CN109246406B - Method for realizing automatic white balance of color camera - Google Patents

Abstract

The invention relates to a method for realizing automatic white balance of a color camera. The method improves the convergence speed of the automatic white balance algorithm of the color camera. The method mainly comprises the following steps: 1. obtaining a current gain and target gain relational expression according to the RGB and YCbCr space conversion relation; 2. detecting white points of the current frame image; 3. estimating the color temperature of the current frame image N, and judging whether color cast occurs or not; 4. calculating a target gain according to the current gain and the target gain relational expression in the step 1; 5. the gain adjustment is made for the R, B components of each pel of the N +2 frame image.

Description

Method for realizing automatic white balance of color camera

Technical Field

The invention relates to an image processing technology, in particular to a method for realizing automatic white balance of a hue machine.

Background

The basic concept of white balance is to restore a white object to white regardless of the light source. The spectral characteristics are different from each other under different light sources or weather conditions, which is called color temperature. Physically, a person sees an object in a particular color because light reflected from the object appears in the human visual system when the light is directed at the object. That is, the color of an object that can be seen depends not only on its reflective properties, but also on the light directed at the object, e.g., a piece of white paper will be orange under tungsten light, green under fluorescent light, and somewhat blue under sunlight. The purpose of automatic white balance is to automatically adjust the proportion of the three primary color components of the image under different color temperature illumination, so that the image looks consistent with the color shot under a standard light source.

Many color cameras with an automatic white balance function perform color temperature evaluation on images in a YCbCr space, and then adjust the gain values of Cb and Cr to make Cb and Cr meet the conditions of white pixel points, thereby completing the automatic white balance function, so that the adjustment method of the Cb and Cr gains is a key point of automatic white balance.

The existing method adjusts the gain step by setting the step length, but when the method is adopted for adjustment, the convergence speed is low, and the use requirement of a high-frequency camera cannot be met.

Disclosure of Invention

In order to overcome the defects of the prior art in the background art, the invention provides a method for realizing automatic white balance of a color camera, which improves the speed of fast convergence of the automatic white balance of the color camera.

The specific technical scheme of the invention is as follows:

a method for realizing automatic white balance of a color camera runs on a processor of the color camera, and the processor is of an FPGA (field programmable gate array) or a CPU (central processing unit) or a DSP (digital signal processor); the method is realized by the following steps:

【1】 Acquiring a relational expression of a current gain R component gain gamma and a target gain R component gain gamma 'and a relational expression of a current gain B component gain alpha and a target gain B component gain alpha' according to the space conversion relation between RGB and YCbCr;

the concrete formula is as follows:

【2】 Setting a white point number threshold value w, detecting each pixel of the current frame image N, judging whether the pixel is a white point, counting the number w 'of the detected white points of the current frame image N, comparing the detected white point number w' of the current frame image N with the white point number threshold value w, if w 'is more than or equal to w, performing the step (3), and if w' is less than w, performing the step (6);

【3】 Estimating the color temperature of the current frame image N, and judging whether color cast occurs or not;

calculating the average value of all white points of the current frame image NAnd

if it is notIf M is set to be 9, judging that the image has color cast, and adjusting the gain of the component B or R, and entering the step (4);

if it is notJudging that the image has no color cast, and entering the step (6);

【4】 When in useCalculating a target gain R component gain gamma' according to a formula (1); when in useCalculating a target gain B component gain alpha' according to a formula (2);

【5】 And (4) carrying out white balance adjustment on the current frame image through the target gain R component gain gamma 'or the target gain B component gain alpha' obtained in the step (4).

【6】 After waiting for the 1 frame image transmission, the steps [ 2 ] to [ 5 ] are repeated from the N +2 frame image.

Further, in the step [ 3 ], the judgment condition that any pixel in the current frame image N is a white point is as follows: y is more than or equal to A, | Cb | is less than or equal to B, | Cr | is less than or equal to C; wherein, A is 2048, B is 10 and C is 20. Since the number of quantization bits of a camera implementing this method is 12 bits, the range of the gradation value is 0 to 4095, and the value of a is set to half of the range of the gradation value in this method, a is set to 2048.

