Computer Graphics Techniques and Applications

1
Computer Graphics Techniques and Applications

• Chapter 4 - Surfacing a Model
• Dr Brett Stevens
• e-mail Brett.Stevens_at_port.ac.uk
• phone [phone]
• address West Wing, Eldon Building
• notes http//techfaculty.port.ac.uk/cgtap

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Last Two Weeks

• Polygon Modelling
• Spline Modelling

• Both include the shape of an object but not its
• Material
• Light source
• Viewpoint

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The Surface of an Object.

• Modelling defines the shape, that a surface
  should fit to, but we still need to know which
  way the surface points.

Normal Vectors specify this.
4
The Surface has Materials

• Surfacing or Shading
• The application of a material to a surface.
• This Includes
• Self-illumination
• Transparency
• Diffuse and Specular Reflections
• Opacity
• Bump Maps
• Displacement Maps

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The Simplest Scene

• A self-luminous object.
• Not visually descriptive.

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Objects with External Lights

• An external light source, shows
• Colour (even if its black or white)
• Structure, through a pattern of shading.

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Reflection

• Shading is a by product of reflection.

Two end points on the reflection continuum
Diffuse (Perfectly Matt) Specular (Perfectly
Shinny)
A material is constructed by combining both 1
other factor
I Iaka IdkdCos(q) IsksCosn(a)
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Specular Diffuse Reflection

• Combination schemes.
• Phong Diffuse objects
• Blinn Specular objects

• The colour of each component can be adjusted,
  along with the spread of the specular component.

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Texture Maps

• Rare to find a flat coloured surface.
• A texture map is a purely visual method of
  varying the colour pattern of an object.
• Its part of the material not something separate.

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Procedural Texture Maps

• Mathematically generated texture maps.
• Fully Scalable and editable.
• Not very detailed and requires some skill to
  master.

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Bitmapped Texture Maps

• The u,v,w coordinates of a 2D bitmap are
  projected onto the x,y,z coordinates of an
  object.
• Allows for fine control of an objects surface
  appearance but requires additional 2D software.

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Global Lighting Algorithms

• Phong and Blinn only consider local reflections
  from a light source.
• Light bounces between objects as well.

• Ray-tracing follows the light from source to
  eye (or the other way around).
• Phong or Blinn still used on the object but
  ray-tracing follows the unused reflections to
  see if they do anything useful.

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Standard vs Ray-Tracing
3 Seconds
15 Seconds
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Transparency
The Petri-Dish was clearly clear.!

• Transparency can be added to the material like
  the diffuse or specular component, either as a
  uniform distribution or as a local map.

• Transparency affects the diffuse and specular
  component

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Displacement Maps

• Actually alters the location and orientation of a
  surfaces faces.
• Useful for creating large, natural, shapes or
  objects.
• A procedural or bitmap can be used.

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Bump Maps

• Normal vector perturbation mapping
• Uses a Height map to simulate texture.
• Dont get confused with a texture map.!!!
• Part of the Material still.

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Normal Maps

• Normal Perturbation are pre-computed and stored
  in RGB space.
• Still look flat at glancing view angles to the
  surface.

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Parallax Maps

• or Virtual Displacement Mapping

Includes depth cue information for
motion-parallax by offsetting pixels. The height
map used to calculate the displacement from the
viewpoint.
Parallax occlusion mapping
The future
Z-correct bump mapping
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Lighting

• Spotlights or omni-lights.
• The omnis intensity and colour can be altered.
• Same for spot light but it can also adjust its
  light cone and falloff to narrow its beam.
• Unlike real life, lights can exclude particular
  objects.

• Lighting is complex and requires many more lights
  than would seem logical.

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Cameras

• A camera can alter their visual properties
  almost at will. The depth of field, and the
  field of view can both be changed.

• Typically changed with a lens metaphor, although
  
• Manual editing is also possible.

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Rendering

• Rending is the process of calculating the diffuse
  and specular colour, the transparency, the
  ray-tracing, the bump or displacement mapping,
  and clipping it all into the view volume of the
  camera in order to create a 3D image of the
  scene.
• The rendering process also calculates whether the
  final view will be Parallel or a perspective
  projection.

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Polygons the Graphics Pipeline
Object coordinates ? world coordinates (world
space) ? the viewing volume (view space) ?
projected onto a 2D surface (projection space) ?
rasterised (converted to a pixel representation)
? the frame buffer.
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Render Effects

• Some effects can only be added during rendering

• The more objects, material effects, and lights
  used, the slower the rending process.

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Stereoscopic Rendering

• Toe-in
• Off-axis

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Tutorial