Scene-based adaptive backlight adjustment method and circuit for local dimming

A image adjustment method applicable to a display includes: defining multiple areas on a display region of the display; obtaining statistics of grayscale of a preliminary image; determining an image type of the preliminary image according to the statistics of grayscale of the preliminary image; generating a Cumulative Distribution Function (CDF) of luminance according to the statistics of grayscale of the preliminary image; individually adjusting a backlight level for each of the areas according to the CDF and the image type of the preliminary image; and generating an output image with each of the areas being individually adjusted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image compensation technique, and more particularly, relating to a technique of dynamically adjusting the backlight level of each area of a light emitting diode (LED) display.

2. Description of the Prior Art

LED-backlit is a flat display technique using LED backlighting instead of cold cathode fluorescent (CCFL) backlighting. A LED-backlit display may offer reduced energy consumption, better contrast and brightness.

The capability of individually controlling backlight level of LEDs allows for the local dimming technology which makes the dark portions in an image to provide pure dark sensations.

Conventional local dimming methods suffer from disadvantages such as high power consumption and decreased luminance. In addition, conventional local dimming methods may introduce unwanted side effects that deteriorate the image quality. For example, the conventional local dimming methods may cause the halo effect or Mosaic effect, when makes the images seem blocky or pixelated.

In view of the above, there is a need for a novel method to solve the aforementioned problems.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a scene-based adaptive backlight adjustment method and circuit for local dimming. More specifically, the present invention provides a method and an associated apparatus for dynamically adjusting the backlight level of each area of LED display in order to solve the above problems.

According to an embodiment of the present invention, an image adjustment method applicable to a display is provided. The image adjustment method comprises: defining multiple areas on a display region of the display; obtaining statistics of grayscale of a preliminary image; determining an image type of the preliminary image according to the statistics of grayscale of the preliminary image; generating a Cumulative Distribution Function (CDF) of luminance according to the statistics of grayscale of the preliminary image; individually adjusting a backlight level for each of the areas according to the CDF and the image type of the preliminary image; and generating an output image with each of the areas being individually adjusted.

According to an embodiment of the present invention, an image adjustment circuit applicable to a display is provided. The image adjustment circuit comprises a storage unit; and a processor. The process is arranged to perform following steps: defining multiple areas on a display region of the display; obtaining statistics of grayscale of a preliminary image; determining an image type of the preliminary image according to the statistics of grayscale of the preliminary image; generating a CDF of luminance according to the statistics of grayscale of the preliminary image; individually adjusting a backlight level for each of the areas according to the CDF and the image type of the preliminary image; and generating an output image with each of the areas being individually adjusted.

DETAILED DESCRIPTION

Some phrases in the present specification and claims refer to specific elements; however, please note that the manufacturer might use different terms to refer to the same elements. Further, in the present specification and claims, the term “comprising” is open type and should not be viewed as the term “consists of.” The term “electrically coupled” can refer to either direct connection or indirect connection between elements. Thus, if the specification describes that a first device is electrically coupled to a second device, the first device can be directly connected to the second device, or indirectly connected to the second device through other devices or means.

Please refer toFIG. 1A, which is a diagram illustrating an image adjustment circuit100according to an embodiment of the present invention, wherein the image adjustment circuit100is applicable to a display, such as an LED monitor. As shown inFIG. 1A, the image adjustment circuit100comprises a processor110and a storage unit120. The processor110may be used to run program codes and perform various kinds of operations, especially the steps shown inFIG. 2. The storage unit120may be used to store a look-up table (LUT)125.FIG. 1Bshows an example of the aforementioned display, wherein the display1000comprises a display region1050that can be divided into multiple areas as those shown in the grid pattern inFIG. 1B, and the light emitted from each area is attributed to multiple LEDs inside.

FIG. 2is a flowchart illustrating an image adjustment method200corresponding to the image adjustment circuit100shown inFIG. 1A. If the result is substantially the same, the steps may not necessarily be executed in the exact order shown inFIG. 2. The image adjustment method200is summarized as follows.

Step204: Define multiple areas on a display region of the display;

Step206: Obtain statistics of grayscale of a preliminary image;

Step208: Determine the image type of the preliminary image according to the statistics of grayscale of the preliminary image;

Step210: Generate a Cumulative Distribution Function (CDF) of luminance according to the statistics of grayscale of the preliminary image;

Step212: Generate an inverse CDF according to the CDF;

Step214: Individually adjust the backlight level for each of the areas according to the inverse CDF of luminance and the image type of the preliminary image;

Step216: Generate an output image with each of the areas being individually adjusted.

