EP2347571A2

EP2347571A2 - Contrast enhancement of images

Info

Publication number: EP2347571A2
Application number: EP09787357A
Authority: EP
Inventors: Erno H. A. Langendijk; Remco T. J. Muijs; Harold A. W. Schmeitz; Leo J. Velthoven
Original assignee: Koninklijke Philips Electronics NV
Current assignee: TP Vision Holding BV
Priority date: 2008-10-13
Filing date: 2009-10-05
Publication date: 2011-07-27
Also published as: WO2010043996A3; JP2012505460A; KR20110083663A; WO2010043996A2; CN102187657A

Abstract

An apparatus is arranged to perform contrast enhancement for one or more color images. The apparatus comprises an image source (101) which provides the color image having color values for image areas, where each image area may specifically correspond to a pixel. A contrast enhancement processor (103) performs contrast enhancement on the color image by modifying a luminance characteristic for at least some image areas in response to the color value for the image area. The contrast enhancement processor (103) may specifically generate compensated luminance values for pixels by normalizing luminance values relative to a nominal luminance value for the color of the pixels. A luminance contrast enhancement is then performed on the compensated luminance values. The approach may allow improved image quality and may in particular reduce introduction of unnatural shadowing effects due to color variations.

Description

Contrast enhancement of images

FIELD OF THE INVENTION

The invention relates to contrast enhancement of images, and in particular, but not exclusively, to contrast enhancement of moving images of a color video signal.

BACKGROUND OF THE INVENTION

Contrast enhancement of images is widely applied to both still and moving images to improve image quality for images perceived to have suboptimal contrast. Conventional contrast enhancement methods work by modifying the luminance (brightness) characteristics for the images, e.g. by applying a non-linear function that makes bright areas brighter and dark areas darker. For example, a luminance signal for a video signal may be modified by applying a dynamic range expander function. However, although this approach may provide suitable performance for many applications, it tends to be suboptimal for many color images and may cause image quality degradation or distortion in some scenarios. E.g., saturated red and blue colors are associated with intrinsic low luminance (typically about 20 % and 10 % of white respectively) and conventional contrast enhancement techniques tend to result in the undesired effect that such colors are further decreased in brightness resulting in the introduction of unnatural shadow regions.

In the outside world, objects are seen by a viewer when they are illuminated by a light source and light is reflected into the viewer's eyes. The spectrum of the reflected light can be calculated as the product of the spectra of the source and the object's reflectivity, respectively. Using well-known color matching functions, the CIE 1931 xy chromaticity coordinates can then be calculated from the spectrum of the light that enters the eye. If an object in a scene reflects 100% of the light and the light source has a flat spectrum (or a relatively flat spectrum as that of the sun), a white object will be seen. This object has the highest possible brightness of all reflecting objects in the scene. When an object reflects only a narrow spectral band, it will have a very saturated color (e.g., 630 nm for saturated red). However, its brightness will be very low, because all the light at the other wavelengths is absorbed. Furthermore, the perceived brightness will depend on the specific color as different wavelengths with the same light intensity are perceived to have different levels of brightness. Therefore, for e.g. saturated colors, even relatively bright objects will have relatively low luminance values and the contrast enhancement will tend to reduce the brightness level further. Accordingly, for many scenes, the contrast enhancement will tend to degrade the image quality, and specifically may introduce additional shadow effects and degrade the differentiation between different colors.

Hence, an improved system for contrast enhancement of images would be advantageous and in particular a system allowing increased flexibility, facilitated operation or implementation, improved image quality and/or improved performance would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to a first aspect of the invention there is provided an apparatus for contrast enhancement of a color image, the apparatus comprising: means for providing the color image having color values for image areas; and contrast enhancement means for performing contrast enhancement for the color image by modifying a luminance characteristic for each image area of at least some image areas in response to a color value for the image area. The invention may allow improved contrast enhancement for images. In particular, the invention may provide improved contrast enhancement for images having color areas that correspond to low perceived brightness and may in many embodiments prevent or reduce shadowing effects and color perception degradations. The invention may in particular allow contrast enhancement which provides improved darkening of shadow regions while boosting illuminated areas. The approach may in many embodiments provide improved independence of the contrast enhancement to the specific color values. The invention may accordingly allow a contrast enhancement that provides a more vivid yet realistic appearance of a scene portrayed by the color image.

