Composing Two Elements Background Holdout Matte Foreground Traveling

Composing Two Elements + = * Background + Holdout Matte = * Foreground Traveling Matte Copyright Ó Pat Hanrahan

Optical Printing From: “Industrial Light and Magic, ” Thomas Smith (p. 181) From: “Special Optical Effects, ” Zoran Perisic Copyright Ó Pat Hanrahan

“Pulling a Matte” - Matte Creation • From digitized images o Blue-screen matting (Petro Vlahos) n o Video or chroma-keying n o Range of chromaticities marked transparent Image processing n • Separate blue background from foreground image Set of colors marked transparent, region growing. . . From computer generated images o Coverage o Transparency Copyright Ó Pat Hanrahan

Blue Screen Copyright Ó Pat Hanrahan

Blue-Screen Matte Extraction • Given: C - Observed color at each pixel CB - Backing color (perhaps at each pixel) • Compute: C’F = (a. F RF, a. F GF, a. F BF, a. F) • G Matte Equation CF C = CF + (1 -a. F) CB • B=a. G C (3 equations in 4 unkowns) CB Vlahos Assumption K B Copyright Ó Pat Hanrahan

Porter-Duff Compositing Algebra C’ = FAC’A+FBC’B Open. GL blendfunction Specify src and dst F’s 0, 1, As, Ad, 1 -As, 1 -Ad, min(As, 1 -Ad), Cs, Cd, 1 -Cs, 1 -Cd, Copyright Ó Pat Hanrahan

Painting – how are each of these strokes done? Copyright Ó Pat Hanrahan