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opengldepth-testing

Objects shake when rotating

I'm facing a problem in my opengl code

I'm trying to build a house, and rotate it 360°, for simplicity let's assume the house has the front wall with window and dor, and a back wall.

I'm using DEPTH_BUFFER not to see the back wall when viewing the front wall, and the other way around, but when I rotate the house the door and window start to shake and get distorced.

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColor3f(0.0, 0.0, 0.0);
glLoadIdentity();
gluLookAt(0.0, 0.0, 40.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
glRotatef(angle, 0.0, 1.0, 0.0);

glEnable(GL_DEPTH_TEST);

glColor3f(0.0, 0.0, 0.0);
glBegin(GL_POLYGON);
glVertex3f(8.0, 3.0, 0.0);
glVertex3f(8.0, -10.0, 0.0);
glVertex3f(1.0, -10.0, 0.0);
glVertex3f(1.0, 3.0, 0.0);
glEnd();

glColor3f(0.0, 0.0, 0.0);
glBegin(GL_POLYGON);
glVertex3f(-9.0, -4.0, 0.0);
glVertex3f(-9.0, 3.0, 0.0);
glVertex3f(-2.0, 3.0, 0.0);
glVertex3f(-2.0, -4.0, 0.0);
glEnd();

glColor3f(1.0, 0.0, 0.0);
glBegin(GL_POLYGON);
glVertex3f(10.0, -10.0, -20.0);
glVertex3f(-10.0, -10.0, -20.0);
glVertex3f(-10.0, 10.0, -20.0);
glVertex3f(10.0, 10.0, -20.0);
glEnd();


glColor3f(1.0, 0.0, 0.0);
glBegin(GL_POLYGON);
glVertex3f(10.0, -10.0, 0.0);
glVertex3f(10.0, 10.0, 0.0);
glVertex3f(-10.0, 10.0, 0.0);
glVertex3f(-10.0, -10.0, 0.0);
glEnd();


glDisable(GL_DEPTH_TEST);


glutSwapBuffers();
Solution

  • The issue is called Z-fighting. This is caused, because depth of the "door", "window" and "wall" are equal. The vertiex coordinates are transformed by the model view matrix and projection matrix and interpolated for each fragment which is covered by the polygon. This results in inaccuracies of the final z coordinate (depth).

    Enable the polygon fill offset (glPolygonOffset) by before drawing the walls, to solve the issue:

    glEnable(GL_DEPTH_TEST);
glDisable( GL_POLYGON_OFFSET_FILL );

// draw door and window
// ...

glEnable( GL_POLYGON_OFFSET_FILL );
glPolygonOffset( 1, 1 );

// draw walls
// ...

    Polygon fill offset manipulates the depth of a fragment by a minimum amount. This causes that the depth of the "walls" is different to the depth of the "window" and "door", even after the transformation by the model view and projection matrix.
    Since an offset is added to the depth of the "wall", the "wall" is always behind the window and the door, independent of the point of view.