Back face culling

Normals for individual faces are compared with the view vector. Visible surfaces are those whose intersection with the view vector forms an angle of less than 90 degrees. Those which intersect at angles of 90 degrees or more are flagged as back facing. Back facing polygons then are not considered for rendering purposes.

• Z buffering
  
  • In addition to color information, Z value is maintained for each pixel
    
  • All Z values are initialized to far clipping plane
    
  • All polygons are drawn
    
  • As each pixel for each polygon is drawn, its Z value is compared to that already in the Z buffer
    
  • If the new Z value is nearer to the viewer than the existing Z value, the pixel drawn
    
  • If the new value is further away than the existing Z value, the pixel is ignored
    
  • Z buffering can be done incrementally
    
  • Drawing order is unimportant (except in the case of a ``tie'')
    
  • Z buffering is widely implemented in both software and hardware
    
  • Z buffering does a good job of rendering visible surfaces
    
  • Z buffering does not address how light interacts between objects
    
    
    

• Ray tracing
  
  • A ray is traced from the eye location through each pixel location into the scene.
    
  • The ray projects into the scene until it encounters a surface.
    
  • If the surface is reflective, a portion of the ray bounces off the surface as determined by the surface properties, and continues into the scene until it encounters another surface.
    
  • If a reflected ray encounters another surface before encountering a light source, the original surface is in shadow.
    
  • If the surface is transparent, a portion of the ray passes through the surface and continues into the scene.
    
  • Each surface encountered contributes a proportion of the color that ultimately will be applied to the pixel through which the ray was traced.
    
  • When the ray has encountered the number of surfaces specified by the trace depth, a weighted average of all surface properties encountered is applied to the pixel in question. The process is repeated for each pixel in the rendered image.
    
    
• Ray tracing is used to simulate:
  • inter-object reflection
  • refractive materials
  • object shadows
    
• Ray tracing is not very efficient in simulating diffuse lighting
  

• Radiosity - In addition to light originating from primary sources, light that reflects from surfaces in the scene is considered as secondary source light in the rendering scheme.
  
  
  
  
  
• Scene is decomposed into polygons
  
• The portion of light from each polygon that reaches each other polygon is computed - form factors
  
• All polygons except light sources are set to an unilluminated state
  
• Light from source polygons cascades through the scene - surface polygons reflectively illuminate more surface polygons, etc.
  
• Process continues for number of steps set by algoritm or user - final apparent color values are obtained
  
• Polygons are drawn, usually with Z buffer algorithm
  
• Radiosity can be thought of as a color pre-processing step rather than a rendering algorithm per se
  

Volume rendering

Stanford Computer Graphics Laboratory

Back to 3D Color outline

This file was last modified on January 29, 2002.