WO2005020130A2 - Procede d'accentuation de couleurs avec compensation et ajustement - Google Patents

Procede d'accentuation de couleurs avec compensation et ajustement Download PDF

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Publication number
WO2005020130A2
WO2005020130A2 PCT/US2004/005640 US2004005640W WO2005020130A2 WO 2005020130 A2 WO2005020130 A2 WO 2005020130A2 US 2004005640 W US2004005640 W US 2004005640W WO 2005020130 A2 WO2005020130 A2 WO 2005020130A2
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color
color component
component
space
pixel
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WO2005020130A3 (fr
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Paul Reed Smith
Gary E. Gilbert
Ted Sabety
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Paul Reed Smith Guitars LP
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Paul Reed Smith Guitars LP
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Priority claimed from PCT/US2001/025654 external-priority patent/WO2002017229A2/fr
Priority claimed from PCT/US2003/025692 external-priority patent/WO2004017261A2/fr
Application filed by Paul Reed Smith Guitars LP filed Critical Paul Reed Smith Guitars LP
Priority to US11/060,294 priority Critical patent/US20050213125A1/en
Publication of WO2005020130A2 publication Critical patent/WO2005020130A2/fr
Publication of WO2005020130A3 publication Critical patent/WO2005020130A3/fr
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N9/00—Details of colour television systems
    • H04N9/64—Circuits for processing colour signals
    • H04N9/646—Circuits for processing colour signals for image enhancement, e.g. vertical detail restoration, cross-colour elimination, contour correction, chrominance trapping filters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/46—Colour picture communication systems
    • H04N1/56—Processing of colour picture signals
    • H04N1/60—Colour correction or control
    • H04N1/6016—Conversion to subtractive colour signals

Definitions

  • the present invention relates generally to color processing systems and more specifically to a color accentuation system and a component of a color processing system.
  • Color processing falls into two general categories, namely light projections or displays which are known as additive color systems and pigment or printing systems which are known as subtractive color systems.
  • Color correction systems have been developed to correct for errors in the reader or scanner of the original material, signal transmission or limitations of the display or printing process. In the printing process, the correction can be directed to ink migration and physical color discontinuity. In an image or a video display, color correction can be for errors in the processing system and/or for changing the quality or color of the picture to meet certain criteria and/or tastes.
  • the present color accentuation system will help improve digital cameras, TV and other video display devices, video recording devices, and HDTV picture quality in both large and small formats. It will also improve color image printing. Digital still cameras and digital video cameras may have a button or command that triggers various levels of accentuation that would improve the picture quality. For example, one might take a picture on a dull, overcast day. When the accentuation button is pressed, the image will look like it was taken on a bright day. In another example, pictures taken with florescent lighting will look as if they were taken with more natural light.
  • the present invention is directed to the concept of accentuating the ultimate color image to be more vivid, color diverse, interesting to the eye and having higher color contrast.
  • the present invention would be compatible with almost any video or print media.
  • This patent description translates well to the CMYK color space, which is the system generally associated with the printing industry.
  • CMYK stands for Cyan, Magenta, Yellow, and Black. These colors are related to the primary colors, red, yellow and blue, with black being considered by this invention as the absence of color.
  • TV's and video use the RGB (Red, Green, and Blue) and Y Cb Cr and its related color spaces.
  • the color accentuation system described herein can be converted into any known or new color space or system, whether additive (light) or subtractive (ink, paint, etc.) using well-known algebraic transformations.
  • additive color spaces the same equations can be used if certain adjustments are made to mitigate for the fact that the color components in some of these color spaces are very different hues from the primary colors. This approach achieves the benefit of the invention, with computational efficiency at the sacrifice of precision, which may be an acceptable trade off in some applications.
  • the primary colors are red, yellow, and blue.
  • Rainbow colors are generally considered the vivid, bright colors and are either a primary color or two primary colors mixed at some ratio/percentage.
  • a subtractive primary color space or process as the percentage of the lowest percentage third color component increases, the overall color becomes more dirty and eventually becomes shades of grays and/or browns. This directly relates to additive color processes and spaces through color space conversion.
  • the system determines the relative magnitude of each color component.
  • the color components are the set of colors that are the axes in a given color space. In Red, Blue, Yellow, RBY (the primary color space), R, B and Y are the color components.
  • the invention selects and adjusts the magnitude of one or more of the colors as a function of the determined relative magnitudes of each color component. The type and amount of the adjustment is a function of the relative magnitude differences. One or more of the magnitudes is adjusted to change the relative magnitudes.
  • the difference in a subtractive color space e.g., CMY(K)
  • RGB additive color space
  • the lowest color component is reduced in the subtractive color space, and the highest color component is increased in the additive color space. No adjustment is made if only two colors are present in the area or pixel being investigated. In CMY(K), black (K) is not considered a color in the initial accentuation step.