Each image area may correspond to/comprise one or more pixels. Furthermore, the at least some image areas may cover the entire image area and/or may be non-overlapping image areas. Specifically, the at least some image areas may correspond to all pixels of a digital color image. The color image may be a still image or may be an image of a sequence of images, such as a frame of a video signal. The color value may be represented by a color representation or a combined luminance and chroma representation. For example, for an analog video signal, the luminance values may be provided by signal values of a luminance signal and the color values may correspond to values of one or more chroma signals. The color value may be a composite value comprising separate values for different color components, such as a value for each of a plurality of primary colors of a display.

In accordance with an optional feature of the invention, the contrast enhancement means comprises: compensating means for generating a compensated luminance value for each of the at least some image areas in response to the color value of the image area; and means for performing a luminance contrast enhancement on the compensated luminance values.

The feature may provide an improved and/or facilitated contrast enhancement. In particular, the feature may allow improved image quality in many scenarios while providing a low complexity implementation and operation.

The compensated luminance value may specifically reflect a relative perceptional brightness for the color represented by the color value relative to other colors, and specifically relative to the color white.

In accordance with an optional feature of the invention, the compensating means further comprises: nominal luminance means for determining a nominal luminance value for a first image area as a function of a color value for the first image area; and luminance determining means for generating a compensated luminance value for the first image area as a function of the nominal luminance value and a luminance value for the first image area.

The nominal luminance value may specifically reflect a relative perceptional brightness for the color represented by the color value relative to other colors (and specifically to the color white) for a given (predetermined) luminance level of the image area. Thus, the color value may be compensated to correspond to a specific luminance level and the nominal luminance value may indicate the perceived brightness of the color for this specific luminance level. Specifically, for a multi- component color values, the nominal luminance value may correspond to a perceived relative brightness for a given combined value of the individual multi-component color value. E.g. the nominal luminance value may correspond to a perceived relative brightness for the color represented by the color value but with a predetermined magnitude (while maintaining the same relationship between the multi- component color values).

In accordance with an optional feature of the invention, the compensating means is arranged to generate the compensated luminance value for the first image area by normalizing the luminance value for the first image area relative to the nominal luminance value.

This may provide a particularly advantageous compensation for color variations in the image.

In accordance with an optional feature of the invention, the nominal luminance means comprises a data store for storing a look-up table comprising nominal luminance values for a plurality of color values and is arranged to determine the nominal luminance value by a table look-up in the look-up table.

This may allow facilitated operation in many embodiments and/or may reduce complexity and/or resource consumption. The look-up table may specifically comprise predetermined luminance values for specific colors.

In accordance with an optional feature of the invention, the nominal luminance value is a maximum luminance value for the color value of the first image area.

This may allow a particularly advantageous contrast enhancement and may in particular provide a highly efficient compensation for impact of perceptual brightness variations as a function of colors. Alternatively or additionally, it may provide for facilitated implementation and/or operation as a maximum luminance value for a given color value may be relatively easy to determine.

In accordance with an optional feature of the invention, the nominal luminance means is arranged to determine the nominal luminance as a function of a plurality of linear color component values of the color value, each color component value corresponding to a display primary color.

For example, the color value may be provided by a linear Red, Green and Blue primary color component value (i.e. in accordance with a linear RGB color scheme corresponding to an RGB color scheme with a Gamma value of 1). The approach may provide an efficient way of determining suitable nominal luminance values reflecting the perceptual brightness variations as a function of colors. The linear color component values may specifically be used to determine a maximum luminance value for the color value. In some embodiments, the apparatus may comprise means for converting the color values from non linear color component values (such as e.g. RGB values with a gamma different than one) to the linear color component values.

In accordance with an optional feature of the invention, the nominal luminance means is arranged to: generate normalized color component values by normalizing the linear color component values relative to a maximum value of the linear color component values; and determine the nominal luminance as a combination of the normalized color component values, the combination comprising a weighting corresponding to a perceptual luminance contribution for each color corresponding to a linear color component value. This may provide a particularly advantageous determination of the nominal luminance. In particular, a facilitated and/or improved determination of the nominal luminance may be achieved.

