Image Processing: Compositing, Alpha Channel, and Semi-Transparent Objects, Slides of Computer Graphics

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Image Processing

Overview

 Image compositing

 Blue-screen mattes

 Alpha channel

 Porter-Duff compositing algebra

 Image morphing

 Specifying correspondences

 Warping

 Blending

Blue-Screen Matting

 Composite foreground and background

images

 Create background image

 Create foreground image with blue background

 Insert non-blue foreground pixels into background

Alpha Channel

 Encodes pixel coverage information

 α = 0 : no coverage (or transparent)  α = 1 : full coverage (or opaque)  0 < α < 1 : partial coverage (or semi-transparent)

 Example : α = 0.

Partial coverage Semi-Transparent or

Pixels with Alpha

 Alpha channel convention :

 (r, g, b, α) represents a pixel that is α covered by the color C=(r/α, g/α, b/α)  Color components are premultiplied by α  Can display (r, g, b) values directly  Closure in composition algebra

 What is the meaning of the following?

Pixels with Alpha

 Alpha channel convention :

 (r, g, b, α) represents a pixel that is α covered by the color C=(r/α, g/α, b/α)  Color components are premultiplied by α  Can display (r, g, b) values directly  Closure in composition algebra

 What is the meaning of the following?

 (0, 1, 0, 1) = full green, full coverage  (0, ½, 0, 1) = half green, full coverage  (0, ½, 0, ½) = full green, half coverage  (0, ½, 0, 0) = no coverage

Semi-Transparent Objects

 Suppose we put A over B over background G

 How much of B is blocked by A?

 How much of B is shows through A?

α A A B G

Semi-Transparent Objects

 Suppose we put A over B over background G

 How much of B is blocked by A?

 How much of B shows through A?

 How much of G shows through both A and B?

α A (1 – α A

A B G

Opaque Objects

 How do we combine 2 partially covered pixels

 3 possible colors (0, A, B)  4 regions (0, A, B, AB)

B

A

A B

AB

Composition Algebra

 12 reasonable combinations

clear A B A over B B over A A in B

B in A A out B B out A A atop B B atop A A xor B

Image Composition Example

Jurassic Park

Overview

 Image compositing

 Blue-screen mattes

 Alpha channel

 Porter-Duff compositing algebra

 Image morphing

 Specifying correspondences

 Warping

 Blending

Cross-Dissolving

 Blend image with “over” operator

 Alpha of bottom image is 1.

 Alpha of top image varies from 0.0 to 1.

blend  i , j    1  t  src i , j   t dst i , j   0  t  1 

Image Morphing

 Combines warping and cross-dissolving