  • the invention can be applied to an image on a pixel by pixel basis (where the accentuation function is calculated and applied to each pixel individually) or on an area by area basis (where the function is calculated for an area of the image and the same function applied to each pixel in the area).
  • An area in an image is a set of adjacent pixels in the image that have substantially the same color, in other words, substantially the same color component magnitudes.
  • a practitioner of ordinary skill will recognize that the benefit of the invention can be attained by determining the accentuation function once for all the pixels in an area because the adjacent pixels have substantially the same color component magnitudes.
  • Scaling functions and compensations may also be used. These include brightness compensation, whiteness compensation, scale-back or scaling compensation and saturation compensation or adjustments using various scaling functions. This may include two color component accentuation or adjustment.
  • Figure 1 is a color processing system in which the present invention can be incorporated.
  • Figure 2 is a subtractive space color wheel.
  • Figure 3 is a single slice color wheel for RYB from color pipe of Figure 5 with scaling functions.
  • Figure 4 is a look up table in CMYK correlating the original to the accentuated color.
  • Figure 5 is a conceptual view of the color pipe.
  • Figure 6 is a flow chart of color accentuation according to the principles of the present invention.
  • Figure 7 shows graphs of a scaling function and its components incorporating the principles of the present invention.
  • Figure 8 shows graphs of various scaling functions incorporating the principles of the present invention.
  • Figure 9 shows graphs of additional scaling functions incorporating the principles of the present inventions.
  • Figure 10 is a conceptual view of the whiteness of a color pipe. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
  • the core of this invention is developed from the primary colors (Red, Yellow, Blue). However, the system functions in both additive and subtractive color spaces through mathematical color-space transforms.
  • the present invention can be used in two modes.
  • a first mode an image, that is encoded using any first color space, is converted into the color component magnitudes of a second color space and has the accentuation function applied in that color space.
  • the resulting image can be converted back to the original color space.
  • the accentuation function can be determined in a first color space and then the accentuation function is transformed to any other color space so that an image need not be converted— the transformed function is applied to the image in the color space of the image. It is also possible to approximate the calculation in such a way that the image and scaling function is not entirely transformed into another color space, but parts of the algebraic transformation are used to calculate intermediate results that provide a close approximation of the invention.
  • CMY and RGB RGB being used in video applications and which is also an additive color space.
  • CMY(K) a subtractive color space
  • RGB and CMY(K) color spaces have known direct mathematical relationships to each other.
  • magenta in the CMYK color space has a small blue component, operations on magenta affect two colors (red and blue), not one (red).
  • the present system looks at the relative differences between the colors and makes the correction based on a function.
  • CMYK K
  • the black is not adjusted in the initial function. But black may still be part of the color percentage, so that the conversion of CMYK to another color space is accurate.
  • RGB CMY
  • the conversion of RGB to CMY does not include the black component.
  • CMY color that is substantially equal in color, they are "dirty" in color.
  • the lowest magnitude value color component of the three colors creates, in the combination the other two colors, a pastel dirtiness, grayness, brown-ness or a perceived lack of contrast, vividness or perceived sharpness.
  • the present invention creates a higher color contrast, sharper, clearer picture or color and reduces the effect of the lower of the three color components, pixel by pixel or area by area.
  • the accentuation adjustment may be to one or more of the three colors.
  • a color space component (CSC) is provided at 30.
  • the accentuation process is applied in at least one of two ways.
  • the relative magnitude of the color components is determined at 34a or 34b.
  • the relative magnitude determination at 34 may be a single step and may be performed before the determination of the type of color space at 32.
  • the color space is pre-defined, and the method would include only one leg 34-42. If it is an additive color space, the difference between the MAX and MED is determined at 36a. If it is a subtractive color space, the difference between the MED and the MLN is determined at 36b. These differences are used at 40a and 40b, respectively, to modify the color components as a function of these differences.
  • the output is provided as a new color space component (CSCnew) at 44.
  • various scaling functions 38a or 38b may be used in modifying the color components, as well as various compensations at 42a or 42b.
  • the compensation at 42a and 42b may be part of the color component modification 40a and 40b or may be a post-process modification.
  • the selection of scaling may precede the steps of 34, 36 or 40 and may be incorporated as part of step 40. In the determining the relative magnitude of the color components, it may require normalizing the color component value ranges in cases where the color components do not have the same numerical range.
  • the color space components may be converted to a different color space using the previously developed modified color component as a function of the differences, or the function of the differences may be converted to other color spaces.
  • the interaction of the various components of the method will be described more fully below. It is possible to approximate the result by using any additive color space components whose value ranges are normalized in the equation otherwise derived, for example, for RGB, as further discussed below.
  • a pixel containing the collection of values for individual color components can be analyzed in percentage magnitudes of those color components. For example, in a 24 bit, 3 color space like RGB, the R, G or B value is divided by 255 to calculate the percentage. Similarly, where the percentage is unity, the number 255 can be used if the color accentuation equation is being calculated without normalization.
