It is in MP4 format. If you are interested, you can download/view it by clicking here.

    ETA: here is another video of the object rotating. It shows more of the problem.

    I am using this Matrix and Vector classes found here.

    This is sloppy, sloppy code, I know -- But, I am just trying to get this to work before I start organizing it so I know how to organize it.

    Please watch the video above to better understand what I am trying to do...

    Code :
    void CPCSView::OnLButtonDown(UINT nFlags, CPoint point){
        Vector3 vecRayNear,vecRayFar;
        { // find the ray based on click point
            //GLdouble dblMatMV[16];
            GLdouble dlbMatPM[16];
            GLint nArrViewPort[4];
            //glGetDoublev(GL_MODELVIEW_MATRIX,dblMatMV); // don't need this, it's stored in m_matModelView
            glGetDoublev(GL_PROJECTION_MATRIX, dlbMatPM);
            glGetIntegerv(GL_VIEWPORT, nArrViewPort);
            double dblArrNear[3];
            gluUnProject(point.x, point.y, 0.0, m_matModelView.getTransposed(), dlbMatPM,
                         nArrViewPort, &dblArrNear[0], &dblArrNear[1],&dblArrNear[2]);
            vecRayNear = Vector3((float)dblArrNear[0],(float)dblArrNear[1],(float)dblArrNear[2]);
            double dblArrFar[3];
            gluUnProject(point.x, point.y, 1.0, m_matModelView.getTransposed(), dlbMatPM,
                         nArrViewPort, &dblArrFar[0], &dblArrFar[1],&dblArrFar[2]);
            vecRayFar = Vector3((float)dblArrFar[0],(float)dblArrFar[1],(float)dblArrFar[2]);
        // calculate the triangle points and normal
        Vector3 vecTri[3];
        Vector3 vecNormal;
            // original triangle
            //glVertex3f(1, 0, 0);
            //glVertex3f(0, 1, 0);
            //glVertex3f(0, 0, 2);
            const Matrix4 mvT(m_matModelView);
            vecTri[0] = Vector3(1,0,0)*mvT;
            vecTri[1] = Vector3(0,1,0)*mvT;
            vecTri[2] = Vector3(0,0,2)*mvT;
            vecNormal = (vecTri[1]-vecTri[0]).cross(vecTri[2]-vecTri[0]);
        { // this is normal "where does a line (in this case the ray) intersect a triangle" code
            const float flDist1 = (vecRayNear-vecTri[0]).dot(vecNormal);
            const float flDist2 = (vecRayFar-vecTri[0]).dot(vecNormal);
            if((flDist1 * flDist2) >= 0.0f){
                TRACE0("line doesn't cross the triangle.\n");
            if(flDist1 == flDist2){
                TRACE0("line and plane are parallel.\n");
            const Vector3 vecIntersect  = vecRayNear+(vecRayFar-vecRayNear)*(-flDist1/(flDist2-flDist1));
            bool bFoundIntersection = true;
            Vector3 vecTest;
            vecTest = vecNormal.cross(vecTri[1]-vecTri[0]);
                TRACE0("No intersection 1\n");
                bFoundIntersection = false;
            vecTest = vecNormal.cross(vecTri[2]-vecTri[1]);
                TRACE0("No intersection 2\n");
                bFoundIntersection = false;
            vecTest = vecNormal.cross(vecTri[0]-vecTri[2]);
                TRACE0("No intersection 3\n");
                bFoundIntersection = false;
                TRACE3("Found Hit: %g; %g; %g\n",vecIntersect.x,vecIntersect.y,vecIntersect.z);
        CGLEnabledView::OnLButtonDown(nFlags, point);

    Any insight would be greatly appreciated!
    Last edited by sonicmouse; 04-08-2013 at 12:06 AM. Reason: Added more comments and another video

  2. #2
    Advanced Member Frequent Contributor
    Join Date
    Mar 2009
    Karachi, Pakistan
    What is your question? You need insights about what?

  3. #3
    Junior Member Newbie
    Join Date
    Jun 2011
    The question is in the video I made.

    I will type out the question/problem when I get home, it was just easier to make a video than try to explain with text.

  4. #4
    Junior Member Newbie
    Join Date
    Jun 2011
    OK... i found the problem. First off, i shouldn't be using the model view transform to alter the triangle before testing if the ray intersects it, secondly, gluUnProject's 2nd parameter shouldn't simply be point.y, since OGL's Y axis is inverted, i had to change it to nArrViewPort[3]-point.y. That's what was causing it to invert.

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts