System and method of contrast enhancement

1. A system for contrast enhancement, comprising:

a processor configured to implement a contrast enhancement function on an input pixel, thereby resulting in a contrast-enhanced pixel;

a delta unit configured to subtract the input pixel from the contrast-enhanced pixel, and to produce a difference value;

a delay unit operable to generate a plurality of delayed difference values with respect to the difference value;

a low-pass filter arranged to generate a refined difference value according to the difference value and at least a portion of the delayed difference values; and

an adding unit configured to add back the refined difference value to the input pixel, and having an output to produce an output pixel.

2. The system of claim 1 , wherein the contrast enhancement function comprises a gray-to-gray transfer lookup table (LUT) which maps a gray level of the input pixel to another gray level of the contrast-enhanced pixel.

3. The system of claim 2 , wherein the gray-to-gray transfer lookup table possesses a transfer curve that is determined according to an image histogram of a plurality of the input pixels.

4. The system of claim 1 , wherein the delay unit comprises a one-dimensional delay unit for obtaining the plurality of delayed difference values that include a plurality of delayed variants of the difference value, and a plurality of advanced variants of the difference value.

5. The system of claim 1 , wherein the low-pass filter comprises:

an adder configured to add weighted delayed difference values; and

an averaging unit arranged to obtain an average of sum of the weighted delayed difference values, thereby resulting in the refined difference value.

6. The system of claim 5 , wherein the refined difference value Rδ(x,y) is obtained according to an expression as follows:

R δ( x,y )=[δ( x,y )* w 0 +δ −1 ( x,y )* w −1 +δ −2 ( x,y )* w −2 + . . . +δ −n ( x,y )* w −n +δ 1 ( x,y )* w 1 +δ 2 ( x,y )* w 2 + . . . +δ m ( x,y )* w m ]/W

where δ(x,y), δ −1 (x,y), . . . , δ −n (x,y), δ 1 (x,y), . . . , δ m (x,y) are the delayed difference values, w −n to w m are weightings of the delayed difference values respectively, and W is the sum of the weighted delayed difference values W=w 0 +w −1 + . . . +w −n +w 1 + . . . +w m , with m and n being integers.

7. A method for contrast enhancement, comprising:

processing an input pixel with a contrast enhancement function, thereby resulting in a contrast-enhanced pixel;

subtracting the input pixel from the contrast-enhanced pixel, thereby resulting in a difference value;

generating a plurality of delayed difference values with respect to the difference value;

generating a refined difference value by low-pass filtering according to the difference value and at least a portion of the delayed difference values; and

adding back the refined difference value to the input pixel, thereby resulting in an output pixel.

8. The method of claim 7 , wherein the contrast enhancement function comprises a gray-to-gray transfer lookup table (LUT) which maps a gray level of the input pixel to another gray level of the contrast-enhanced pixel.

9. The method of claim 8 , wherein the gray-to-gray transfer lookup table possesses a transfer curve that is determined according to an image histogram of a plurality of the input pixels.

10. The method of claim 7 , wherein the step of generating the delayed difference values comprises performing one-dimensional delay for obtaining the plurality of delayed difference values that include a plurality of delayed variants of the difference value, and a plurality of advanced variants of the difference value.

11. The method of claim 7 , wherein the low-pass filtering step comprises:

adding weighted delayed difference values; and

obtaining an average of sum of the weighted delayed difference values, thereby resulting in the refined difference value.

12. The method of claim 11 , wherein the refined difference value Rδ(x,y) is obtained according to an expression as follows:

R δ( x,y )=[δ( x,y )* w 0 +δ −1 ( x,y )* w −1 +δ −2 ( x,y )* w −2 + . . . +δ −n ( x,y )* w −n +δ 1 ( x,y )* w 1 +δ 2 ( x,y )* w 2 + . . . +δ m ( x,y )* w m ]/W

where δ(x,y), δ −1 (x,y), . . . , δ −n (x,y), δ 1 (x,y), . . . , δ m (x,y) are the delayed difference values, w −n to w m are weightings of the delayed difference values respectively, and W is the sum of the weighted delayed difference values W=w 0 +w −1 + . . . +w −n +w 1 + . . . +w m , with m and n being integers.