  • the modifying or scaling function f(%MID - %MLN) result may be set to zero if the difference between MID and MIN is very small.
  • the scaling function f may be a constant times (%MID - %MLN), as in equation (a).
  • the modifying function f(%MLD - %MIN) may also increase, decrease or change the adjustment signified by the difference as a function of any of the color components present or the specific percentage relationship of the color components.
  • the maximum color component MAX may be increased. Also, both MLN may be decreased and MAX increased.
  • the function or its equivalent modifies one or more of the component color values based on the difference between the two lowest percentage color component values. This can be algebraically converted to any other color space using well-known mathematical conversions.
  • scaling functions of any type may be applied to equations (a) - (f) based on any combination of the color component values. Some circumstances may require that more or less scaling occur, for example, as discussed with respect to equation (b). This applies also to equations (d)- (f).
  • a dominant color compensation, whiteness compensation and display or printing device specific compensation or scaling functions discussed below may be used.
  • Further scaling compensation may be applied so that the output avoids full saturation when the MAX color component value is already large (in an additive space) or the MEN component is very small (in a subtractive space.) This may include two color component accentuation or adjustment
  • Accentuation may be equivalently performed based on lookup tables.
  • the new color component values are determined by matching the original color component values to those in the table and reading the new color component values out of the table for that color component set.
  • the middle two columns of Figure 4 would be a partial example of such a lookup table.
  • %MED - %MEN may be utilized as an offset into a lookup table or a two- dimensional index of (%MAX, %MED).
  • the compensations and adjustments described herein may further affect the (%MED-%MIN) or (%MAX- %MIN) value as a means of improving code execution speed.
  • Common assembly- level computing instruction sets and higher-level languages such as C, C++, and many others have inherent indexing capabilities that make such an implementation very efficient. The need for complicated mathematical calculations being performed during run time can be eliminated or reduced.
  • FIG. 1 illustrates a color processing system 20 for reproducing a color image 10, as image 12, on a media 14. If this is a printing process, then media 14 is the object on which the printing is performed. If it's a display like a television or CRT, then media 14 is a display.
  • the color processing system 20 generally includes a lens 22 providing input signals of the image 12 to a color separator 24.
  • the color separator 24 provides a minimum of three colors and in this example, four color signals to the signal processor 26.
  • the signal processor 26 then provides appropriate drive signals to projectors or printers 28, depending upon whether it is a printer or a light projector.
  • projectors/printers are shown but other projector or printers may be used depending upon the number of colors being processed. For example, it could be a three color additive system, a four color separation system, or a six color system.
  • the color processing system 20 can be thought of as a combination of components to process the color signal.
  • the lens 22 would introduce a color image to a color encoding system 24 that color separates a pixel into color components for a given color space.
  • the encoded image information is presented to a signal processor 26 that applies scaling functions that affect the color accentuation and also applies color space transformations.
  • the image information is transferred to the projector/printer 28 to recombine color components through either a light projection, ink printing system, or other recombinant method to form the processed image 12.
  • digital data comprising the original encoded image or the processed image can be stored as digital files on digital recording media and/or transmitted as digital files such that the components of the system depicted in Figure 1 may be separated physically and not reside within a single apparatus.
  • the color accentuation of the present method would be in the signal processor 26.
  • the signal processor 26 may be part of the original camera or scanner and/or may be in the signal processor 26 for the projector or printer.
  • the signal processor 26 may be part of a device that either plays back pre-recorded video media or processes video signals received by the device. These may include, for example, television or other display devices, as well as DVD-R or other video storage devices.
  • the signal processor 26 may include well-known signal correction software modified to incorporate the present invention.
  • CMYK color separation system
  • RGB color formats
  • RYB color spaces
  • LCH hue
  • S saturation
  • L luminance or lightness
  • C chrominance
  • U V; Cb, Cr; Pb, Pr
  • Saturation is the degree of color intensity.
  • Hue is also known as the name of the color and luminance is the degree of light/dark of the color.
  • any color on the outside of the wheel is vivid and/or pure. Any color on the outside of the wheel is either one primary color or combinations of two primary colors, as in a rainbow. If any amount of a third primary color is added to the outside of the wheel, the color starts becoming dirty, less vivid, and moves into the interior of the wheel. As it approaches the center where all three colors have substantially the same or equal value, it becomes dirty gray or brown, depending on its component colors. Eventually, as the color component percentages become large and near equal, the color becomes dirty gray which is the center of the wheel.
  • the Figure 3 wheel is the 100% slice through a solid color cylinder ("color pipe"), the surface of which contains the three primary colors Red, Yellow, Blue, equally spaced along the circumference.
  • the slice of the color cylinder ranges in intensity from 0% at one end of the cylinder to 100% at the other end.
  • Figure 5 shows a conceptual view of the color pipe.