Further, the threshold number of white points must be enough to ensure the effect and correctness of white balance, where w is set to 1/500 of the total number of pixels;

the invention has the beneficial effects that:

the invention adopts the method of directly calculating the gain to ensure that the automatic white balance can be converged quickly, the convergence can be realized through 2 or several frame periods, and the white balance can be realized quickly for cameras with frame frequency of tens of frames/second or hundreds of frames/second, thereby greatly improving the working efficiency.

In the general white balance method, when the gain is adjusted, a large step size is set to ensure the convergence speed, so that the convergence cannot be achieved. The method for directly calculating the gain can realize that the step length of the adjacent two times of gain adjustment is very small, and the accuracy of the white balance method is ensured.

Drawings

FIG. 1 is a schematic flow chart of the present invention;

FIG. 2 is a linear relationship between Cb and gain α;

fig. 3 is a linear relationship between Cr and gain γ.

Detailed Description

The invention provides a method for quickly and efficiently realizing automatic white balance of a color camera by directly calculating gain, which comprises the following specific implementation steps:

【1】 Obtaining a relational expression of a current gain R component gain gamma and a target gain R component gain gamma 'and a relational expression of a current gain B component gain alpha and a target gain B component gain alpha'; the specific implementation steps of the step are as follows:

determining a formula for converting an RGB space into a YCbCr space;

Y＝a*γ*R+b*G+c*α*B (1)

Cb＝α*B-Y＝-a*γ*R-b*G+(1-c)*α*B (2)

Cr＝γ*R-Y＝(1-a)*γ*R-b*G-c*α*B (3)

wherein, Y is the brightness component of the pixel point in YCbCr space; cb is a blue offset component of the pixel point in the YCbCr space; cr is a red offset component of the pixel point in a YCbCr space; r is a red component of the pixel point in the RGB space, G is a green component of the pixel point in the RGB space, B is a blue component of the pixel point in the RGB space, a is a coefficient of the R component in the space conversion formula, B is a coefficient of the G component in the space conversion formula, c is a coefficient of the B component in the space conversion formula, gamma is a gain of the R component in the automatic white balance, and alpha is a gain of the B component in the automatic white balance.

As can be seen from equations (1), (2) and (3), α is a constant when γ is gain-adjusted; when the gain adjustment is performed on α, γ is taken as a constant. Thus, γ is linearly related to Cr, and α is linearly related to Cb, and specific linear relationships are shown in fig. 2 and 3.

[ 1.2 ] according to the formulas (1), (2) and (3):

Y′＝a*γ′*R+b*G+c*α′*B (4)

Cb′＝α′*B-Y′＝-a*γ′*R-b*G+(1-c)*α′*B (5)

Cr′＝γ′*R-Y′＝(1-a)*γ′*R-b*G-c*α′*B (6)

wherein, Cr' is a red offset component, i.e. a target value, when the automatic white balance method converges, and is a constant; cb' is a constant, which is the blue shift component when the automatic white balance method converges, i.e., the target value; γ 'is the gain of the R component at the time of convergence of the automatic white balance method, and α' is the gain of the B component at the time of convergence of the automatic white balance method.

Can be obtained by the arrangement of the formula (5) and the formula (6)

It can be seen that the target gains γ 'and α' can be calculated from the current gains γ and α.

【2】 Setting a white point number threshold value w, detecting each pixel of the current frame image N, judging whether the pixel is a white point, counting the number w 'of the detected white points of the current frame image N, comparing the detected white point number w' of the current frame image N with the white point number threshold value w, if w 'is more than or equal to w, performing the step (3), and if w' is less than w, performing the step (6); (ii) a 1/500 for the total number of w detector pixels in this embodiment. (ii) a

The judgment condition that any pixel in the current frame image N is a white point is as follows: y is more than or equal to A, | Cb | is less than or equal to B, | Cr | is less than or equal to C; wherein, A is 2048, B is 10 and C is 20. Since the number of quantization bits of a camera implementing this method is 12 bits, the range of the gradation value is 0 to 4095, and the value of a is set to half of the range of the gradation value in this method, a is set to 2048.

