Image analysis – Color image processing
Reexamination Certificate
1997-06-06
2001-10-09
Boudreau, Leo (Department: 2621)
Image analysis
Color image processing
C382S163000, C382S284000, C348S586000, C345S111000
Reexamination Certificate
active
06301382
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for “pulling” or extracting an image of one or more foreground objects from a composite image in which the foreground objects are in front of a background, and more particularly, to a method for pulling the one or more foreground objects from recorded images of the one or more objects disposed in front of two differently colored backgrounds.
BACKGROUND OF THE INVENTION
In the motion picture industry, pulling a matte of a foreground object from a scene, i.e., extracting an image of the object from a background, and combining (compositing) the pulled matte with another entirely different background has proven to be difficult. Historically, an image of a foreground object was pulled with a matte created from a strip of monochrome film that was transparent for those portions of a scene that contained the desired object and opaque everywhere else. In this way, when the matte was placed together with another strip of film that included the complete scene (foreground object and background), only the foreground object would be illuminated (visible on a projection screen). A holdout matte is the complement of a matte because it is opaque for those portions of a scene that contain the foreground object(s) and transparent everywhere else. Thus, the holdout matte may be employed to pull a matte of the image of the background so that another foreground object may be combined with the pulled matte to create a new composite image. Furthermore, a matte and a holdout matte may have regions that are partially transparent, so that the foreground object and/or the background may be partially illuminated.
New scenes are often created by compositing an extracted (uncomposited) foreground object with a separately recorded background. A significant benefit of compositing is that if the foreground object needs alteration, the entire scene does not have to be re-recorded with the background. Instead, the individual foreground object may be manipulated and separately re-recorded. Also, if an object is not recorded with a camera, but has been optically scanned into a video memory, compositing may be the only way to incorporate the object into a composite image of a desired scene.
In the prior art, several techniques have been employed to extract an image of a foreground object from a background with varying degrees of success. One technique employs special types of light sources, such as sodium, infrared, and ultraviolet, to simultaneously record multiple images of a foreground object disposed over a single background on multiple strips of film. The multiple film images provide information about particular attributes of the foreground object so that the object's matte may be more easily pulled. However, employing special light sources to simultaneously record multiple images of a foreground object on different strips of film with multiple cameras or with a single camera that is modified to record on multiple strips of film simultaneously is difficult and expensive.
Another prior art technique teaches recording one image of a foreground object onto a single strip of film. The technique employs a uniform backing color behind a foreground object to pull a matte of the object. Petros Vlahos, an early pioneer of the motion picture and video industries, invented the Color Difference Technique™, which is the most commonly employed single film image technique for pulling a matte of a foreground object. Vlahos developed this technique so that editors in the film industry could inexpensively produce an image of separately recorded foreground objects combined with a new background. The technique provides for recording an image of a foreground object(s) in front of a background that has a pure and uniform color, such as blue, green, or yellow. Since blue is the most frequently employed pure and uniform color for backgrounds, the Color Difference Technique™ is commonly referred to as “blue screen matting.” Next, an operator manipulates a set of controls to pull a matte of those portions (foreground objects) of the image that do not contain the pure and uniform color. The pulled matte is the image of the foreground object and the matte may be combined with another separately recorded background to create a new composite image. Thus, the Color Difference Technique™ enables the separate recording of a foreground object within a film studio, so that a pulled matte of the object may be added to a new background at a later date. This aspect of the Color Difference Technique™ can be particularly important when creating a composite image that has a background, which is inherently expensive or dangerous to produce.
One disadvantage of the Color Difference Technique™ is that a background having a pure and uniform color is expensive to manufacture, and the impurities in the color of the background will adversely effect the quality of the matte pulled of the foreground object. Another disadvantage is that the coloring of the foreground object is restricted to those colors not represented in the background. Since the Color Difference Technique™ can only pull a matte of those portions of a foreground object that do not have the same color as the background, the matte will have a “hole” in any portion of the object that includes the background's color. For example, if a user of the Color Difference Technique™ employs a blue background to pull a matte of an actor having blue eyes of the same blue color as the background, the eyes of the actor will appear as transparent holes in the matte. Thus, the Color Difference Technique™ is an imperfect solution to the problem of pulling a matte of a foreground object.
Vlahos also developed an apparatus, the Ultimatte™, that applied the concepts of the Color Difference Technique™ to the production of video images. In addition to determining the color coordinates for each point (pixel) of an uncomposited foreground object, the Ultimatte™ provides for calculating an alpha channel of the object so that a digital matte may be accurately pulled of the object for video. The alpha channel is the digital equivalent of the holdout matte that is used to remove a foreground object from a scene in the film industry. However, the alpha channel is used in video to provide shape and transparency to a foreground object. A zero value in the alpha channel value for a pixel causes the pixel to be transparent, and as the alpha value approaches 100%, the pixel becomes increasingly more opaque, at the color specified for the pixel.
SUMMARY OF THE INVENTION
In accord with the present invention, a method is defined for extracting an image of a foreground object from composite digitized images, using a computer. The method includes the steps of recording a first composite image in which the foreground object is disposed in front of a first background having a first arbitrary coloring. A second composite image is then recorded that includes the foreground object disposed in front of a second background having a second arbitrary coloring that is different than the first arbitrary coloring. A set of color coordinates are obtained for each point in the first composite image and in the second composite image; the color coordinates define a color for each point. Finally, the set of color coordinates for each point in each of the first and the second composite images is triangulated, returning an alpha value and a set of color coordinates for each point in the image of the foreground object, to define the image of the foreground object apart from any background.
One of the features of the present invention is that the image of the foreground object may include a shadow of the foreground object that is cast on the first and the second backgrounds.
REFERENCES:
patent: 4100569 (1978-07-01), Vlahos
patent: 4835532 (1989-05-01), Fant
patent: 5140416 (1992-08-01), Tinkler
patent: 5194941 (1993-03-01), Grimaldi et al.
patent: 5249039 (1993-09-01), Chaplin
patent: 5343252 (1994-08-01), Dadourian
patent: 5355174 (1994-10-01), Mishima
patent: 5428401 (1995-06
Blinn James F.
Smith Alvy Ray
Anderson Ronald M.
Boudreau Leo
Microsoft Corporation
Sherali Ishrat
LandOfFree
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