In accordance with an optional feature of the invention, the nominal luminance means is arranged to determine the luminance value for the first image area as a combination of the linear color component values, the combination comprising a weighting corresponding to a perceptual luminance contribution for each color corresponding to a linear color component value.

This may provide a particularly advantageous determination of the luminance value. In particular, a facilitated and/or improved determination of the luminance value may be achieved.

In accordance with an optional feature of the invention, the contrast enhancement means further comprises: means for de-compensating contrast enhanced luminance values resulting from the luminance contrast enhancement, the de-compensating corresponding to a reverse operation of the compensating means. The feature may provide an improved and/or facilitated contrast enhancement.

In particular, the feature may allow improved image quality in many scenarios while providing a low complexity implementation and operation.

For example, the de-compensation may be such that the resulting relative modification of the luminance value corresponds to the relative modification applied to the compensated luminance value by the luminance contrast enhancement.

In accordance with an optional feature of the invention, the contrast enhancement means further comprises: means for, for at least a first image area, determining a relationship between a compensated luminance value for the first image area and an enhanced luminance value resulting from the luminance contrast enhancement; and modifying means for modifying a color value for the first image area in response to the relationship.

In accordance with an optional feature of the invention, the modifying means is arranged to multiply color component values of the color value for the first image area by a ratio between the compensated luminance value for the first image area and the luminance value for the first image area, each color component value corresponding to a display primary color.

This may provide a particularly efficient and low complexity operation. In accordance with an optional feature of the invention, the apparatus further comprises means for converting non-linear multi color component color values into linear multi color component color values; and wherein the luminance means is arranged to determine luminance values in response to the linear multi color component color values.

In accordance with an optional feature of the invention, the contrast enhancement means comprises means for determining an illumination intensity value for each image area in response to the color value and to perform the contrast enhancement on the illumination intensity value.

In accordance with another aspect of the invention, there is provided a method of contrast enhancement for a color image, the method comprising: providing the color image having color values for image areas; and performing contrast enhancement for the color image by modifying a luminance characteristic for each image area of at least some image areas in response to a color value for the image area. These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 is an illustration of an example of a contrast enhancement apparatus in accordance with some embodiments of the invention;

Fig. 2 is an illustration of an example of a contrast enhancement processor in accordance with some embodiments of the invention;

Fig. 3 is an illustration of an example of a luminance transfer characteristic for a contrast enhancement algorithm; and

Fig. 4 is an illustration of an example of a method of contrast enhancement for a color image in accordance with some embodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 illustrates an example of a contrast enhancement apparatus for performing contrast enhancement on one or more color images. In the following, the contrast enhancement performed by the apparatus will be described with focus on contrast enhancement for a single digital still color image. However, it will be appreciated that in other embodiments, contrast enhancement may for example be applied to sequences of images, such as video frames of a digital video signal, or directly to analog representations of color images, such as to an analog color video signal.

The contrast enhancement apparatus comprises a color image source 101 which is arranged to provide a color image which in the specific example is a digital still image represented by a color value for each pixel of the image. The color value is specifically a set of multi-component color values corresponding to a specific color space. Thus, a value is provided for each primary color of a display to present the image. For example, the color value may contain three multi-component color values corresponding to e.g. the color components of an XYZ or RGB color space.

The following description will focus on an example where the color value for each pixel contains three drive values for respectively the Red, Green and Blue colors of an RGB color space. However, it will be appreciated that other color representations may be used in other embodiments. For example, in some scenarios the image may be represented by luminance and chroma values that may further be analog or digital representations.

It will also be appreciated that in other examples, a separate color value may not be provided for each image pixel but rather a color value may provide color information for an image area that may correspond to a plurality of pixels. For example, luminance data may be provided on a per pixel bases whereas the color values are provided on a per pixel block basis, where each pixel block comprises a plurality of pixels (e.g. color data may be provided for blocks of four pixels). Thus, an image area may be a pixel block comprising one or more pixels. The color image source 101 may for example provide the color image by retrieving it from an internal source, such as a hard disk or other storage means, or may e.g. comprise a receiver for receiving the color image from an external source.

