lynwood dunn (1904-1998) visual effects pioneer acme-dunn optical printer
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Lynwood Dunn(1904-1998)• Visual effects pioneer• Acme-Dunn optical printer
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The Over Operator
• How to indicate which parts of front picture are clear and which are opaque
• Use alpha channel to indicate opacity [Smith]
• Over operator [Porter & Duff S’84]• A over B:
CA over B = aA CA + (1 – aA) aB CB
aA over B = aA + (1 – aA) aB
• Note that aA CA used in color eqs,so store aA CA instead of CA
• A over B w/premultiplied alpha
CA over B = CA + (1 – aA) CB
aA over B = aA + (1 – aA) aB
B
A
C = (aR aG aB a)
25% transparent yellow
75% transparent blue
?
CA over B = .75(1,1,0) + .25(.25)(0,0,1)= (.75,.75,0) + (0,0,.125)= (.75,.75,.125)
aA over B = .75 + .25(.25) = .875CABW = .875(.75,.75,.125) +
.125(1)(1,1,1) = (.66,.66,.11)+(.125,…) =(.78,.78,.24)
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Academy of Motion Picture Arts & SciencesScientific and Engineering AwardTo Alvy Ray Smith, Tom Duff, Ed Catmull and Thomas Porterfor their Pioneering Inventions in Digital ImageCOMPOSITING.PRESENTED MARCH 2, 1996
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Accumulating Opacity• Depends on order of accumulation• Back to front
– Over operator
Cout = a C + (1 – a) Cin
aout = a + (1 – a) ain
– No need to maintain a• Front to back
– Under operator
Cout = ain Cin + (1 – ain) C
aout = ain + (1 – ain) a– Need to maintain a
Cin,ainCout,aout
C,a
Cout,aoutCin,ain
C,a
view
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Volume Rendering Methods• Data stored as 3-D array of samples
C – color
a – opacity
N – normal, etc.• Splatting
– “Forward:” voxels to pixels– Overlay planes from volume
• Ray casting– “Backward:” pixels to voxels– Ray marches from voxel to voxel
from SGI’s Volumizer, 1999