In step206, statistics of grayscale can be referred toFIG. 3A, which is a diagram illustrating an original histogram before the equalization, wherein the horizontal axis represents the grayscale. The histogram shown inFIG. 3Acan be converted into the CDF shown inFIG. 3Bby the processor110, as described in step210. The CDF of luminance is statistics for providing the information about how many pixels in each grayscale, and the CDF of luminance can be used to determine the image type of the preliminary image, wherein the preliminary image can be realized as an unprocessed image or data. Image types may be roughly classified as either dark-dominant or light-dominant images. Most pixels of a dark-dominant image have low grayscale (e.g. closer or equal to 0), while most pixels of a light-dominant image have high grayscale (e.g. closer or equal to 255). In other words, a dark-dominant image is an image that comprises a majority of low grayscale pixels and dark-dominant areas are areas that comprise a majority of low grayscale pixels. Similarly, a light-dominant image is an image that comprises a majority of high grayscale pixels, and light-dominant areas are areas that comprise a majority of high grayscale pixels.

To be more specific, image types may be classified as dark scene images, text images and webpage images. In general, a dark scene image, such as a cinema image, requires dark pixels to look even darker, so that some detailed curves and edges can be revealed more clearly. Hence, reducing the backlight levels of those dark-dominant areas may improve the overall image quality.

However, regarding a light-dominant image, such as a text image or webpage image, dark pixels only occupy a small part of the entire display region1050. In this situation, reducing the backlight levels of those unnoticeable dark-dominant areas simply makes them more noticeable without providing an advantage. For example, the entire image may seem even more blocky or pixelated, which deteriorates the image quality and greatly lowers the user experience.

To address the above problem encountered in related art techniques, the present invention further takes the image type of the preliminary image (i.e. the image not outputted yet) into account in order to make the light-dominant image look smooth and natural.FIG. 3Ais an original histogram of a dark-dominant image according to an embodiment of the present invention. As can be seen from the histogram, the number of low grayscale pixels (e.g. approximately ranging from 0 to 50) is much more than the number of high grayscale pixels. Hence, in Step208, the image type of the preliminary image is determined according to the histogram shown inFIG. 3A. Further, in Step210, the CDF of luminance (e.g. the CDF shown inFIG. 3B) can be generated according to the histogram shown inFIG. 3A, and the CDF of luminance can later be used to generate the inverse CDF in Step212.FIGS. 4A-4Dshow various examples of the CDF of luminance, each having a difference distribution.FIGS. 5A-5Dshow the inverse CDFs respectively converted from the CDFs inFIGS. 4A-4D, wherein the term “backlight” is briefed as “B/L” inFIGS. 5A-5D. The inverse CDF can be stored into the LUT125of the storage unit120of the image adjustment circuit100, for follow-up use. The inverse CDFs shown in inFIGS. 5A-5Dmay be used to perform local dimming, that is, to adjust the backlight in an area-by-area manner.

In Step214, the backlight level of each of the areas is individually adjusted according to the inverse CDF of luminance and the image type of the preliminary image. As explained above, referencing the image type of the preliminary image is crucial for reducing the blocky or pixelated effect resulted from the local-dimming operations, and the detailed utilizations of the image type of the preliminary are as follows.

When the image type is determined as a dark-dominant image rather than a light-dominant image, the backlight level of all of the areas will be reduced by a first extent, wherein the dark-dominant image is an image that comprises a majority of low grayscale pixels, and the dark-dominant areas are areas that comprise a majority of low grayscale pixels.

On the other hand, when the image type is determined as a light-dominant image rather than the dark-dominant image, the backlight level of any of dark-dominant areas amongst the areas will be reduced by a second extent smaller than the first extent, wherein the light-dominant image is an image that comprises a majority of high grayscale pixels, thereby alleviating the aforementioned blocky or pixelated effect. In another example, the backlight level of any of dark-dominant areas amongst the areas can be even raised or remained unchanged based on different user modes or settings, in order to further suppress the aforementioned blocky or pixelated effect.

After the compensation made to preliminary is done, Step216outputs an output image with each of the areas being individually adjusted.

FIG. 6shows a hardware architecture600according to an embodiment of the present invention, wherein the hardware architecture600may be adopted by the aforementioned image adjustment circuit100, display1000and image adjustment method200. Block620is marked with “HGL” which represents statistics, such as the histogram shown inFIG. 3A, retrieved from the preliminary image (marked with Img in) from Block610. Block630represents an LUT unit which generates an inverse CDF of luminance in Block633from the CDF in Block632which is further based on the statistics in Block631. Block640(marked with “Spatial filter”) is used to perform some image processing, such as Fourier Transformation, etc. Block650is used for receiving the processing result from Block640and the inverse CDF from Block630. Next, Block660utilizes the information from Block650to compensate the preliminary image so as to generate the compensated output image in Block670.

In view of the above, embodiments of the present are capable of: reducing the halo effect for the dark scene image due to purified dark-dominant areas; and improving smooth in high bright scene or web scene images, e.g. mitigating the clipping effect due to steep grayscale difference between adjacent areas of the display.