【3】 Estimating the color temperature of the current frame image N, and judging whether color cast occurs or not;

calculating the average value of all white points of the current frame image NAnd

if it is notIf M is set to 9, judging that the image has color cast, and B or R components need to be subjected to gain adjustment, and entering step 4;

if it is notJudging that the image has no color cast and entering step 6;

【4】 When in useCalculating a target gain R component gain gamma' according to a formula (1); when in useCalculating a target gain B component gain alpha' according to a formula (2);

【5】 And (4) carrying out white balance adjustment on the current frame image through the target gain R component gain gamma 'or the target gain B component gain alpha' obtained in the step (4).

【6】 After waiting for the 1 frame image transmission, the steps [ 2 ] to [ 5 ] are repeated from the N +2 frame image. The reason for this is that after new gains γ 'and α' are calculated from the nth frame image, if the frame rate is fast enough, the N +1 th frame image is already output and the new gain does not work. The new gain will be applied to the N +2 th frame image. So in order to ensure the correctness of the automatic white balance, the method is adopted.

Because many parameters of w, A, B, C and the like are set by self, if the set parameters are not proper, the automatic white balance can also be incapable of converging, and the adjusting times T of the automatic white balance can be set. If the threshold value is exceeded, the automatic white balance is temporarily stopped, and after a period of time, the gain is restored to the default value, and the calculation is restarted.

Claims (2)

1. A method for realizing automatic white balance of a color camera is characterized by comprising the following steps:

【1】 Acquiring a relational expression of a current gain R component gain gamma and a target gain R component gain gamma 'and a relational expression of a current gain B component gain alpha and a target gain B component gain alpha' according to the space conversion relation between RGB and YCbCr;

the concrete formula is as follows:

cb is a blue offset component of a pixel point in a YCbCr space; cr is a red offset component of the pixel point in a YCbCr space; g is the green component of the pixel point in the RGB space, B is the blue component of the pixel point in the RGB space, a is the coefficient of the R component in the space conversion formula, B is the coefficient of the G component in the space conversion formula; cr' is a red offset component when the automatic white balance method converges, namely a target value, and is a constant; cb' is a constant, which is the blue shift component when the automatic white balance method converges, i.e., the target value;

【2】 Setting a white point number threshold value w, detecting each pixel of the current frame image N, judging whether the pixel is a white point, counting the number w 'of the detected white points of the current frame image N, comparing the detected white point number w' of the current frame image N with the white point number threshold value w, if w 'is more than or equal to w, performing the step (3), and if w' is less than w, performing the step (6);

【3】 Estimating the color temperature of the current frame image N, and judging whether color cast occurs or not;

the judgment condition that any pixel in the current frame image N is a white point is as follows: y is more than or equal to A, | Cb | is less than or equal to B, | Cr | is less than or equal to C; wherein, A is 2048, B is 10, and C is 20;

calculating the average value of all white points of the current frame image NAnd

if it is notIf M is set to be 9, judging that the image has color cast, and adjusting the gain of the component B or R, and entering the step (4);

if it is notJudging that the image has no color cast, and entering the step (6);

【4】 When in useCalculating a target gain R component gain gamma' according to a formula (1); when in useCalculating a target gain B component gain alpha' according to a formula (2);

【5】 Performing white balance adjustment on the current frame image through the target gain R component gain gamma 'or the target gain B component gain alpha' obtained in the step (4);

【6】 After waiting for the 1 frame image transmission, the steps [ 2 ] to [ 5 ] are repeated from the N +2 frame image.

2. The method for realizing automatic white balance of a color camera according to claim 1, wherein: w is 1/500 of the total number of detector pixels.