  • Figure 10 shows a conceptual view of the whiteness of a color pipe, as discussed below.
  • the percentage shown on the color pipe signifies the maximum value of any of the three primary colors. Thus, if Red is the maximum color at 80%, the color wheel would be the 80% wheel of the color pipe.
  • a set of scaling function adjustments S 2 , S and S 4 are also shown. They illustrate that the scaling function varies as the color moves from dirty, for example, toward pure. S 2 shows adjustment for an original color close to a pure color. S 3 and S 4 show additional smaller adjustments.
  • the arrows show the adjustment of the value of the color components using a scaling function that modifies the total color component values so that the total color moves towards the outside vivid portion of the circle.
  • the scaling function is based on differences between color component values.
  • the length of the arrow represents the relative adjustment for one example scaling function.
  • the amount of accentuation relates directly to the arrow length for that pixel accentuation. The closer a color is to the outside of the wheel, the more it is accentuated towards a vivid pure color on the outside of the wheel.
  • the color can saturate the range, so the scaling function may become attenuated for color values close to the maximum in the range, thus causing the accentuation to occur primarily in an annular ring of the color wheel.
  • Si, S 2 , S 3 and S 4 are shown as radii since the adjustment is of only one color component. If changes are made to more than one color component, the result may not be a radii as shown by S 5 , depending on the function.
  • This example scaling function S 5 shows a curved adjustment favoring one of the primary colors.
  • the scaling function can change in any dimension in the color pipe.
  • a scaling function may also be used which moves circumferentially and changes the color or hue. This would result by changing two of the three colors.
  • the color pipe and wheel also have a fourth dimension of white/black (not shown). The fourth dimension adds amounts of white or black to lighten or darken/dull any color on the pipe/wheel by degrees of colorless gray (black into white).
  • Figure 10 shows the degree of white of a color pipe/wheel. It begins from black at zero and proceeds to white at 100%. These are the same percentages slices as described with respect to Figure 5. As discussed previously, the center of the slice represents all three colors having the same value. It should also be noted that the size of the slices is illustrated in Figure 10, which illustrates the amount of differences that may occur from one of the colors being zero and at least one of the other colors being a maximum on the surface of the circumference of the slice. A compensation scheme based on the amount of whiteness will be discussed below, with respect to Figure 10.
  • the pipe/wheel dimensions are red, yellow, blue, and black.
  • the example is shown as reducing the percentage of the lowest color, the other color components may also be adjusted.
  • the highest may be increased by itself or in combination with lowering the lowest.
  • the middle color can be raised. All of these reduce the effect or contribution of the third or lowest color.
  • the scaling function may be a modification of the numerical difference of the middle and lowest percentage of color components, as discussed with respect to equations (b)-(f) for a subtractive color space.
  • the primary colors have different degrees of dirtiness. Blue contributes more dirtiness than red which contributes more than yellow for example. Thus if blue is the lowest percentage color component it will be reduced more than if red or yellow was the lowest percentage color component.
  • different colors saturate quicker than others and differently in different color spaces. For example, in typical media devices, red often saturates quicker or more than green or blue and, thus, would use a different scaling function. Saturation is discussed in detail below. Also, if the to be adjusted color component is large, a scaling compensation may be required as discussed in detail below.
  • equations describing calculations in a given color space may be transformed algebraically into different but functionally equivalent calculations in a different color space using well-known mathematical transformations such that the results are substantially equivalent.
  • equation (b) which is defined for use in a subtractive color space (e.g. CMYK)
  • CMYK subtractive color space
  • an additive color space including RGB
  • %MAX New %MAX + a*(l - e ( " b * (%MA ⁇ - %M ' D)) ) *o /oMA ⁇ * (100 o /o . %MAX) where a and b are numerical constants.
  • the nonlinear of equation (h) can be transformed into any other color space.
  • the scaling function for RGB can be itself transformed into other color spaces using well-known transformations, including to YCbCr, YUV.
  • this implementation makes it possible to experiment with a variety of scaling functions, f, without reprogramming extensively.
  • the compensations to the scaling function can be easily implemented by simple offsets to the input indexes of the table expressing the scaling function f.
  • equation (g) is an example of equation (i).
  • the form of equation (i) may provide computational efficiencies and is easier to manipulate in conversions between color spaces.
  • %MAX is the number equal to the value of the MAX color component divided by its range.
  • f(%MAX-%MED) for RGB (or other additive color spaces) can be used that avoids over-saturating color by having a shape that rolls-off when the MAX-MID approaches its maximum.
  • This scaling function has six parameters, a, b, c, d, g, and h that can be adjusted to change the shape of the scaling function.
  • the 255 divisor in these equations will be changed to be equal to 2 (# b,ts er componen ) .
