Objectives

• Explore the rendering of quads, quadstrips and polygons.
• Perform some simple experiments with the scissors buffer

Setup

• Create a new lab09 project as usual.

• Use the following setRC:

  glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
  glColor3f(0.0f, 1.0f, 0.0f);
  glOrtho (-100.0f, 100.0f, -100.0f, 100.0f, -100.0f, 100.0f);

  glPolygonMode(GL_FRONT,GL_LINE);
  glPolygonMode(GL_BACK, GL_LINE);
  glShadeModel(GL_FLAT);
  glEnable(GL_DEPTH_TEST);

• Define a function for drawing a QUAD - essentially a four sided polygon:

void drawQuad()
{
  glBegin(GL_QUADS);
    glVertex3f(-50.0f,  0.0f, 0.0f);
    glVertex3f( 0.0f,  50.0f, 0.0f);
    glVertex3f( 50.0f,  0.0f, 0.0f);
    glVertex3f( 0.0f, -50.0f, 0.0f);
  glEnd();
}

• Also build a quad strip:

void drawQuadStrips()
{
  glBegin(GL_QUAD_STRIP);
    glVertex3f(-50.0f,  -50.0f, 0.0f);
    glVertex3f( -50.0f,  50.0f, 0.0f);
    glVertex3f( 0.0f, -50.0f, 0.0f);
    glVertex3f( 0.0f,  50.0f, 0.0f);
    glVertex3f( 50.0f, -50.0f, 0.0f);
    glVertex3f( 50.0f, 50.0f, 0.0f);
  glEnd();
}

• and a general Polygon:

void drawPolygon()
{
  glBegin(GL_POLYGON);
    glVertex3f(-50.0f,  50.0f, 0.0f);
    glVertex3f( 20.0f,  50.0f, 0.0f);
    glVertex3f( 50.0f,  30.0f, 0.0f);
    glVertex3f( 50.0f, -50.0f, 0.0f);
    glVertex3f(-50.0f, -50.0f, 0.0f);
  glEnd();
}

• Build and run these individually

Edges

• The following method draws a four pointed star:

void star()
{
  glBegin(GL_TRIANGLES);

    glVertex2f(-20.0f, 0.0f);
    glVertex2f(20.0f, 0.0f);
    glVertex2f(0.0f, 40.0f);

    glVertex2f(-20.0f,0.0f);
    glVertex2f(-60.0f,-20.0f);
    glVertex2f(-20.0f,-40.0f);

    glVertex2f(-20.0f,-40.0f);
    glVertex2f(0.0f, -80.0f);
    glVertex2f(20.0f, -40.0f);

    glVertex2f(20.0f, -40.0f);
    glVertex2f(60.0f, -20.0f);
    glVertex2f(20.0f, 0.0f);

    glVertex2f(-20.0f, 0.0f);
    glVertex2f(-20.0f,-40.0f);
    glVertex2f(20.0f, 0.0f);

    glVertex2f(-20.0f,-40.0f);
    glVertex2f(20.0f, -40.0f);
    glVertex2f(20.0f, 0.0f);

  glEnd();
}

• If we set polygonmode to fill

  glPolygonMode(GL_FRONT,GL_FILL);
  glPolygonMode(GL_BACK, GL_FILL);

• then we see this:

• Resetting to line mode, however, produces this:

• As you can see, the edges are showing. Wether these edges are displayed or not can be controlled by this api call:

• glEdgeFlag

• Try this version:

void starWithoutEdges()
{
  glBegin(GL_TRIANGLES);

    glEdgeFlag(GL_FALSE);
    glVertex2f(-20.0f, 0.0f);
    glEdgeFlag(GL_TRUE);
    glVertex2f(20.0f, 0.0f);
    glVertex2f(0.0f, 40.0f);

    glVertex2f(-20.0f,0.0f);
    glVertex2f(-60.0f,-20.0f);
    glEdgeFlag(GL_FALSE);
    glVertex2f(-20.0f,-40.0f);
    glEdgeFlag(GL_TRUE);

    glVertex2f(-20.0f,-40.0f);
    glVertex2f(0.0f, -80.0f);
    glEdgeFlag(GL_FALSE);
    glVertex2f(20.0f, -40.0f);
    glEdgeFlag(GL_TRUE);

    glVertex2f(20.0f, -40.0f);
    glVertex2f(60.0f, -20.0f);
    glEdgeFlag(GL_FALSE);
    glVertex2f(20.0f, 0.0f);
    glEdgeFlag(GL_TRUE);

    glEdgeFlag(GL_FALSE);
    glVertex2f(-20.0f, 0.0f);
    glVertex2f(-20.0f,-40.0f);
    glVertex2f(20.0f, 0.0f);

    glVertex2f(-20.0f,-40.0f);
    glVertex2f(20.0f, -40.0f);
    glVertex2f(20.0f, 0.0f);
    glEdgeFlag(GL_TRUE);

  glEnd();
}

Buffering

• So far your application have been single buffered.

• This is configured in main with GLUT_SINGLE

  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);

• and in renedScene() where the single buffer is flushed:

void renderScene(void)
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

  starWithoutEdges();

  glFlush();
}

• For complex models and animations it may be preferable to switch to double buffering. This involves specifying the GLUT_DOUBLE display mode:

  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);

• and using this methdod:

• glutSwapBuffers

• instead of

• glFlush

• This would be the default for many applications.

• In some circumstances you may require single buffering.

Exercises

Review the glScissor

Rework the rendering in this lab to only render the a rectangular portion of the screen.