Double buffering

• I can't stand the flickering any longer so
• glutInitDisplayMode( GLUT_RGBA |GLUT_DOUBLE );
• glutSwapBuffers()
  • Does a glFlush()
  • And buffer management.
• glFlush()
  • Tells OpenGL to flush all buffers
  • Should be called when you wish to have the previously called commands finished.
  • Like at the end of the display function.

A Historic Way of Drawing Things

• Most of these commands are deprecated, but there is a HUGE code base out there which employs these.
• Most of the techniques don't change, just the way you submit the commands.
• void glBegin(GLenum mode);
  • Start drawing a primitive or set of primitives.
  • MODE:
    • GL_POINTS
      • Each point is drawn
    • GL_LINES
      • Each pair of vertexes are drawn as a line
      • point 0 to point 1, point 2 to point 3, ...
    • GL_LINE_STRIP
      • Connect the dots.
    • GL_LINE_LOOP
      • Connect the dots and go back to the beginning
    • GL_TRIANGLES
      • independent triangles.
      • point0, point 1, point 2,
    • GL_TRIANGLE_STRIP
      • A strip of triangles.
      • Point 0, point 1, point 2
      • Point 1, point 2, point 3
    • GL_TRIANGLE_FAN
      • All triangles start at point 0
      • First 0,1,2
      • Second 0,2,3
      • ...
    • GL_QUADS
      • Four vertexes define a quad, in order.
      • quads are independent.
    • GL_QUAD_STRIP
      • As in triangle strip, but with four vertexes.
    • GL_POLYGON
      • All points form a single shape.
  • It ends with a void glEnd(void)
  • Data is specified via the void glVertex*() family.

        glBegin(GL_LINE_STRIP);
        for (i=0; i<data.size()-1; i+= skip ){
               glVertex2f(data[i][0],data[i][1]);
        }
        glVertex2f(data[i][0],data[i][1]);
    
        glEnd();
    	     

  • Attributes can be specified at any time:
    • void glColor*()
    • void glPointSize(Glfloat size);
    • void glLineWidth( GLfloat width)
    • There are other attributes, which we will discover as we go along.
    • Remember, attributes are like a switch, they stay in place until they are changed.
    • So if you set the color to red, everything after that will be red until you change the color.
• A few other commands
  • void glClearColor(GLfloat red, GLfloat green, GLfloat blue, GLfloat alpha);
    • Value to set the buffers to when a clear occurs.
    • RGBA values should be in [0,1]
    • Do this once in initialization
    • Unless you want to clear the screen to different colors.
  • void glClear(GLbitfield mask)
    • GL_COLOR_BUFFER_BIT - set the screen to the clear color.
    • Call this once per screen update.
    • At the beginning of the update.
  • void glFlush(void)
    • But we already have this one.
• An Example:
  • Draw a circle, or something like one.
  • This uses the parametric equations
    • P_x =c_x + r cos(θ)
    • P_y =c_y + r sin(θ);
    • 0 ≤ θ ≤ 2π
  • Keypresses
    • a-j change drawing mode
    • C draw in a random color
    • q quit
  • Code is located in two files
    • Illustrate how to use Make to compile everything.
    • Illustrate how to share a global between two files.
  • Code:
    • Makefile
    • keys.C
    • keys.h
    • main.C