  • the parameters are typically set so that there is an initial concave upward or monotonically increasing section near where MAX-MED is close to zero that then enters a concave downward or plateau peak area where the color accentuation effect is at its maximum, which then rolls off back down or monotonically decreases toward the minimum effect to be applied where the maximum input for MAX-MID is reached.
  • Figure 8 shows examples of four other scaling functions using equation (j).
  • the constants are as follows:
  • Figure 9 shows two other examples (super 1 and super2) of the scaling function compared to an exponential (exp) and linear (liner.4) version.
  • the constants for exp, superl and super2 for equation (j) are:
  • the linear f(MAX-MED) is 0.4 times (MAX-MED).
  • a review of the curve for superl shows a peak or relative plateau above 0.5 in the range of 80 to 150 or approximately 27% of the total difference range.
  • Super2 peaks or plateaus above 0.3 in the range of 55 to 145 or 35% of the total difference range.
  • superl rises over 40% of the difference range and decreases over 22% of the difference range
  • super2's rises and falls are more equal over 22% of the difference range. Both are at zero over 30% of the difference range.
  • This general scaling function shape can be adjusted to optimize the color accentuation effect to meet the requirements of particular storage, transmission, display or output devices.
  • the practitioner of ordinary skill will recognize that a variety of algebraic functions can be devised that produce an equivalent shape that provides maximum color accentuation in a region between the lowest values and highest values for MAX-MED (in an additive color space).
  • the equation (i) can be transformed into any other color space. Alternatively, it can be used in some additive color spaces, for example, Y Cb Cr or Y U V, as an approximation, as described below.
  • the algebraic transformation of the equation from a subtractive space to an additive space converts the comparison of the two minimum color component magnitudes to examining the magnitudes of the two maximum color components and scaling the color component values based on the difference between the maximum and middle values of the three color components.
  • lowering the magnitude of the minimum color in CMYK is the equivalent of raising the magnitude of the maximum color in RGB space.
  • R-G 2.409*(Cr - 128) + .391*(Cb - 128)
  • R-B 1.596*(Cr - 128) - 2.018*(Cb- 128)
  • G-B -.813*(Cr - 128) - 2.409*(Cb - 128)
  • a logic table determines whether R, G or B is the MAX or MED, and MAX-MED is already calculated. This difference is used in a look-up table to determine the scaling function F of RGB for the corresponding MAX. For the present example, green G is assumed to be the MAX and, consequently, green G is to be accentuated.
  • R is MAX
  • the adjustment of the three color components to Ynew, Crnew, Cbnew would use the coefficients of R (.257, .439, -1.148) from the above YCrCb equations and, if B is MAX, the coefficients (.098, .-071, .439) are used.
  • B is MAX
  • the coefficients (.098, .-071, .439) are used.
  • the MAX-MED is calculated once for two pixels because Y falls out of the equation. The practitioner of ordinary skill will recognize that many video systems share a single Cb and Cr value pair between two different pixels, where the Y value is the only color component difference between the two pixels.
  • This approach is equivalent to using three normalized axes on the color wheel other than R Y or B, but which each represent pure hues comprised of two primary colors.
  • the adjustment moving vectors Si, S 2 , S or S 4 in Fig. 3 toward the outer rim of the wheel can be accomplished by adjusting the component value along one of such axes, even if it is not a pure primary color axis.
  • the MAX color component is the substantially predominant in the pixel
  • the movement of the pixel color value on the color wheel is substantially radial, and thus this approximation is accurate.
  • the scaling function can be designed to compensate.
  • One compensating scaling function takes the MAX-MED argument, but also adjusts the value of the MED component so that the pixel color position on the color wheel moves radially outward. Adjustment of the MAX produces an equivalent effect.
  • a threshold THRESH MID is defined as the magnitude of MED above which adjustment occurs relative to the amount of color accentuation in MAX (or MEN).
  • This threshold can be defined as a parameter under manual control, such as an absolute distance from MAX (or MEN), or be fixed. Alternately, and the preferred embodiment, is an automatically calculated distance based on MAX and MEN values.
  • the desired effect in this embodiment is as follows: Case 1 : When MED is less than THRESH MID , no change occurs to MED. Case 2: When MED is greater than THRESH MID , MED is boosted relative to MAX.
  • THRESH MID Less adjustment occurs as MED gets closer to THRESH MID - More occurs as MID is closer to MAX.
  • the dynamic in MID is subject to two inputs: first, the amount of change in MAX and second, the relative position of MED to the THRESHMID-
  • An alternate calculation of THRESH MID could be MIN + (MED -MEN)/A.
  • Practitioners of ordinary skill will recognize that instead of an absolute threshold described above, a continuous adjustment can be calculated where the magnitude of adjustment of MED decreases as MED approaches toward MEN can be used instead.
  • An illustrative example of this kind of calculation to replace thresholding is explained below with regard to the scale-back function.
  • THRESH MID is set and used in equation (m) the calculation may be performed even for Case 1
  • brightness compensation acts to offset the overall brightness change in an image after the initial color accentuation takes place.
