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Hidden Surfaces Removal

Hidden Surfaces Removal. Prof. Ravindra R Patil (15CS62). Visible-Surface Detection. Problem: Given a scene and a projection, what can we see?. Visible-Surface Detection. Two main types of algorithms: Object space: Determine which part of the object are visible

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Hidden Surfaces Removal

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  1. Hidden Surfaces Removal Prof. Ravindra R Patil (15CS62)

  2. Visible-Surface Detection Problem: Given a scene and a projection, what can we see?

  3. Visible-Surface Detection Two main types of algorithms: Object space: Determine which part of the object are visible Image space: Determine per pixel which point of an object is visible Object space Image space

  4. Visible-Surface Detection Four algorithms: • Back-face elimination • Depth-buffer • Depth-sorting • Ray-casting But there are many other.

  5. N V Back-face elimination 1 We cannot see the back-face of solid objects: Hence, these can be ignored

  6. Back-face elimination 2 We cannot see the back-face of solid objects: Hence, these can be ignored V N

  7. Partially visible front faces Back-face elimination 3 • Object-space method • Works fine for convex polyhedra: ±50% removed • Concave or overlapping polyhedra: require additional processing • Interior of objects can not be viewed

  8. Normalized view volume front = visible pixel Depth-Buffer Algorithm 1 • Image-space method • Aka z-buffer algorithm Algorithm: Draw polygons, Remember the color most in front. yv xv zv View plane

  9. Sorting Depth-Buffer Algorithm 2 varzbuf: array[N,N] ofreal; { z-buffer: 0=near, 1=far } fbuf: array[N,N] ofcolor; { frame-buffer } For all 1<= i, j <=N do zbuf[i,j]:= 1.0; col[i,j]:= BackgroundColour; For all polygons do { scan conversion } For all covered pixels (i,j)do Calculate depth z; If z < zbuf[i,j]then { closer! } zbuf[i,j] := z; fbuf[i,j] := surfacecolor(i,j);

  10. polygon scan line y x x+1 Depth-Buffer Algorithm 3 Fast calculation z: use coherence. display

  11. Depth-Buffer Algorithm 4 + Easy to implement + Hardware supported + Polygons can be processed in arbitrary order + Fast: ~ #polygons, #covered pixels - Costs memory - Color calculation sometimes done multiple times - Transparancy is tricky

  12. END

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