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# OpenGL provides strange Output

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2 replies to this topic

### #1PGmeiner  Members

Posted 19 June 2001 - 08:13 AM

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

### #2wikidsmot  Members

Posted 19 June 2001 - 08:42 AM

gluPerspective( angle, fAspect, .01f, 10000 );

### #3SirKnight  Members

Posted 19 June 2001 - 10:07 AM

Yes, like wikidsmot showed. The problem is that your near clip plane is too far away. If you need that near clip plane to be that far away, then you would need to translate your whole scene farther out down the -z axis.

-SirKnight

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