  • color accentuation acts to brighten a pixel by adjusting one color component upward in value based on the color accentuation function.
  • the accumulation of accentuation across an image therefore increases the "brightness" of the image.
  • the brightness compensation may affect all three components in a pixel proportionally to the amount of accentuation on the accentuated component in the pixel.
  • a user controlled or set scaling multiplier BrightnessScale is a parameter used to further scale the magnitude of the brightness compensation operation.
  • brightness compensation may also be performed in RYB, CMY and CMYK color spaces as well as using the brightness, luminance or lightness L of other polar color spaces for scaling. The operation of this process may be transformed through standard color space conversions and are equivalent.
  • Brightness compensation may also be performed to preserve the characteristic brightness as described by the "Y" value in color spaces YUV, YCbCr, etc.
  • the process is as follows: • Perform the color accentuation step on the image in RGB or any other color space, after conversion from YUV or YCbCr, as the case may be, to RGB (or that other color space). • Calculate the original and new Y values Y and Ynew through the color space conversion equations that define the conversions from RGB to YUV, or RGB to YCbCr (if the other color space is RGB, for example). • Using the arithmetical relation of Y and Ynew (for example, their ratio), scale the magnitudes of the two color components of the pixel that have not been adjusted by color accentuation (e.g., MEDnew and MENnew in RGB color space).
  • the resulting three components are the pixel output in RGBnew (or whatever the other color space is). • Transform the new RGBnew result (or whatever the other color space is) to YUVnew or YCbCrnew using well-known transform arithmetic, resulting in a YUV or YCbCr brightness compensation.
  • the brightness compensation in Y may scale all three color components or just one color component.
  • the number of color components scaled may be a function of the transformation equations for Y.
  • the brightness correction should not substantially diminish the results of the color accentuation.
  • Dominant color compensation begins with measuring the image as a whole for the relative prominence of a particular color component.
  • RGB space an additive color space, this test is for Red, Green, and Blue.
  • CMY or CMYK color spaces subtractive color spaces, the test is for Cyan, Magenta, or Yellow, where black is ignored in the CMYK color space.
  • One preferred embodiment is to average the separate color component values across the entire image. A result for each color is obtained. For the RGB color space, an average value for each of Red, Green, and Blue are obtained. The highest value is considered the prominent color.
  • the difference between the highest average color value and the next-highest average color value is used to scale the amount of color accentuation applied to any pixel.
  • This difference between the highest and next-highest average color values are inputs to a mathematical function, which then creates a color prominence multiplier. This is implemented by multiplying the result of the average-value difference function by the resulting scaling function for that pixel. It is applied only to those pixels where the maximum color component for the pixel is the same color component as the maximum average color component of the image.
  • %MAXnew %MAX + f(%MAX - %MED) * g(AVGmax - AVGmid) * (100% - %MAX) * %MAX
  • AVGmax is the averaged color component value across all pixels in an image that has the largest resulting value
  • AVGmid is the averaged color component value across all pixels in an image that has the next-largest resulting value
  • g( AVGmax - AVGmid) is a function that calculates the amount of color prominence scaling that should be applied to an image when the MAXold color and AVGmax color are the same color component.
  • This adjustment for color dominance in a region can also be achieved using convolution.
  • a convolution is performed that integrates over all the neighboring pixels within some radius R, the cumulative sum of each magnitude of the same color component divided by their corresponding distances from the given pixel.
  • the scaling function applied to the given pixel is multiplied by a coefficient inversely proportional to the convolution result. En this manner, when a pixel resides within a region where the same maximum color component is heavily dominant, the scaling function is reduced in effect.
  • similar convolution results can be achieved by using the distance to some power, or the color component magnitude to some power or some combination thereof.
  • X 0 ,Y 0 are the dimensions of the region of convolution, and N is the selected sharpness of the convolution function.
  • Cx,y is large.
  • the scaling function can be attenuated by a number inversely proportional to Cx,y or Cx,y raised to some degree.
  • this technique is equivalent to convolving across any shape surrounding the point X,Y.
  • the computationally easiest form is the rectangular shape, with the convolution calculated discretely, as demonstrated above.
  • the scaling functions can be applied such that a different scaling function is applied depending on which color component is the MAX (in the case of an additive color space) or MIN (in the case of a subtractive color space).
  • each color can have a different scaling function for the same differences. This is another way of dealing with the over saturation problem in at least some colors. For example, often R is accentuated too much, while the G and B are acceptable when all three use the same scaling function.
  • each color component can have its own scaling function such that when a given pixel has a color component selected as MAX (in the additive case), then the scaling function for that color is used for that pixel.
  • the R scaling function is less than the scaling function for G and B.
  • the selection and shape of the scaling functions for the different color components will depend on the characteristics or requirements of the particular display or image output device, storage device or where the color coding and decoding signal process is situated.
