JP2006295623A - Image processing method and image display device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image processing method capable of improving contrast while holding features of an original image such as luminance distribution and average luminance, and an image display device. <P>SOLUTION: The image processing method comprises a step 1 for applying color/space conversion to an inputted image signal and obtaining a converted image signal, a step 2 for preparing the histogram of the converted image signal, a step 4 for applying signal conversion to the converted image signal in accordance with the prepared histogram, and a step 5 for applying color/space conversion to the signal-converted image signal. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image signal processing method and apparatus, and an image display apparatus using them.

  Conventionally, processing by histogram smoothing is known as a method for improving image quality by making the luminance distribution of a display image uniform in an image display device. If this process is applied to an image whose luminance distribution is biased toward the dark side or an image whose luminance side is biased toward the bright side, the luminance will be distributed over the entire light and dark, improving the contrast and improving the image quality. Can be made. As described above, the histogram smoothing can be expected to be particularly effective for an image with uneven or uneven luminance distribution.

  However, according to this conventional method, since the luminance distribution of the display image is made uniform, it is difficult to adjust the degree of contrast improvement, the average luminance of the display image changes, and the relative brightness is reduced. There was a problem that it changed greatly.

  The present invention has been made to improve the above-described problems, and an object thereof is to improve contrast while maintaining the characteristics of an image. Therefore, in the present invention, the contrast can be improved while maintaining the characteristics of the original image such as the luminance distribution and the average luminance as compared with the conventional method.

  The present invention is an image processing method including a step of creating a histogram of an input image signal, and a step of performing signal conversion on the converted image signal according to the created histogram.

  The present invention also includes a step of performing color space conversion on an input image signal composed of a plurality of color components, a step of calculating a histogram for the signal subjected to the color space conversion, and a signal corresponding to the histogram An image processing method including a step of performing conversion and a step of performing color space conversion on the signal subjected to the signal conversion.

  The image processing method sets an upper limit value and a lower limit value and performs signal conversion when the input signal is within the range of the upper limit value and the lower limit value.

  Furthermore, the present invention is an image processing method in which the upper limit value is determined based on a histogram of an input image.

  Further, the present invention is an image processing method in which the lower limit value is determined based on a histogram of an input image.

  The present invention is the image processing method in which the signal conversion is nonlinear conversion.

  Furthermore, the present invention provides an image display device that displays an image based on an input image signal, a histogram creation unit that creates a histogram of the input image signal, and signal conversion of the image signal converted according to the created histogram. And an image display device including a signal conversion unit.

  As described above, by using the image signal processing method or the image signal processing apparatus according to the present invention, it is possible to improve the contrast while maintaining the characteristics of the image.

The best mode for carrying out the present invention will be described.
Embodiments of an image processing method and an image display apparatus according to the present invention will be described with reference to the drawings.

  Example 1 will be described. Hereinafter, an image processing apparatus and method according to the present embodiment will be described in detail with reference to the drawings. First, the image processing apparatus according to the present embodiment will be described with reference to the drawings. FIG. 2 is a block diagram showing the configuration of the image processing apparatus 2 in the present embodiment. The image processing apparatus 2 includes a color space conversion unit 21, a histogram creation unit 22, a memory 23, a statistic calculation unit 24, a signal conversion unit 25, and a color space conversion unit 26.

  The color space conversion unit 21 converts the input image signals R1, G1, and B1 input from the (R, G, B) color space to the (Y, Cb, Cr) color space by matrix calculation. Image signals Y1, Cb1, and Cr1 are output. The histogram creation unit 22 creates a histogram (frequency distribution of the luminance signal Y1) of the input image based on the image signals Y1, Cb1, and Cr1 of each pixel input from the color space conversion unit 21, and stores them in the memory. The statistic calculator 24 calculates an average value and a standard deviation based on the histogram of the input image stored in the memory 23. The signal conversion unit 25 performs signal conversion processing on the image signals Y1, Cb1, and Cr1 input from the color space conversion unit 21 using the average value and standard deviation of the histogram input from the statistic calculation unit 24. The image signals Y2, Cb2, and Cr2 are output. The color space conversion unit 26 converts the image signals Y2, Cb2, and Cr2 input from the signal conversion unit 25 from the (Y, Cb, Cr) color space to the (R, G, B) color space by matrix calculation. The converted image signals R2, G2, and B2 are output.

  Next, an algorithm of the image processing method in the present embodiment will be described with reference to the drawings. FIG. 1 is a flowchart showing the flow of the image processing method in this embodiment. The three types of image signals R1, G1, and B1 input to the image processing apparatus in the present embodiment are subjected to color space conversion processing in Step 1 and converted into image signals Y1, Cb1, and Cr1. Next, in step 2, a histogram of an input image that is a frequency distribution of the luminance signal Y1 is created. Next, based on the histogram created in Step 3, the average value and standard deviation of the luminance signal Y1 are calculated. Next, in step 4, the image signals Y1, Cb1, and Cr1 are subjected to signal conversion processing based on the calculated average value and standard deviation of the luminance signal Y1 to obtain Y2, Cb2, and Cr2. Next, in step 5, the image signals Y2, Cb2, and Cr2 are subjected to color space conversion processing to obtain output image signals R2, G2, and B2.