The color image source 101 is coupled to a contrast enhancement processor 103 which is arranged performing contrast enhancement on the image. The contrast enhancement includes modifying a luminance characteristic for at least some image areas depending on a color value for the individual image area. Thus, the contrast enhancement processor 103 performs contrast enhancement which does not only consider luminance information but also depends on the color of the image area being modified. For example, the adjustment of a pixel value depends not only on the brightness but also on the color of that pixel.

In particular, the contrast enhancement processor 103 normalizes the luminance characteristic of image areas such that the luminance of the image area is normalized with respect to the maximum luminance of the color of the image area prior to the contrast enhancement. This normalization procedure effectively provides an approximation of the illumination intensity in a scene.

The contrast enhancement processor 103 then proceeds to perform contrast enhancement on the normalized image by darkening shadow regions and increasing the brightness in illuminated areas regardless of the color. The resulting contrast enhanced image can then be de-normalized to provide a contrast enhanced image corresponding to the original image.

As the contrast enhancement takes into account the dependency of the perceptual luminance for the specific color, the approach allows the contrast enhancement to be adapted to the local color conditions in order to provide an improved contrast enhancement. Specifically, the contrast enhancement of different colors may be harmonized to provide a more consistent contrast enhancement. This can in particular prevent darkening of relatively bright colors that have low luminance values due to the color rather than the relative brightness. E.g. in contrast to conventional contrast enhancement approaches, saturated colors associated with an intrinsic low maximum luminance (e.g. red and blue) are not darkened more than colors with a high intrinsic maximum luminance, such as green and yellow. The result may be a clearer and more vivid image with realistic shadow effects and clearer color differentiation.

The compensation of the luminance may specifically result in the compensated luminance corresponding to a non-color specific illumination intensity for an image area rather than a color specific luminance value for the image area. Thus, the compensated luminance may reflect the strength of white light falling on an image object rather than the perceived luminance resulting from light being reflected by the image object. Thus, in the example of FIG. 2, the contrast enhancement process is applied to illumination values rather the original luminance values. Fig. 2 illustrates more detailed elements of the contrast enhancement processor

103 of Fig. 1.

In the example, the contrast enhancement processor 103 receives data in the form of multi color component color values comprising three non-linear component values. A three component non-linear color format is used in the specific example, such as e.g. an XYZ format. The following description will focus on the color values comprising a Red,

Green and Blue component corresponding to three primary colors for displays presenting the image. Specifically, the color values are provided as RGB values in accordance with the RGB color space. Furthermore, the RGB color space is in the example a non-linear color space with a gamma value different than one. A typical gamma value may e.g. be 2.2. The contrast enhancement processor 103 accordingly comprises a color scheme converter 201 which receives the non- linear RGB image data from the color image source 101. The color scheme converter 201 then proceeds to convert the color scheme into a linear color scheme, which in the specific example is a linear RGB color scheme. Thus, the color scheme converter 201 performs the conversion: where R_υ, G_l} and B_1J denote the input red, green and blue sub-pixel drive values at row i and pixel positiony, respectively, and RiJ"¹, G₁J¹" and B₁J¹" represent the subpixel drive values in the linear light domain.

The color scheme converter 201 is coupled to a luminance compensation processor 203 which generates compensated luminance values for the image areas where the compensated luminance values depend not only on the luminance values of the image areas but also on the color values of the image areas.

The luminance compensation processor 203 comprises a nominal luminance processor 205 arranged to determine a nominal luminance value for each image area, and specifically for each image pixel. The nominal luminance value is specifically a relative luminance value for the color relative to a white color. Thus, the nominal luminance value may reflect a perceptual brightness or luminance level for the color for a given light intensity.

Thus, different colors will have different nominal luminance values as the eye will perceive different colors with different brightness despite them having identical light intensities. For example, the same amount of light of a blue color will tend to be perceived to be ten times less bright than the same amount of light of a green color.

The nominal luminance processor 205 may for example comprise a data store which stores a look-up table comprising predetermined nominal luminance values for different color values at a given light intensity. The nominal luminance processor 205 may then normalize the color value to that light intensity and perform a table look-up that identifies the stored nominal luminance value for the color closest (or identical) to the color represented by the color value.

In the specific example of FIG. 2, the nominal luminance value is a maximum luminance value for the given color. Thus, the nominal luminance value is a relative value for the perceived luminance value for the color assuming that the light intensity is at the maximum level that can be represented by that color representation.

