Image analysis – Color image processing
Reexamination Certificate
1999-05-06
2002-07-16
Mehta, Bhavesh (Department: 2624)
Image analysis
Color image processing
C382S166000
Reexamination Certificate
active
06421460
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is related to the compositing or blending of graphical image data.
Many graphics or image processing computer programs, such as Adobe® Photoshop®, available from Adobe Systems Incorporated, of San Jose, Calif., build a final image by compositing two or more image layers together. The image layers may be thought of as stacked sheets of acetate. The density of the ink on the acetate controls the transparency of the sheet, i.e., the extent to which that sheet obscures the images from the underlying sheets. In the computer program, the color and density of the ink on the acetate sheet are represented by a color value and an alpha value (representing opacity), respectively. Normally, an alpha value of zero means the corresponding region is fully transparent, and a value of one means it is fully opaque.
Each layer typically includes image data and compositing controls. Typically, the image data is represented by an array of pixels, with each pixel having a color and, optionally, an opacity. Similarly, if there is a mask, the mask is represented by an array of pixels, with each pixel having an opacity. Alternatively, the image data or the mask or both can be defined analytically, e.g., by using shape outlines, or by other functions which map positions to color and opacity. In addition, the image data and the mask can be dynamic, that is, computed from other data at the time the layers are composited.
The compositing controls may include a transfer mode, also known as a blending mode. The transfer mode of an image layer determines how the color in the image layer mixes with the color accumulated from the underlying layers in the same image position. More specifically, it is a recipe for blending colors that can be expressed as a function taking an upper color and lower color to produce a third color. Image manipulation computer programs, such as Photoshop®, generally provide a wide variety of predefined transfer modes. The basic transfer mode equation is:
new_color=&agr;·
T
(lower_color, upper_color)+(1−&agr;)·lower_color
where &agr; is the opacity of the upper layer, T is the selected transfer mode function, and lower_color and upper_color are the color values of the lower layer (which in some contexts is called the accumulation layer) and the upper layer for the graphical element whose color new_color is being calculated. The color values are not pre-multiplied by the opacity. Transfer modes should not be confused with the well-known Porter-Duff compositing operations, which define how the alpha values determine how much of the blended colors survive in the result, but which assume essentially normal mode color mixing. T. Porter and T. Duff, “Compositing Digital Images”, SIGGRAPH 84, pp. 253-59 (1984).
The following table shows examples of transfer mode functions T. Each of the example transfer modes works componentwise; that is, each new colorant value is calculated based solely on the corresponding argument colorant values and independently of the values of different colorants. However, in the general case, transfer mode functions are not so limited. For example, there are hue, color, saturation, and luminosity modes that are based on doing a conversion to a hue, saturation, and luminance form (not exactly the same as either HSL or HSB because luminance is computed as a weighted average of red, green, and blue or cyan, magenta, and yellow multiplied by black), taking an appropriate mixture of the components from the upper and lower colors, and then performing a hue preserving clip to convert the color back into the RGB or CMY form. As is conventional, component colorants are treated as ranging from zero to one, with zero representing the darker end of the range, although in a typical implementation the actual values representing zero to one generally range across the values available at the colorants' bit depth. The exact effect of the blending mode will depend on the color space. The convention that zero is darker than one for each colorant means that 100% ink in CMYK is represented by zero and 0% ink is represented by one, whereas in RGB, no phosphor excitation is represented by zero and full excitation is represented by one.
T (L, U)
Formula
Normal (L, U)
= U
Multiply (L, U)
= L · U
Screen (L, U)
= U + L − U · L = 1 − (1 − U) · (1 − L)
Difference (L, U)
= |U − L|
ColorDodge (L, U)
= Min (1,L/(1−U)), with 0/0 treated as 0
ColorBum (L, U)
= 1 −Min (1, (1 − L)/U), with 0/0 treated as 0
Darker (L, U)
= Min (L, U)
Lighter (L, U)
= Max (L, U)
Add (L, U)
= Max(1 ,L + U)
Overprint (L, U)
= 1 −Add(1 − L,1 − U)
SofiLight (L, U)
= L − (1 − 2 · U) · L · (1 − L) if U ≦ 0.5
= L + (2 · U − 1) · (D(L) − L) if U ≧ 0.5,
where
D(x) = ((16 · x − 12) · x + 4) · x if x ≦
0.25 = sqrt (x) if x ≦ 0.25
Image layers are generally composited in order from bottom to top. The general process for compositing an image layer begins with calculation of any dynamic data in the image layer, such as the color of the pixels in the image and the opacity of the pixels in the mask. Then, the opacity is determined for each pixel from the mask, the global opacity, if any, and the image data. Finally, the color of each pixel in the layer is combined with the color of the corresponding pixel in an accumulation buffer to generate a new composited color. The combination is controlled by the opacity of the pixel and the transfer mode.
Some computer programs, in addition to having predefined transfer modes, also give users the ability to define their own transfer modes. One such system is described in commonly-owned U.S. patent application Ser. No. 09/008,269 filed Jan. 16, 1998 to Hamburg, incorporated here by this reference, which discloses methods and apparatus, including computer programs, for use by a user to create new transfer modes (called compound transfer modes) based on existing (or newly-created) transfer modes and a scalar parameter, as appropriate.
In this specification, the terms color, alpha, and graphic element are used. A color is a representation of a particular color. The representation can be in any form suitable for computation and need only have the property that colors can be interpolated. Alpha is a quantity that characterizes the opacity of a graphic element. An alpha value of zero indicates total transparency. An alpha value of one indicates complete opacity. When working with graphic elements described with a color and alpha value, the color is free to be undefined or arbitrary if the alpha value is zero. A graphic element is a piece of the compositing process. Two graphic elements each having a color and alpha value are composited to produce a third graphic element. In a raster context, the primitive graphic element is a pixel. In a vector graphics context, it could be a region of solid color, such as a region defined by a PostScript path.
Raster accumulation is one example of compositing. In raster accumulation, the primitive graphic element is the pixel. The accumulation buffer is a collection of pixels arranged in a regular pattern—generally a grid. The elements to be accumulated are also collections of pixels. Every pixel has a color and may have an alpha value. To composite a single element into the accumulation buffer, each pixel in the accumulation buffer is composited with the corresponding pixel in the element to produce a new value for the accumulation buffer pixel. A stack of elements is composited by initializing the accumulation buffer and then compositing each element from bottom to top into the accumulation buffer. The final contents of the accumulation buffer represent the final image.
At least one prior art system provided compositing with transfer mode T for color blending without alpha in an accumulation buffer (i.e., with an implicit alpha va
Adobe Systems Incorporated
Do Anh Hong
Fish & Richardson P.C.
Mehta Bhavesh
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