Image Compositing and Matting

Introduction Matting and compositing are important operations in the production of special effects. These techniques enable directors to embed actors in a world that exists only in imagination, or to revive creatures that have been extinct for millions of years. During matting, foreground elements are extracted from a film or video sequence. During compositing, the extracted foreground elements are placed over novel background images.

Traditional approaches to matting Traditional approaches to matting include blue-screen matting and rotoscoping. The former requires filming in front of an expensive blue screen under carefully controlled lighting The latter demands talent and intensive user interaction.

Rotoscoping Rotoscoping --- the process of tracking contours in a video sequence

Basic problem of blue screen matting Given an image of a foreground object shot in front of a backing color (blue screen or green screen or other colors) Obtain a matte of the foreground object so that the foreground object can be blended into a new background image using the matte to produce a new composite image.

Notations Let C = [R, G,B] denote a color with 0 ≤ R, G, B ≤ 1. Let  denotes a transparency value with 0 ≤  ≤ 1. Foreground image color: C f = [R f,G f,B f ],  f = 1 Backing (screen) color: C k = [0, 0,B k ],  k = 1 (assuming blue screen) Original foreground object color: C o = [R o,G o,B o ] Background image color: C b = [R b,G b,B b ],  b = 1 Composite image color: C c = [R c,G c,B c ]

Problem Statement Given C f and C b at corresponding pixels, and C k a known backing color, and assuming C f =  o C o + (1 −  o )C k Determine  o and C o, which then gives the composite color C c =  o C o + (1 −  o )C b at the corresponding point, for all points that C f and C b share in common.

Solution Since C f =  o C o + (1 −  o )C k, we have: 4 unknowns, 3 equations… R f =  o R o G f =  o G o B f =  o B o + (1 −  o )B k.

Case 1: No Blue There is no blue in C o, i.e., B o =0, and B k ≠0. Then This case is very restrictive. It rules out many colors, including grays because grays have blue.

Case 2: Gray and Skin Color Assume R o =aB o (or G o =aB o ) and B k ≠0 Then where

Example 1: Gray Then This case applies to science fiction movie in which the spaceships are mostly gray.

Example 2: Skin Color This case applies to human faces, hands, legs, etc.

Solution for General Cases To solve the matting problem in general: Need to take the same image with two different backing colors. This gives four equations for solving the four unknowns Case 1: Use Two Different Shades of Blue. Case 2: Use Two Different Backing Colors.

Case 1 Use two different shades of blue B k1 and B k2 as backing colors. Then so

Case 2 Use two different backing colors C k1 and C k2. Or Over-constrained, 1 unknowns, three equations

Case 2: Solution 1 Adding up 3 equations:

Case 2: Solution 2 Apply least squares method Define: Then Least-squares

Least-Squares Set

Case 1 Example

Case 2 Example

Other Developments in Matting Matting Without Blue Screen A method proposed by Ruzon and Tomasi User specify object region and boundary region. Alpha value of object region is set to 1. Alpha value of boundary region is computed by estimating the contributions of neighboring objects’ colors.

Example

Bayesian Approach

Shadow Matting Pull a matte of shadow. Acquire photometric and geometric properties of the target scene by sweeping oriented linear shadows across it. Then, composite the shadow onto the scene.

Example

Environmental Matting Left: alpha matte. Middle: environment matte. Right: photo.

References 1. Y.-Y. Chuang, B. Curless, D. H. Salesin, and R. Szeliski. A bayesian approach to digital matting. In Proc. IEEE CVPR, pages II–264–II–271, Y.-Y. Chuang, D. B. Goldman, B. Curless, D. H. Salesin, and R. Szeliski. Shadow matting and compositing. ACM Transactions on Graphics, 22(3):494–500, July M. A. Ruzon and C. Tomasi. Alpha estimation in natural images. In Proc. IEEE CVPR, pages 18–25, A. R. Smith and J. F. Blinn. Blue screen matting. In Proc. ACM SIGGRAPH, pages 259–268, D. E. Zongker, D. M. Werner, B. Curless, and D. H. Salesin. Environment matting and compositing. In Proc. SIGGRAPH, pages 205–214, 1999.

Digital Compositing Digital compositing means “digitally manipulated integration of at least two source 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.

Example 1

Example 2

More examples op.htm op.htm

Main Topics Alpha blending: blending foreground and background Keying: separating foreground and background Luma, chroma, difference keying Rig removal: removing unwanted elements

Alpha Blending C = [α F + (1 – α) B] 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. If α ranges from 0 to 255, then the formula becomes: C = [α F + (1 – α) B] / 255

Example: No Background

Example: With Background

Note For shadow,  must take fractional value (0 < α < 1). Otherwise, shadow looks unreal.

Boundary area  at boundary area should also be fractional. Otherwise, have dark fringes; unrealistic.

Summary A good matte has fractional  in shadow, and along object boundaries and shadow boundaries.

Keying Separating foreground from background, 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 fractional a in shadow, and along object boundaries and shadow boundaries.

Basic methods Luma keying: based on luminance (i.e., intensity) Chroma keying: based on color (i.e., blue screen, green screen) Difference keying: requires a clean plate, i.e., a background image without the foreground element.

Basic Idea Compute difference between foreground and background (based on luma, chroma, or color) Very small diff   = 0. Very large diff   = 1. Intermediate diff  intermediate 

Luma Keying Key out the background based on luminance. Useful when background has a uniform luminance that is very different from foreground luminance.

Result

Chroma Keying Key out the background based on color. Useful when background has a uniform color that is very different from foreground color. Example: Image shot with blue screen.

Characteristics of blue screen image

Difference Keying More general than luma and chroma keying. Key out background based on pixel-wise color difference between foreground and background footage.

Final Composition

Rig Removals Rigs are equipment that support the actors or the props. Sometimes, rigs cannot be removed by keying alone. So, have to apply masking technique to remove rigs. Need clean plate of background footage. If camera moves, then need motion-controlled camera: Computer controls camera to move the same way twice: Without foreground objects; get clean plate. With foreground objects.

Basic Idea Apply a mask to mask out the rig. Then, replace pixels in masked area by corresponding pixels in clean plate background. If rig moves in footage, then have to animate the mask accordingly.