It will be appreciated that the maximum luminance value in this case may reflect both an impact of the color on the perceived luminance (due to the eye and brains relative brightness perception for different colors) as well as an impact of the varying ability of the color scheme to provide maximum light intensities for different colors. For example, for an RGB color scheme, a white signal may be represented by the component values (1,1,1) whereas the maximum light intensity that can be provided for e.g. the color red may correspond to (1,0,0), i.e. the maximum light intensity that can be provided will in this case be reduced for red relative to some composite colors. In the specific example, the nominal luminance processor 205 calculates the nominal luminance value for a color value directly from the linear multi component color values, i.e. from the linear RGB values. The calculation specifically comprises an initial normalization of the color values relative to a maximum value of the linear color component 5 values followed by a combination of the normalized values taking into account the perceptual luminance contribution for each color corresponding to a linear color component value.

Specifically, the nominal luminance processor 205 proceeds to normalize the linear RGB values by dividing each value by the largest of the multi component values:

i n / pmax ^max nmax | _ / nhn /~ihn nhn \ I τ, r Λ γ/ nhn /~ihn nhn \

¹⁰ \^R _y > <>:, ' ^βy ) - (^R _y ' ^y > ^B,_J )l^MΛX K^K,j > <>,, > ^B _y ) '

Thus, the relative RGB values corresponding to the maximum intensity for that color is determined (assuming that RGB values are represented as values in the range of [0,1]).

15 It will be appreciated that this maximum luminance also reflects the intensity limit of the color scheme for different colors (e.g. for a pure green light, the resulting maximum value will be (0,1,0) and for a composite white light, the resulting maximum value will be (1,1,1)).

It will also be appreciated that in other embodiments, other normalization 0 parameters may be used. For example, the normalization may be relative to the combined luminance for the pixel value, such as e.g.:

( nmax ^max nmax | / T₃Un _/~ιhn nhn \ / [ nlu f . t_/~ιhn γ- . I nhn V- K,_j ' ^_y ' B_y J = (X_y ' V_1J , B_υ )/ ^ [X_y J + [(J_1J J + [B_1J ) 5 The nominal luminance processor 205 then proceeds to calculate the perceived luminance for the normalized color value taking into account the eye's and brain's relative perception for the three primary display colors. Specifically, the nominal luminance processor 205 combines the multi component values with each value being weighted by a value reflecting the perceived brightness contribution from the corresponding primary color.

30 In the example, the nominal luminance processor 205 uses a weighted summation:

ymax _ τψ nmax , _W ^max , rp nmax where Yy^max is the nominal luminance value for the pixel and the weights W are set to reflect the perceptional brightness contribution for the individual primary color. For example, for a Rec 709 gamut the values may be set as W_R = 0.2126, W_G = 0.7152 and W_B = 0.0722.

The luminance compensation processor 203 furthermore comprises a luminance processor 207 which calculates a luminance value for each image area (and specifically pixel in the current example) in response to the color value for that image area. In the specific example, the same combination is used to determine the perceived luminance value as was used for determining the nominal luminance value. Thus, the luminance processor 207 calculates the luminance for pixel (i,j) as: .

The nominal luminance processor 205 and the luminance processor 207 are coupled to a compensated luminance processor 209 which proceeds to calculate a compensated luminance value for the pixels as a function of the nominal luminance value and the luminance value for the pixel. The compensated luminance value may be calculated in response to only these parameters without taking any other parameters into account.

In the specific example, the compensated luminance processor 209 calculates the compensated luminance value for a pixel by normalizing the luminance value for the pixel relative to the nominal luminance value for the pixel. In particular, the compensated luminance processor 209 can calculate the compensated luminance as a relative luminance of the pixel relative to a maximum luminance value. Specifically, the compensated luminance may be calculated as:

This value corresponds to an estimation of the illumination intensity of the pixel (i.e. is compensated for the brightness effect of the color information) and is specifically calculated by normalizing the luminance of the input pixel relative to the maximum luminance achievable for the color of the pixel. In the specific example, the RGB values may be in the range of [0,1] resulting in the illumination intensity value I also being in the range of [0,1]. The luminance compensation processor 203, and specifically the compensated luminance processor 209, is coupled to a luminance contrast enhancement processor 211 which is fed the compensated luminance values, and specifically the illumination intensity values I_u, and which proceeds to perform a luminance (only) contrast enhancement on the compensated luminance values.

