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Shadows

Shadows. CS418 Computer Graphics John C. Hart. Shadowing. Shadows indicate light occlusion Fix object positions relative to each other. Perspective Distortion. screen. y. y view. y clip. -z. z view. 1. Shadow Buffer. Williams, SIGGRAPH 78 Render scene twice from viewpoint

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Shadows

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  1. Shadows CS418 Computer Graphics John C. Hart

  2. Shadowing • Shadows indicate light occlusion • Fix object positions relative to each other

  3. Perspective Distortion screen y yview yclip -z zview 1

  4. Shadow Buffer • Williams, SIGGRAPH 78 • Render scene twice from viewpoint (x,y,z)screen = WPV(x,y,z)world • Once with light source on • Once only with ambient • Save a z-buffer • Render scene from light position (x,y,z)light= WlPlVl(x,y,z)world • Save only z-buffer • For each viewpoint image pixel compute (x,y,z)vis=WlPlVl V-1P-1W-1 (x,y,z)screen • If zvis < zlight at (x,y)vis= (x,y)light • Then pixel is in shadow RGB Depth Viewpoint Light Position

  5. Shadow Buffer • Williams, SIGGRAPH 78 • Render scene twice from viewpoint (x,y,z)screen = WPV(x,y,z)world • Once with light source on • Once only with ambient • Save a z-buffer • Render scene from light position (x,y,z)light= WlPlVl(x,y,z)world • Save only z-buffer • For each viewpoint image pixel compute (x,y,z)vis=WlPlVl V-1P-1W-1 (x,y,z)screen • If zvis < zlight at (x,y)vis= (x,y)light • Then pixel is in shadow

  6. Shadow Buffer • Williams, SIGGRAPH 78 • Render scene twice from viewpoint (x,y,z)screen = WPV(x,y,z)world • Once with light source on • Once only with ambient • Save a z-buffer • Render scene from light position (x,y,z)light= WlPlVl(x,y,z)world • Save only z-buffer • For each viewpoint image pixel compute (x,y,z)vis=WlPlVl V-1P-1W-1 (x,y,z)screen • If zvis < zlight at (x,y)vis= (x,y)light • Then pixel is in shadow

  7. Shadow Buffer • Williams, SIGGRAPH 78 • Render scene twice from viewpoint (x,y,z)screen = WPV(x,y,z)world • Once with light source on • Once only with ambient • Save a z-buffer • Render scene from light position (x,y,z)light= WlPlVl(x,y,z)world • Save only z-buffer • For each viewpoint image pixel compute (x,y,z)vis=WlPlVl V-1P-1W-1 (x,y,z)screen • If zvis < zlight at (x,y)vis= (x,y)light • Then pixel is in shadow

  8. Shadow Buffer • Williams, SIGGRAPH 78 • Render scene twice from viewpoint (x,y,z)screen = WPV(x,y,z)world • Once with light source on • Once only with ambient • Save a z-buffer • Render scene from light position (x,y,z)light= WlPlVl(x,y,z)world • Save only z-buffer • For each viewpoint image pixel compute (x,y,z)vis=WlPlVl V-1P-1W-1 (x,y,z)screen • If zvis < zlight at (x,y)vis= (x,y)light • Then pixel is in shadow

  9. Numerical Analysis

  10. Percentage Closer Filtering • Reeves, Salesin, Cook, SIGGRAPH 87 • Shadow aliasing occurs when numerical inaccuracy and discretization causes sample to compare to the wrong depth near an edge • Can be fixed by comparing sample in a neighborhood (e.g. 3x3 pixels) and filtering the binary shadow answer • Can also be extended to generate soft shadows

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