OpenGL - 3D Shapes

Using the same files you have been working on, make changes to view your first 3D object. A cube.

• The first thing you have to do is set up the environment for 3D viewing.
  To do this you have to:
  • In resizeGL():
    • We draw inside an area of the screen known as the viewing volume. In the last assignment the viewing volume was more like a viewing plane (or like a sheet of paper).
    • Change the projection mode from gluOrtho2D(). Notice the '2D' bit.
    • We can change this to a 3D orthographic projection by using:
      • glOrtho( -1.0, 1.0, -1.0, 1.0, 5.0, 15.0 );
      • The parameters are (left, right, top, bottom, near, far).
      • An orthographics projection is much like a technical drawing where no account for perspective is taken into the drawing.
      • With an Orthographic projection the viewing volume is a parallelpiped (a box).
      • Objects are drawn the same size regardless of their distance from the viewer.
    • The cube would appear end on in the top right corner of the screen as such:
      
      Or if it is rotated slightly:
      
    • We usually like to use a persepctive type drawing for a more realistic view of the object.
      
    • Only objects inside the viewing volume are drawn.
    • Objects closer to the viewer are drawn larger.
    • In a perspective projection the viewing volume is a frustum of a pyramid, ie a truncated pyramid. You can specifuy this in one of two ways:
      • glFrustum()
      • gluPerspective()
    • In this example I use glFrustum( -1.0, 1.0, -1.0, 1.0, 5.0, 15.0 );
      The parameters are (left, right, top, bottom, near, far).
      NB. the near and far are not z-coordinates as the left, right, near, far parameters are. Near and far are distances from the eye to the front and back clipping planes of the scene.
    • If an objects, or part of an objbect, moves outside of this volume it is clipped (that part of the object is not rendered).Observe what happens when the corner of the cube is closer to the eye/camera than the near clipping plane.
      
  • setXRotation() & setYRotation()
    • I have also added setXRotation() and setYRotation() routines that are almost exactly the same as the setZRotation() from the 2D lab exercise.
    • The mouseMoveEvent() function has changed to allow rotations on the 3 axes (rememeber our discussion in lab about sensible rotations for a 2D scene).
      The changes are:
      • using the left mouse button for rotations, intead of changing the calues for the zRot variable they now change the values for xRot and yRot.
      • the middle moouse button now can perform a zoom operation, ie a translation on z by changing the value of zTrans.
  • The bulk of the changes are made to the paintGL() function. These are:
    • First we need to draw a cube. Instead of using glBegin(GL_POLYGON); to draw a square, use 6 polygons or quads (or some variation) to draw a cube. Hint: draw it on paper first and think about what the vertex coordintes will be. Think about inside and outside facing polygons and the anti-clockwise ordering scheme.
    • Remember our discussion in lab about buffers and what glClear( GL_COLOR_BUFFER_BIT); does.
      • In 3D we also need to use the depth buffer. The depth buffer is required for doing 3D models. This enables drawing in the correct order so that polygons/objects/etc closer to the eye/screen are drawn on top of the things in the distance.
      • Without depth buffering the image may look like this:
        
      • We need to add the depth buffer information. We do this by changing the above glClear() function call to include the depth buffer:
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
      • Then, we need to enable the depth test in the drawing routine so that it tests to see if a polygon, or indeed part of a polygon should be drawn. This is accomplished by:
        glEnable(GL_DEPTH_TEST);
      • We add in the code to make sure the transformations are applied. This means the mouse movements and the new sliders for the y and z rotations. This should be straight forward enough, for you to do on your own.
    
    
  Remember to check the documentation.

Read chapter 3 of the OpenGL Programming Guide: Viewing. Pay special attention to Matrices and how OpenGL deals with them.

Here is an updated and re-commented version of the files. www.cpsc.ucalgary.ca/~pj/453/box/3Dcube/

Useful References

OpenGL Programming Guide (The Red Book).

OpenGL Index in Alphabetic Order (This is pretty long)

The OpenGL Website

The OpenGL Website - tutorials

Contact me
email: pj@cpsc.ucalgary.ca
Tel:  (403) 220 7041.