Thus, the contrast enhancement applied to the image by the luminance contrast enhancement processor 211 considers only compensated luminance characteristics and does not consider any color parameters or characteristics.

It will be appreciated that any suitable contrast enhancement process based on luminance may be used. For example, a simple non- linear function may be applied to the compensated luminance values resulting in a reduction of low luminance values and an increase in high luminance values. For example, the transfer characteristic of FIG. 3 may be applied. It will be appreciated that a large number of luminance contrast enhancement algorithms will be known to the skilled person and that any suitable such algorithm may be used without detracting from the current invention.

Thus, in the example, the contrast enhancement is applied to the illumination signal I instead of being applied to the luminance Y. Denoting the contrast enhancement operation by F_contrast{}, and the resulting luminance values by I_enh, the enhancement operation may be represented by: J ^*

In the example of FIG. 1 and 2, the contrast enhancement processor 103 further comprises a de-compensation processor 213 which is capable of compensating the contrast enhanced luminance data for the compensation of the original luminance values performed by the luminance compensation processor 203. Thus, the de-compensation processor 213 may perform a de-compensation which corresponds to the reverse operation of the luminance compensation processor 203. Specifically, the normalization relative to the nominal luminance value may be reversed. As a specific example, the de-compensated luminance value for a pixel may be determined by multiplying the contrast enhanced compensated luminance value by the nominal luminance value associated with the color of that pixel.

In the specific example of FIG. 2, the de-compensation processor 213 performs an indirect de-compensation by directly scaling the color coordinates of the pixel dependent on the contrast enhanced color value. Specifically, the de-compensation processor 213 comprises a luminance relationship processor 215 which determines a relationship between the contrast enhanced luminance value and the compensated luminance value prior to the contrast enhancement. The relationship may specifically be a ratio indicating the relative change in the luminance value resulting from the contrast enhancement. In the specific example of FIG. 2, the luminance relationship processor 215 calculates the ratio:

The luminance relationship processor 215 is coupled to a color value modifier

217 which is also coupled to the color scheme converter 201. The color value modifier 217 proceeds to modify the color values of the image to reflect the relationship between luminance values which has been introduced by the contrast enhancement.

In the specific example where the color values are given by three multi color component values, the color value modifier 217 may simply multiply the color component values of a color value for a pixel by the ratio between the luminance values of the pixel before and after contrast enhancement. Thus, in the specific example of FIG. 2, the color value modifier 217 is a multiplier multiplying the individual linear RGB color values by the ratio between the compensated luminance value (the illumination intensity) before and after contrast enhancement.

enh

( rthn,out _/~ihn,out jyhn,out \ JΛ Γ / JΛ IIΠ _/~ihn j-thn \ ' ¹J

(Λ, , (^j _y , ^B _y )

Thus, the color values of the enhanced output are generated by scaling the input linear RGB-data values by the ratio of the enhanced and original illumination.

In the system, the luminance compensation processor 203 may accordingly be considered to perform a transformation from a luminance domain to an illumination intensity domain and the de-compensation processor 213 may be considered to perform the complementary transformation from the illumination intensity domain to the luminance domain.

The resulting RGB color values are then fed to a second color scheme converter 219 coupled to the color value modifier 217 and arranged to convert the linear RGB values to non-linear RGB values corresponding to the format used for the input data to the first color scheme converter 201.

The described approach may provide an improved contrast enhancement and may for example reduce the risk of introducing unnatural shadow effects to color images. The approach allows critical areas to be identified as having the same illumination intensity allowing them to be mapped in a consistent manner. Furthermore, the illumination map consisting of compensated luminance values provide clearer differences between illuminated areas and shadow areas. Contrast enhancement of the illumination values will therefore increase the shadow intensity while maintaining a natural balance between objects. Fig. 4 illustrates an example of a method of contrast enhancement for a color image.

The method initiates in step 401 wherein a color image is provided having color values for image areas.

