Strange problem about OpenGL drawing in a QGraphicsView.

[superwave]

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:
7920

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:
7921

Why this weird display occur ?

Thanks a lot

[superwave]

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?