  • a particular display or printing device may have particular visual characteristics, in other words, its color response function may have non-linear aspects. Therefore, the scaling function can be modified to complement these effects. For example, where the display device would appear to over saturate at certain levels when color accentuation is applied, the scaling function can be modified to level off when %MED-%MIN reaches a certain threshold and roll-off when it reaches a second threshold. Similarly, when %MED-%MIN is less than a certain threshold, the scaling function can be set to a set amount. Practitioners of ordinary skill in the art can construct smooth transitions from the scaling function domain across the threshold to the domains where the scaling function value is set to a different function.
  • Whiteness compensation addresses artifacts that become visible after Color Accentuation is applied under certain conditions in where the color components of a region are "near white".
  • the term “near white” will apply to the colors which are at or near the white end of the pipe/ wheel of Figure 10, and also to those colors close to the center of the pipe where all three color components are somewhat close in value.
  • Color Accentuation may cause a pixel, or group of pixels, to stand out against it's surroundings because of local variations in the value of (MAX-MED). This results in some neighboring pixels being accentuated more than others.
  • the apparent brightness or contrast between neighboring groups of pixels after accentuation may be more than before, and sufficiently so to be more perceptible.
  • This effect is typical in images that have color noise or compression artifacts.
  • This effect is more noticeable when the color component values are higher in value, i.e. closer to white values (closer to pixel values of 255 in 24-bit, 8-bit per pixel, RGB conditions).
  • the black end of the color pipe of figure 10 the effect is not as perceptible.
  • the differences can be larger and thus the differences between neighboring accentuated pixels or regions is more perceptible.
  • the whiteness compensation adjustment scales back the amount of color accentuation under certain conditions.
  • the reduction is made relative to the MEN component value in the pixels for an additive color space and the MAX component value for a subtractive color space.
  • the following will discuss the additive color space as an example and is applicable the subtractive color spaces as discussed for other compensations and adjustments.
  • the MEN component value of a pixel is larger, then the entire pixel is closer to the white end of the color pipe.
  • more scaling back of the accentuation should occur when the MIN value gets larger, regardless of the MAX-MED value in an RGB color space.
  • this method uses a near white compensation or offset "OFFSET nwo " to the indexing of (MAX-MED) in the f(MAX- MED) color accentuation algorithm.
  • OFFSET nwo the indexing of (MAX-MED) in the f(MAX- MED) color accentuation algorithm.
  • a lookup table it literally is an offset of the index into the (MAX-MID) portion of the table.
  • a maximum offset value is scaled by the value of MEN.
  • Higher MIN means more offset of the (MAX- MED) value.
  • this embodiment is as follows: Assume that for each color plane, R, G and B, there is a constant controlling the magnitude of the whiteness compensation effect.
  • the value of _whiteness compensation offset is limited to be less than or equal to (MAX-MID) so that the index does not become less than zero.
  • the same scaling can be applied to all pixel values in a universal fashion.
  • a second embodiment of the whiteness compensation a linear or non-linear scaling is applied to the amount of accentuation.
  • the whiteness compensation of f(MAX-MED) is calculated as: g(MEN) * f(MAX-MED)
  • Max scaling compensation (or in a subtractive space, MIN scaling compensation, referred to here as "scale-back") addresses an effect observed with images or video where the color component values are large.
  • the amount of color accentuation applied to the MAX color component is equal to the function f(MAX-MED) times the value of MAX, as described above in equation. Regardless of the actual value of MAX, the calculated amount of accentuation is the same. However, since the core function uses the return accentuation value multiplied by the MAX value, a larger MAX means that the change in the maximum color component value increases in absolute value when MAX gets larger.
  • scale-back En order to avoid this saturation, a compensation function called scale-back or scale-back compensation is used. Max-scaling decreases the amount of color accentuation monotonically as the value of MAX increases in an additive color space. In a subtractive color space, this would be an adjustment that reduces the amount of color accentuation as the value of MIN decreases toward zero.
  • a variety of functions for scale-back can be used. In the preferred embodiment, a simple linear calculation has proven very effective. Also, different colors may require unique scale-back function characteristics. Thus, the scale-back functions may be different for each color.
  • the parameter p defines the sharpness of the scale-back effect: that is, how quickly the amount of color accentuation is reduced as the value of MAX approaches 255, and how much of a reduction occurs.
  • f accen (MAX-MID) [a *(l. e (-b((MAX-fnwc)-MID)/2 55))] * [-(((MAX-fhwc)-MED)/(255*c)) d +l]*...
  • the last step would be to calculate the Brightness Compensation, using equations (n), (o), and (p).
  • the technique of Color Accentuation can be combined with other image enhancement techniques when applied to a subject image or video or film frame.
  • One example is the use of histogram equalization.
