Hello everywhere, I had some problems with Drawing rectangles in OpenGL. When I define a Perspective Projection-Matrix and set the PolygonMode to GL_LINE and rotate the rectangle around the y-axis then sometimes in some special positions the Lines who create the Rectangles disappears partial. What can be the reason for this strange phenomena. Here is the Projection and Rendering-Code is use for the above described example: Projection-Function: GLfloat fAspect = (GLfloat) w/(GLfloat) h; glViewport( 0, 0, (GLsizei) w, (GLsizei) h); glMatrixMode (GL_PROJECTION); glLoadIdentity (); double angle = calculateAngle(h, 40); gluPerspective(angle, fAspect, 5.0f, 400); glMatrixMode (GL_MODELVIEW); glLoadIdentity (); gluLookAt(2.0, 1.0, 3.0, // eye 0.0, 5.0, -1.0, // center 0.0, 0.0, 1.0);// dir.vector Render-function: glPolygonMode(GL_FRONT_AND_BACK, GL_FILL); glClear(GL_COLOR_BUFFER_BIT); glRectd(-5 ,-5, 5, 5); glRotatef(0.3f, 0.0, 1.0, 0.0); I hope somebody can tell me the reason for that. regards peter gmeiner Peter Gmeiner