1. Improving the appearance of 3D OpenGL scenes
Brian Farrimond
Robina Hetherington

2. What we shall do Hidden line removal Lighting and shading Materials
Enable nearer objects to hide distant objects
Lighting and shading
Improve the sense of depth with shading
Materials
Add colour
Importing 3DSMax models
Add realistic models

3. Hidden line removal
Unless told otherwise, OpenGL draws on top of other objects
regardless of how far away they are
Suppose we want to draw this 

4. First attempt

5. The effect
Because OpenGL draws the cube then the sphere, the sphere is drawn on top even though it is further away
Ex07

6. Hidden line removal
OpenGL uses depth-buffering (also known as z-buffering)
Depth buffer is created
It records for each pixel, the distance from the viewer
Initially all set to a very large value

7. Hidden line removal As each object drawn each pixel is generated
its distance from the viewer is compared with the corresponding value in the depth buffer
If smaller than the value already there then
Pixel is updated
Its distance is recorded in the depth buffer
Otherwise
Pixel is not drawn

8. Coding in OpenGL
1. In main – create a depth buffer

9. Coding in OpenGL
2. In reshape – enable depth testing

10. Coding in OpenGL
3. In display – set the depth buffer to high values

11. Results – Ex08

12. Lighting 3D drawing aims to look realistic
Realism includes realistic lighting effects
Light in the real world is very complicated
Optics is a whole branch of Physics
Light involves quantum mechanics!
OpenGL uses simplified light calculations
Results are acceptable

13. Default lighting OpenGL default is to have no lighting
Switch on the lighting with:
glEnable(GL_LIGHTING);
OpenGL has 8 lights available named GL_LIGHT0, GL_LIGHT1, .., GL_LIGHT7
Switch on a light like this:
glEnable(GL_LIGHT0);

14. Ex09
Here is init modified to switch on GL_LIGHT0

15. Ex09
Ex08 – no lighting
Ex09 – with lighting

16. Ex09 Sphere has its polygons visible
No colours – glColor3f is ignored in a lit scene

17. Setting the shade model
We can make the sphere show its polygons with
glShadeModel(GL_FLAT);
When drawing a polygon, OpenGL chooses one of the polygons vertices and colours all the polygon's pixels the same colour as this vertex
Put the command into init

18. Setting the shade model
We can make the sphere look smoother with
glShadeModel(GL_SMOOTH);
When drawing a polygon, OpenGL computes a colour for each vertex then colours the polygon's interior pixels by interpolating the vertex values to provide the smooth effect.
This is the OpenGL default.
Put the command into init

19. Defining materials to get colour
When we use lights we need to specify an object’s colour in a more sophisticated way
Interaction of a surface with light is scary physics
In OpenGL we simplify by using the concept of material properties

20. Lights with colour

21. OpenGL material properties
Material property
Description
Ambient
Light scattered by the rest of the scene. It is omnidirectional.
Diffuse
Reflected light from light sources. The more directly the surface faces a light, the more light is reflected.
Emissiveness
Light generated by the object. Used mainly to simulate lamps or the Sun in a scene
Specular
Specular reflection produces highlights which are brightest at the angle of reflection between the light and the viewpoint.
Shininess
The brightness and sharpness of the highlight.

22. Specifying diffuse colour
Defining colours
as 4 element arrays of floating point numbers:
red,
green,
blue,
alpha

23. Specifying diffuse colour
Each array element is a floating point number

24. Specifying diffuse colour
Indicates the variable is an array instead of a single value

25. Specifying diffuse colour
Puts values into the array

26. Specifying diffuse colour
Setting the colour for the red cube

27. Specifying diffuse colour
Setting the colour for the green sphere

28. Notes We need to use arrays to define material colours
GL_FRONT specifies that the colour should be applied to the front of the object’s polygons
Alternatives are GL_FRONT and GL_FRONT_AND_BACK

29. Defining the light Default colour is white
Default position is (0, 0, 1)
Change the position like this …

30. Array containing x, y, z coords of light position
Defining the light
Default colour is white
Default position is (0, 0, 1)
Change the position like this …
Array containing x, y, z coords of light position
plus w value
w = 0 : light at specified point
w = 1 : light is at infinity in direction from origin through (x, y, z)

31. Light is moved to the new position
Defining the light
Default colour is white
Default position is (0, 0, 1)
Change the position like this …
Light is moved to the new position

32. Nate Robbins tutorial
LightMaterial

33. ExLoad3DS
Illustrates changing the light position interactively

34. Using 3DS Models in OpenGL
Complex models can be imported into OpenGL if the file format is understood
3DSMax has a facility for exporting models as .3ds files.
Web sites contain clues as to the structure of this file format
In this module we shall use the vertex and polygon information found there

35. Using 3DS Models in OpenGL
We need to
Load the 3DS data into a suitable data structure
Use the data structure to draw OpenGL polygons

36. Procedure: adding files
Put a copy of the files
load3dsbtf.cpp
load3dsbtf.h
into your project folder
Add load3dsbtf.cpp to your project tree

37. Procedure: programming the OpenGL
Add a variable that forms the data structure
Use the call loadBTF3DS to load a model from the .3ds file
Use the call display3DSObject to display the model in OpenGL

38. Include file with definitions needed for 3DS
ExLoad3ds
Include file with definitions needed for 3DS

39. 3DS file may contain more than one object
ExLoad3ds
3DS file may contain more than one object

40. Array of pointers to 3ds object data structures
ExLoad3ds
Array of pointers to 3ds object data structures

41. Count of the number of 3ds objects
ExLoad3ds
Count of the number of 3ds objects

42. A function to set the colour

43. Here we load the 3DS object from its file
init
Here we load the 3DS object from its file

44. display

45. Display each of the objects read from the 3DS file