WO2007072321A1

WO2007072321A1 - Color image enhancement for small displays.

Info

Publication number: WO2007072321A1
Application number: PCT/IB2006/054802
Authority: WO
Inventors: Laurent Lucat
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2005-12-19
Filing date: 2006-12-13
Publication date: 2007-06-28

Abstract

The invention relates to a method and device for color image enhancement particularly suited to use in respect of small displays, wherein the saturation in respect of each pixel is boosted to a greater or lesser extent depending on the saturation value and/or the hue (non-linear and non-uniform saturation boosts). The hue in respect of each pixel is further intensified to a greater or lesser extent depending on the principal contributing colors of the hue (non-linear and non-uniform hue modification).

Description

COLOR IMAGE ENHANCEMENT FOR SMALL DISPLAYS

FIELD OF THE INVENTION

This invention relates generally to a color image enhancement system and method particularly suited for use in small displays, such as the type provided in portable, battery- powered electronic devices like mobile telephones and personal digital assistants (PDAs).

BACKGROUND OF THE INVENTION

There are many constraints placed on the displays used in portable, battery-powered devices, such as mobile telephones and PDAs in order to minimize weight, maximize battery life and maintain competitive costs. As a result, color saturation is often sacrificed in return for better performance in other areas. Furthermore, the current trend toward higher pixel densities, resulting in lower aperture ratios, will further reduce color saturation because the color filters must transmit even more light. In addition to this intrinsic limited color rendition, variations in the display's primary colors must also be taken into consideration. Not only do these colors differ in displays from one supplier to another, but also within a single batch from the same source.

As a result, the primary colors of mobile displays are often different from those dictated by broadcasting standards. In liquid crystal displays (LCDs), the primary colors are typically less saturated and the blue primary color is shifted in hue. In organic light emitting diode (OLED) displays, all primary colors differ in hue and the blue primary color is also less saturated in order to extend lifetime.

The human eye will accept limited deviation in a display's color primaries, even without the application of additional color processing to the video signal. However, without additional color processing, the colors on most portable displays look rather pale and sometimes have incorrect hue. Furthermore, due to the de-facto small size of mobile device displays, it is often desirable, from a user-experience point of view, to artificially enhance the color contrast of the displayed picture or video, even if the display performances are very good.

PCT Application No. WO 2004/068845 describes a method of applying selective chromaticity corrections (i.e. changes in hue, saturation and brightness) to the device- dependent coordinates of pure colors that define the gamut of colorimetric values that the device is capable of producing, the selective chromaticity corrections being applied using a piecewise linear correction function. However, because the chromaticity correction is applied uniformly across an entire image, there is still a trade off between color contrast enhancement and natural color preservation.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a method and device for improved color image enhancement, particularly suited for use in small displays, wherein a high degree of color contrast enhancement is achieved together with a high level of natural color preservation.

In accordance with the present invention, there is provided a method for enhancing a color input image for display, said input image being represented by a plurality of pixels in respect of each of which is provided image data including a saturation value and a hue value, the method comprising the steps of determining the saturation value and/or the hue value of each pixel of said input image and modifying said saturation value and/or said hue value by an amount dependent on said respective value only if if said respective value is determined to meet a predetermined criterion.

Also in accordance with the present invention, there is provided a device for enhancing a color input image for display, said input image being represented by a plurality of pixels in respect of each of which is provided image data including a saturation value and a hue value, the device comprising means for determining the saturation value and/or the hue value of each pixel of said input image and modifying said saturation value and/or said hue value by an amount dependent on said respective value only if said respective value is determined to meet a predetermined criterion.

Thus, modification of the saturation and/or hue value of a pixel of the input image is non-uniform (in the sense that the amount of modification made, if any, is dependent on the saturation value and/or hue value) and non-linear (in the sense that whether or not any modification is made is determined by the saturation value and/or the hue value and, more specifically, whether the respective saturation value and/or hue value meets some predetermined criterion.

In one exemplary embodiment, modification of the saturation value of a pixel comprises amplifying said saturation value. In this case, it is considered that pixels having a relatively low saturation value and those having a relatively high saturation value should be minimally amplified or not amplified at all. Thus, the predetermined criterion may comprise that the saturation value falls within a predetermined medium range of values, in which case the saturation value is significantly amplified, and otherwise the saturation value is minimally amplified or not amplified at all. The gain by which a saturation value is amplified if it meets the predetermined criterion, may be dependent on the hue. In this case, it is desirable to amplify a saturation value in respect of a primarily blue or green hue by a greater extent than that of a primarily yellow or red hue.

In another exemplary embodiment of the present invention, modification of the hue value of a pixel comprises increasing the intensity of the hue by replacing or combining said hue with the closest pure hue, i.e. red, green, blue, cyan, magenta or yellow. In this case, once again, it is considered advantageous to increase the intensity of a primarily blue or green hue by a greater extent than that of a primarily red or yellow hue.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the present invention will be apparent from, and elucidated with reference to, the embodiments described herein.

Embodiments of the present invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

Figure 1 illustrates an exemplary transfer function demonstrating non-linear image processing used in an exemplary method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a method and device for color image enhancement particularly suited to use in respect of small displays, wherein the saturation in respect of each pixel is boosted to a greater or lesser extent depending on the saturation value and/or the hue according to a non-linear and/or non-uniform process. The hue in respect of each pixel is further intensified to a greater or lesser extent depending on the principal contributing colors of the hue according to a non-linear and/or non-uniform hue process.

Thus, an exemplary embodiment of the present invention is based on two principal principles. The first one is non-uniform (in the 2-dimensional color space) and/or non-linear color saturation boost.

