Digital Compositing
CS5245 Vision and Graphics for Special Effects
Leow Wee Kheng
Department of Computer Science
School of Computing
National University of Singapore
Leow Wee Kheng (CS5245) Digital Compositing 1 / 67
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Digital compositing means digitally manipulated integration of at least two images to produce a new image. The new image must appear realistic.
Typology: Lecture notes
2021/2022
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Digital Compositing
CS5245 Vision and Graphics for Special Effects
Leow Wee Kheng
Department of Computer Science
School of Computing
National University of Singapore
Introduction
Introduction
Digital compositing means digitally manipulated integration of at least two images to produce a new image.
The new image must appear realistic. It must be completely and seamlessly integrated, as if it were actually photographed by a single camera.
Introduction
Main Topics Keying Separate foreground elements from background elements. Rig Removal Remove unwanted elements in footage. Alpha Blending Blend foreground elements with new background footage.
Other topics: Refer to [Kel00].
Alpha Blending
Alpha Blending
Basic equation C = αF + (1 − α)B (1)
F : foreground image B: background image C : composite α: opacity or transparency matte: an image of α values The above operation is performed on each corresponding pixel. If α = 1, then C = F , foreground is shown, i.e., foreground is opaque. If α = 0, then C = B, background is shown, i.e., foreground is transparent. 0 < α < 1: semi-transparent, e.g., shadow, smoke, etc.
Alpha Blending
Example: With Background B.
foreground F matte α
background B composite C
Alpha Blending
Example: Foreground with shadow.
foreground F matte α
background B composite C
Alpha Blending
α at boundary should also take intermediate value. Otherwise, have dark fringes.
Good matte has intermediate α at boundary and in shadow.
Alpha Blending
Real images have smooth boundaries, no fringe.
Keying
Keying
Separate foreground elements from background elements, creating a matte of foreground. Also called pulling a matte (of foreground), or keying out (i.e., making transparent) background. Recall: A good matte has intermediate α in shadow and along object boundaries.
Keying
Basic methods Luma keying based on luminance, i.e., intensity. Chroma or color keying based on color, i.e., blue screen, green screen. Difference keying requires clean plate, i.e., background image without foreground element.
Color Keying with After Effects
Color Keying with After Effects
Example images from The Saint (1997).
Color Keying with After Effects
Create a comp with background below foreground in timeline panel.
Select foreground, then select “Effect → Keying → Color Key”. Use dropper to select Key Color. Set Color Tolerance amount.
Color Keying with After Effects
Results: Color keying in AE has limitations.
increase color tolerance
Color Keying with Keylight in After Effects CS
Color Keying with Keylight in After Effects CS
Keylight [Key] An Academy award winning blue and green screen keyer. Core algorithm was developed by the Computer Film Company. Further developed and ported to After Effects by The Foundry.
Use Keylight in After Effects CS Create a comp with background below foreground in timeline panel.