Z-Buffer Algorithm

• Perspective transformation maps viewing pyramid to viewing box in a manner that maps lines to lines
• This transformation also maps polygons to polygons
• Idea: When we scan convert, step in z as well as x and y.
• In addition to framebuffer, we'll have a depth buffer (or z buffer) where we write z values
• Initially, z buffer values set to

  Depth of far clipping plane (usually 1) will also suffice

• Scan convert using the following WritePixel:

  WritePixel(int x, int y, float z, colour)
  if ( z < zbuf[x][y] ) then
      zbuf[x][y] = z;
      frambuffer[x][y] = colour;
  end

• Runtime:
  • : number of scan converted pixels
  • n: number of polygons

Z-buffer in Hardware

Z-buffer is the algorithm of choice for hardware implementation

+

Easy to implement

+

Simple hardware implementation

+

Online algorithm (i.e., we don't need to load all polygons at once in order to run algorithm)

-

Doubles memory requirements (at least)

But memory is cheap!

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Scale/device dependent