Improving the appearance of 3D OpenGL scenes

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Improving the appearance of 3D OpenGL scenes

• Brian Farrimond
• Robina Hetherington

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What we shall do

• Hidden line removal
• 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

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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 -----------?

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First attempt
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The effect

• Because OpenGL draws the cube then the sphere,
  the sphere is drawn on top even though it is
  further away
• Ex07

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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

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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

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Coding in OpenGL

• 1. In main create a depth buffer

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Coding in OpenGL

• 2. In reshape enable depth testing

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Coding in OpenGL

• 3. In display set the depth buffer to high
  values

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Results Ex08
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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

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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)

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Ex09

• Here is init modified to switch on GL_LIGHT0

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Ex09
Ex08 no lighting

• Ex09 with lighting

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Ex09

• Sphere has its polygons visible
• No colours glColor3f is ignored in a lit scene

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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

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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

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Defining materials to get colour

• When we use lights we need to specify an objects
  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

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Lights with colour
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OpenGL material properties
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Specifying diffuse colour
Defining colours as 4 element arrays of floating
point numbers red, green, blue, alpha
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Specifying diffuse colour
Each array element is a floating point number
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Specifying diffuse colour
Indicates the variable is an array instead of a
single value
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Specifying diffuse colour
Puts values into the array
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Specifying diffuse colour
Setting the colour for the red cube
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Specifying diffuse colour
Setting the colour for the green sphere
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Notes

• We need to use arrays to define material colours
• GL_FRONT specifies that the colour should be
  applied to the front of the objects polygons
• Alternatives are GL_FRONT and GL_FRONT_AND_BACK

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Defining the light

• Default colour is white
• Default position is (0, 0, 1)
• Change the position like this

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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)
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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
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Nate Robbins tutorial

• LightMaterial

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ExLoad3DS

• Illustrates changing the light position
  interactively

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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

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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

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Procedure adding files

• Put a copy of the files
• load3dsbtf.cpp
• load3dsbtf.h
• into your project folder
• Add load3dsbtf.cpp to your project tree

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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

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ExLoad3ds
Include file with definitions needed for 3DS
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ExLoad3ds
3DS file may contain more than one object
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ExLoad3ds
Array of pointers to 3ds object data structures
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ExLoad3ds
Count of the number of 3ds objects
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A function to set the colour
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init
Here we load the 3DS object from its file
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display
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display
Display each of the objects read from the 3DS file