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Refresher on Viewing Transformation, cont.

• Clipping after VT is simply along planes defined by

  • (-1 x 1)

  • (-1 y 1)

  • (-1 z 0)

• Depth comparison before VT

  • Compare every two points, e.g., P1 and P2:
    perform calculation to determine if they are on same projector (a lot of work).

  • If (x, y) pairs are on same projector, then z-values are compared.

• Depth comparison after te VT

  • Compare every two points, e.g., P'1 and P'2:
    x'1 must = x'2, and y'1 must = y'2 for one of those points to obscure the other, i.e., to be on the same projector (not much work to perform necessary comparisons).

  • If the (x', y') pairs are equal then the z'-values are compared.

• Projection after VT is just discarding z-values!
  
  

Viewing & Rendering - 09 MAY 95

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