  • This technique which is well known in the art, expands the dynamic range of pixel values in an image by an adaptive process. The probability function of pixel values for a given image is calculated and then a pixel mapping function derived from that function which, when applied to each pixel, improves the contrast and dynamic range of the image.
  • One object of the present invention is to combine Color Accentuation and histogram equalization by either first enhancing the contrast of the image using histogram equalization and then applying Color Accentuation or by first applying Color Accentuation and then applying histogram equalization.
  • a typical histogram equalization method is described in Digital Image Processing, Gonzalez & Woods, Prentice Hall, ⁇ 2002, Chapter 3.3.1, which is incorporated here by reference.
  • the color image is first transformed to a grey-scale image. This is typically done by calculating a grey scale value equal to a linear combination of the R, G, and B values for each pixel.
  • the coefficients for R, G and B, respectively are (.3, .5, .2).
  • the object of the histogram process is to calculate a unique transfer function T that maps an input grey scale level from the original image to an output level grey scale level.
  • One object of the invention is to avoid the banding.
  • One way the invention does this is as follows: the grey scale image is encoded to encompass more than 255 levels of grey scale. This is accomplished at the conversion step described above. Each grey scale calculation is then (((m_weightRed * R) + (m_weightBlue * B) + (m_weightGreen * G))/ ((m_weightRed + m_weightBlue + m_weightGreen)/(GREYSCALE_LEVELS/255)), [000110] Where the m weight values are the coefficients described above and GREYSCALEJLEVELS is the number of different grey scales minus one. In the preferred embodiment, the number of grey scale levels is 1023 By this means, the banding problem is reduced considerably.
  • transfer function T produces a result that is too extreme and therefore impractical. It is a further object of the invention to control the transfer function T so that the results are appropriate for photographs.
  • the transfer function T can be modified as follows:
  • a further problem with histogram equalization is that it can introduce artifacts into an image in certain circumstances. .
  • the denominator used to calculate T is the number of grey scale levels, not necessarily 255 .
  • the output grey scale values are calculated when iterating through the image a second time on a pixel by pixel basis, this time determining the amount of shift D for each pixel.
  • D is divided by 3 (to allocate to R, G and B). This value (whether positive or negative) is first multiplied by the value 255 for GREYSCALE LEVELS and then added to the R, G and B value of the original color image.
  • the multiplication renormalizes the value D from a grey scale range of 0 to GREYSCALE LEVELS to a range of 0 to 255.
  • gray scale values need to be calculated to determine the histogram. These values are used to adjust the components of the original color image.
  • further use of the grey scale image is optional: either the grey scale values can be looked up in order to determine how to change the color components for the color output, or the grey scale values of the color pixels can be recalculated in that step.
  • the value D can be allocated to the values of R, G and B using the weighting or another function.
  • the second iteration of the color image may occur during the same iteration through the color image where color accentuation is applied, thus saving compute time.
  • the present system is considered a color accentuation system, not a color correction system, although it is expected that this process can become a new kind of color correction.
  • Color correction implies that the to be printed or displayed color is corrected to be identical to the original image. In many cases, the image taken has color flaws that depart from the original, or the intended original.
  • the present method or system has used the amplitude of the color components as the parameter to be measured and adjusted.
  • Other parameters of the system may be used for the relative measures and adjustment. They could include any of color, hue, saturation, luminance, chrominance, focus or any other video control.

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Abstract

L'invention concerne un procédé d'accentuation de couleur permettant de déterminer l'importance relative de chaque composante couleur dans chaque pixel ou zone. Ce procédé permet de sélectionner et d'ajuster l'importance d'au moins une couleur comme une fonction de l'importance relative déterminée de chaque composante couleur. Le type et la quantité d'ajustement sont fonction des différences d'importance relative. Au moins une importance est ajustée pour modifier les importances relatives. Généralement, dans un espace de couleur soustractive, il y a une différence entre la couleur d'importance inférieure et la couleur d'importance moyenne et, dans un espace de couleur additive, il y a une différence entre la couleur d'importance supérieure et la couleur d'importance moyenne. En général, la composante couleur inférieure est réduite dans l'espace de couleur soustractive et la composante couleur supérieure est augmentée dans l'espace de couleur additive. On peut utiliser diverses fonctions de mise à l'échelle et de compensations ou d'ajustements de l'accentuation de couleur.
PCT/US2004/005640 2000-08-18 2004-02-26 Procede d'accentuation de couleurs avec compensation et ajustement Ceased WO2005020130A2 (fr)

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US10/362,198 US6947177B2 (en) 2000-08-18 2001-08-17 Method of color accentuation
US44986703P 2003-02-27 2003-02-27
US60/449,867 2003-02-27
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US5911003A (en) * 1996-04-26 1999-06-08 Pressco Technology Inc. Color pattern evaluation system for randomly oriented articles
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US6594387B1 (en) * 1999-04-30 2003-07-15 Texas Instruments Incorporated Enhanced color correction
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