According to the non-linear boost, instead of amplifying the saturation with the same gain over the full dynamic, as in the prior art, because input pixels having a low saturation often better correspond to some light reflections, it is considered advantageous not to amplify the saturation too much. For input pixels already having a high saturation value, it is also considered advantageous not to amplify the saturation too much because it would provide an unrealistic image. Only pixels having middle saturation value may be advantageously affected by a significant saturation gain.

According to the non-uniform boost, it is considered, from a user-experience point of view, more acceptable to modify (boost) the saturation of some given colors than others. Indeed, it is undesirable, for example, to modify the color of the human skin. On the other hand, the color of a blue sky or a green landscape may be intensified. This leads to a saturation gain, which is dependent upon the hue. More explicitly, pixels which hue is around blue or green are provided with a higher saturation boost, than those having a hue around yellow or red.

The second principle is the non-uniform hue modification. In order to better distinguish between various objects in an image, it is considered desirable to better differentiate the various colors of the images. One "extreme" solution would be to substitute the input color hue by the closest pure one, resulting e.g. in 6 pure color hues (red, yellow, green, cyan, blue, magenta) images. This provides a high contrast but a too much cartoon-like "surrealist" picture. Thus, such an approach is advantageously mitigated by a "smoother" processing, such as a combination of the input hue and the closest pure one.

Furthermore, for the same reason as the one mentioned above, such a color modification should advantageously be lower for the yellow/red hues than for the blue/green ones. Indeed, for a small display viewing, it can be useful to enhance the contrast between a green-like landscape and a blue-like sky by setting a (more) pure green landscape and a (more) pure blue sky, as at the same time, it is better not to modify too much the yellow-pink like skin hue.

More specifically, a possible transfer function for the application of chromaticity correction in respect of an exemplary embodiment of the present invention is illustrated in Figure 1. Others are possible, but the global idea remains the same, namely, that the saturation of input pixels having a low saturation value or a high saturation value is amplified by a lower degree than the saturation of pixels having a saturation value in a medium range. The formula describing the illustrated transfer function may be written as: S_out (i) = k(S_in (i))*S_in (i) + A(S_in (i)) where i the the index of a pixel, S_out designates a saturation value of an output pixel, S_in designates a saturation value of an input pixel, and k and A are factors which define a non-linear function such as the one shown in Figure 1. Thus, the amplification of saturation is applied non-linearly across an image, depending on the saturation value of the input pixel.

In addition to the above-described non-linear transfer function, a so-called nonuniform transfer function is also applied to the input pixels in an exemplary embodiment of the invention. The resultant transfer function can be written as:

S_out (i) = k(S_in (i), H_in (i))*S_in (i) + A(S_in (i), H_in (i)) where H_in designates the hue of an input pixel.

With regard to hue modification, it is first necessary to choose the polarization hues. One could, for example, take the three primary hues (R, G, B) or 6 hues (3 primaries + cyan, magenta, yellow), or more. In this case, the polarization hues are equally angularly distributed on the chromatic disc. One could also choose the polarization hues as a function of the content of the input image.

An example of a transfer function for modification of the hue is the following: H_out (i) = (l-k(i))*H_in (i) + k(i)*H_pol (i) where H_pol (i) is the polarization hue for pixel i, i.e. the current polarization hue + neighboring H_in (i), and k(i)is a weighting coefficient which can depend on H_in (i).

Thus, modification of both the saturation S and the hue H is non-uniform and nonlinear, with a typical conversion matrix of RGB or YUV (luminance, red difference, blue difference) to (Brightness, Saturation, Hue) or HSV (Hue, Saturation, Value), and vice versa. This leads to a transformation from YUV => HSV or RGB => HSV. The processing (to modify the saturation S and hue H) is then driven in the HSV color space, and afterwards, the inverse transformation is performed, from HSV => YUV or HSV => RGB. In general, on a chromatic disc, the saturation S corresponds to the radius and the hue H corresponds to the angle, and the following exemplary conversion formulas will be known to a person skilled in the art: _m B) ≠ _n 0

H'HSV is multiplied by 60° to get a hue value H_Hsv in degrees.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word "comprising" and "comprises", and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. The singular reference of an element does not exclude the plural reference of such elements and vice- versa. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. The precise implementation may depend on complexity requirements. In a device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A method for enhancing a color input image for display, said input image being represented by a plurality of pixels in respect of each of which is provided image data including a saturation value and a hue value, the method comprising the steps of determining the saturation value and/or the hue value of each pixel of said input image and modifying said saturation value and/or said hue value by an amount dependent on said respective value only if said respective value is determined to meet a predetermined criterion.

2. A method according to claim 1, wherein modification of the saturation value of a pixel comprises amplifying said saturation value.

3. A method according to claim 2, wherein the predetermined criterion comprises that the saturation value falls within a predetermined medium range of values, in which case the saturation value is significantly amplified, and otherwise the saturation value is minimally amplified or not amplified at all.

4. A method according to claim 2, wherein the gain by which a saturation value is amplified if it meets the predetermined criterion is dependent on the hue.

5. A method according to claim 4, wherein a saturation value in respect of a primarily blue or green hue is amplified by a greater extent than that of a primarily yellow or red hue.

6. A method according to claim 1, wherein modification of the hue value of a pixel comprises increasing the intensity of the hue.

7. A method according to claim 6, wherein increasing the intensity of the hue comprises replacing or combining said hue with the closest pure hue.

8. A method according to claim 7, wherein the intensity of a primarily blue or green hue is increased by a greater extent than that of a primarily red or yellow hue.

9. A device for enhancing a color input image for said display, said input image being represented by a plurality of pixels in respect of each of which is provided image data including a saturation value and a hue value, the device comprising means for determining the saturation value and/or the hue value of each pixel of said input image and means for modifying said saturation value and/or said hue value by an amount dependent on said respective value only if said respective value is determined to meet a predetermined criterion.