Step 401 is followed by step 403 wherein contrast enhancement is performed for the color image by modifying a luminance characteristic for each image area of at least some image areas in response to a color value for the image area

It will be appreciated that the above description for clarity has described embodiments of the invention with reference to different functional units and processors. However, it will be apparent that any suitable distribution of functionality between different functional units or processors may be used without detracting from the invention. For example, functionality illustrated to be performed by separate processors or controllers may be performed by the same processor or controllers. Hence, references to specific functional units are only to be seen as references to suitable means for providing the described functionality rather than indicative of a strict logical or physical structure or organization. The invention can be implemented in any suitable form including hardware, software, firmware or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units and processors.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims do not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example shall not be construed as limiting the scope of the claims in any way.

Claims

CLAIMS:

1. An apparatus for contrast enhancement of a color image, the apparatus comprising: means (101) for providing the color image having color values for image areas; and - contrast enhancement means (103) for performing contrast enhancement for the color image by modifying a luminance characteristic for each image area of at least some image areas in response to a color value for the image area.

2. The apparatus of claim 1 wherein the contrast enhancement means (103) comprises: compensating means (203) for generating a compensated luminance value for each of the at least some image areas in response to the color value of the image area; and means (211) for performing a luminance contrast enhancement on the compensated luminance values.

3. The apparatus of claim 2 wherein the compensating means (203) further comprises: nominal luminance means (205) for determining a nominal luminance value for a first image area as a function of a color value for the first image area; and - luminance determining means (209) for generating a compensated luminance value for the first image area as a function of the nominal luminance value and a luminance value for the first image area.

4. The apparatus of claim 3 wherein the compensating means (203) is arranged to generate the compensated luminance value for the first image area by normalizing the luminance value for the first image area relative to the nominal luminance value.

5. The apparatus of claim 3 wherein the nominal luminance means (205) comprises a data store for storing a look-up table comprising nominal luminance values for a plurality of color values and is arranged to determine the nominal luminance value by a table look-up in the look-up table.

6. The apparatus of claim 3 wherein the nominal luminance value is a maximum luminance value for the color value of the first image area.

7. The apparatus of claim 3 wherein the nominal luminance means (205) is arranged to determine the nominal luminance as a function of a plurality of linear color component values of the color value, each color component value corresponding to a display primary color.

8. The apparatus of claim 7 wherein the nominal luminance means (205) is arranged to: generate normalized color component values by normalizing the linear color component values relative to a maximum value of the linear color component values; and determine the nominal luminance as a combination of the normalized color component values, the combination comprising a weighting corresponding to a perceptual luminance contribution for each color corresponding to a linear color component value.

9. The apparatus of claim 8 wherein the nominal luminance means (205) is arranged to determine the luminance value for the first image area as a combination of the linear color component values, the combination comprising a weighting corresponding to a perceptual luminance contribution for each color corresponding to a linear color component value.

10. The apparatus of claim 2 wherein the contrast enhancement means (103) further comprises: means (213) for de-compensating contrast enhanced luminance values resulting from the luminance contrast enhancement, the de-compensating corresponding to a reverse operation of the compensating means.

11. The apparatus of claim 2 wherein the contrast enhancement means (103) further comprises: means (215) for, for at least a first image area, determining a relationship between a compensated luminance value for the first image area and an enhanced luminance value resulting from the luminance contrast enhancement; and modifying means (217) for modifying a color value for the first image area in response to the relationship.

12. The apparatus of claim 11 wherein the modifying means (217) is arranged to multiply color component values of the color value for the first image area by a ratio between the compensated luminance value for the first image area and the luminance value for the first image area, each color component value corresponding to a display primary color.

13. The apparatus of claim 1 further comprising means (201) for converting nonlinear multi color component color values into linear multi color component color values; and wherein the luminance means is arranged to determine luminance values in response to the linear multi color component color values.

14. The apparatus of claim 1 wherein the contrast enhancement means (103) comprises means for determining an illumination intensity value for each image area in response to the color value and to perform the contrast enhancement on the illumination intensity value.

15. A method of contrast enhancement for a color image, the method comprising: providing the color image having color values for image areas; and performing contrast enhancement for the color image by modifying a luminance characteristic for each image area of at least some image areas in response to a color value for the image area.