Strange problem about OpenGL drawing in a QGraphicsView.



  • OpenGL rendering is done by reimplementing QGraphicsScene's drawBackground() function. I also add two QGraphicsRectItem in the QGraphicsScene, one embeded a dialog with four buttons in it; another is a yellow Solid circle.
    when the opengl codes are as follows included in drawBackground() of QGraphicsScene,

    @MTList=glGenLists(1);
    glEnableClientState(GL_VERTEX_ARRAY);
    Vertex_iterator vi = model.vertices_begin();
    glVertexPointer(3, GL_DOUBLE, sizeof(Vertex), &(vi->point()[0]));
    glDisable(GL_LIGHTING);
    glNewList(MTList, GL_COMPILE);
    glColor3f(1.0,0.0,1.0);
    //glLineWidth(10.0);
    //glDisable(GL_CULL_FACE);
    //glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
    glDrawElements(GL_POINTS, 3*model.size_of_facets(), GL_UNSIGNED_INT, &modelfaces[0]); glEndList();
    glDisableClientState(GL_VERTEX_ARRAY);@

    at this time, the two items in QGraphicsScene display as expected:
    !http://uploads.gamedev5.net/gallery/album_467/gallery_200313_467_12630.jpg(1)!

    However, when I uncomment //glPolygonMode(GL_FRONT_AND_BACK, GL_LINE):

    @glColor3f(1.0,0.0,1.0);
    //glLineWidth(10.0);
    //glDisable(GL_CULL_FACE);
    glPolygonMode(GL_FRONT_AND_BACK, GL_LINE);
    glDrawElements(GL_POINTS, 3*model.size_of_facets(), GL_UNSIGNED_INT, &modelfaces[0]);@

    (I know I should also change GL_TRIAGNLES, however the result is the same.) then the two items as distorted like two slashes:
    !http://uploads.gamedev5.net/gallery/album_467/gallery_200313_467_2080.jpg(2)!

    Why this weird display occur ?

    Thanks a lot



  • I revised the line to : glPolygonMode(GL_BACK, GL_LINE); the problem still exists,

    HHHOoooWWwwever, glPolygonMode(GL_FRONT, GL_LINE); sloved the problem and the two items come out to be as expected.

    Why?


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