ＵＲＧ＝ＲＧ−ＬＲＧ

ＵＢＲ＝１−ＬＢＲ、ＬＴ＝０．１、ＵＴ＝０．９

Further, signal conversion processing for the image signals Y1, Cb1, and Cr1 will be described in detail. Hereinafter, the total number of pixels of the input image is N, the number of steps of the input luminance signal Y1 (the number of gradations) is K, the average value of the histogram is M, and the standard deviation is σ. At this time, the input luminance signal Y1 takes an integer value between 0 and K-1. The minimum value of the input luminance signal Y1 is Ymin, and the maximum value is Ymax. The signal conversion is performed in different cases according to the value of the input luminance signal Y1, and the converted luminance signals are Y2, Cb2, and Cr2. First, each parameter RG, LRG, URG, LBR, UBR, LT, UT, LR, UR, and MID necessary for signal conversion is calculated by the following formula.

URG = RG-LRG

UBR = 1-LBR, LT = 0.1, UT = 0.9

Next, the cumulative frequency of the histogram from signal values 0 to Y is A (Y), and MIN and MAX are determined as follows according to the values of Ymin and Ymax.
When Ymin <LT × K, the minimum signal value Y satisfying A (Y) ≧ N × LR is MIN, and when Ymin ≧ LT × K, MIN = Ymin.

Further, when Ymax> UT × K, the maximum signal value Y satisfying A (Y) <N × (1−UR) is set to MAX, and when Ymax ≦ UT × K, MAX = Ymax. further,
LB = MIN × (1−LBR), UB = MAX + (K−MAX) × UBR
And
Here, S (x) is a sigmoid function defined by the following equation.

（２）ＭＩＮ≦Ｙ１＜ＭＩＤの場合

（３）ＭＩＤ≦Ｙ１≦ＭＡＸの場合

（４）ＭＡＸ＜Ｙ１の場合

これにより、Ｙ２を得ることができる。また、いずれの場合も、Ｃｂ２＝Ｃｂ１、Ｃｒ２＝Ｃｒ１とする。
このようにして、入力画像信号Ｒ１、Ｇ１、Ｂ１を、変換された画像信号Ｒ２、Ｇ２、Ｂ２とすることができる。 Then, the cases are classified according to the value of the input luminance signal Y1, and the following signal conversion is performed.
(1) When Y1 <MIN

(2) When MIN ≦ Y1 <MID

(3) When MID ≦ Y1 ≦ MAX

(4) When MAX <Y1

Thereby, Y2 can be obtained. In either case, Cb2 = Cb1 and Cr2 = Cr1.
In this way, the input image signals R1, G1, and B1 can be converted into converted image signals R2, G2, and B2.

  FIG. 3 is an example of a histogram before the signal conversion process is performed. FIG. 4 is an example of a histogram after the signal conversion process. FIG. 5 is an example of a luminance correspondence graph when the signal conversion process is performed. Comparing FIG. 4 with FIG. 3, it can be seen that the histogram is averaged by performing the signal conversion process, which improves the contrast even for an image with uneven or uneven luminance distribution, It can be understood that can be improved.

Explanatory drawing of the flowchart which showed the flow of the process of the image processing method in this invention. The block diagram which showed the structure of the image processing apparatus 2 in this invention. Explanatory drawing of an example of the histogram before performing the signal conversion process in this invention. Explanatory drawing of an example of the histogram after performing the signal conversion process in this invention. Explanatory drawing of an example of the brightness | luminance corresponding | compatible graph of the signal conversion process in this invention.

Explanation of symbols

2 Image processing device 21 Color space conversion unit 22 Histogram creation unit 23 Memory 24 Statistics calculation unit 25 Signal conversion unit 26 Color space conversion unit

Claims (7)

  An image processing method, comprising: creating a histogram of an input image signal; and performing signal conversion on the converted image signal in accordance with the created histogram.   A step of performing color space conversion on an input image signal composed of a plurality of color components, a step of calculating a histogram for the signal subjected to the color space conversion, and a step of performing signal conversion according to the histogram; And a step of performing color space conversion on the signal subjected to the signal conversion.   The image processing method according to claim 1, wherein an upper limit value and a lower limit value are set, and signal conversion is performed when an input signal is within the range of the upper limit value and the lower limit value.   The image processing method according to claim 3, wherein the upper limit value is determined based on a histogram of an input image.   The image processing method according to claim 3, wherein the lower limit value is determined based on a histogram of an input image.   The image processing method according to claim 1, wherein the signal conversion is nonlinear conversion. In an image display device that displays an image based on an input image signal,
An image display device comprising: a histogram creation unit that creates a histogram of an input image signal; and a signal conversion unit that performs signal conversion on the converted image signal according to the created histogram.

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