Calculation method for the correction of white balance

This Application claims priority of Taiwan Patent Application No. 97109124, filed on Mar. 14, 2008, the entirety of which is incorporated by reference herein.

1. Field of the Invention

The disclosure relates generally to calculation methods for calculating white balance correction values, and, more particularly to calculation methods for calculating white balance correction values applied in an image capture device while using an auxiliary light for shooting pictures/capturing images.

2. Description of the Related Art

Charge-coupled devices (CCD) and Complementary Metal-Oxide Semiconductors (CMOS) have been widely used as image sensors in digital cameras in replace of conventional film. During picture shooting/image capturing, color accuracy determined by the image sensor for the image, may be impacted by the color temperatures of the lights coming from various light sources surrounding the image, as the brightness of the captured environment is provided by the surrounding lights intermixed with various light sources.

Typically, when capturing an image within a light source with lower color temperature value, image color for the captured image will slant toward the red color. Contrarily, the image color for the captured image will slant toward the blue color when capturing an image within a light source with higher color temperature value. Therefore, under light sources with different color temperature values, the image color for the captured image will be somewhat different from that of the actual object so that the color for the captured image should be corrected. Such correction process for correcting the image color is called a white balance process.

When capturing an image within an environment with low brightness, a flash is usually used to compensate for the lack of brightness. Methods for performing the white balance process using the flash, such as US publication No. 20030030730 A1, have already been disclosed. Meanwhile, US publication No. 20030030730 A1 provides a method for adjusting the white balance using the flash, in which, in the invention, a first control value is obtained based on the flash amount shot to the target object and the brightness before shooting. The first control value is applied to a default formula to obtain a second control value, and a white balance process for the image signals outputted by the image sensor is then performed according to the first control value and the second control value. However, the greater the color strength of the provided flash becomes, the larger the effect for color deviation.

Therefore, to overcome the aforementioned disadvantages, a calculation method for calculating the white balance correction values is provided by using the brightness weightings provided by the environment light and the auxiliary light when capturing the image so as to efficiently improve the color deviation effect caused by the auxiliary light source.

It is therefore an objective to provide a calculation method for calculating a white balance correction value for correcting the color deviation impacted by adding an auxiliary light source.

In an embodiment, a calculation method for calculating a white balance correction value is disclosed. When an image capture device utilizes an auxiliary light to capture an image, the weightings of the image color impacted by the color temperature of each light source are obtained by the brightness-rate which is provided by the environment light and the flash, and then the weightings of the color temperatures of the two light sources are balanced to obtain a white balance correction value. Lastly, the white balance correction value is applied to correct the white balance of the image.

With the provided calculation method and white balance correction value, white balance of the image can be precisely performed under a capturing environment with low brightness even if the auxiliary light provides strong brightness compensation.

The invention will become more fully understood by referring to the following detailed description with reference to the accompanying drawings, wherein:

FIG. 1 is a flowchart of an embodiment of a calculation method for calculating a white balance correction value of the invention;

FIG. 2 is a schematic diagram illustrating an embodiment of a relationship table of the pulse width and the default flash brightness of the invention; and

FIG. 3 is a schematic diagram illustrating an embodiment for forming the first image of the invention.

The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims.

Embodiments of the invention provide an image capture device that utilizes auxiliary light to compensate for the environment brightness so as to perform a white balance correction for the image, wherein the auxiliary light of the invention may be any device capable of emitting light such as a flash or an AF auxiliary light. In the invention, the flash is utilized as an example of the auxiliary light and detailed in below for brevity.

Please refer to FIG. 1 . FIG. 1 is a flowchart of an embodiment of a calculation method for calculating a white balance correction value of the invention.

Step 1 : first, a color temperature test of the flash is performed in a darkroom to obtain a default flash color temperature CT 2 and another test, testing the pulse width (in unit of μs) corresponding to the default brightness of the flash is performed to obtain the default brightness of the flash, wherein the pulse width is a pulse width obtained by an image capture device detecting the environment brightness within the darkroom, to drive the flash device that is a device having the flash.

The color temperature test of the flash is performed by the image capture device that performs the image capture operation to a gray card in the darkroom and automatically adjusts the pulse width of the output voltage of the flash to drive the flash device to generate various default brightness of the flash corresponding to different pulse widths so as to perform a brightness compensation for the environment. At the same time, an image is captured by the image capture device and the RGB value of each pixel within a specific area of the captured image are summed and then averaged to generate a calculation result of an average summation thereof. The calculation result represents the RGB value of the default flash color temperature CT 2 (R CT2 , G CT2 , B CT2 ), and the RGB value representing the light intensity of each color of the three primary colors including a red (R) color, a green (G) color and a blue (B) color displayed by pixels within the image.

For example, if the color displayed by one of the pixels within the image is a red color, the light intensity of the RGB value (R, G, B) are assumed to be 255, 0, 0 for representing the light intensity of the red, the green and the blue colors for the RGB values, respectively.

In the test, wherein the pulse width corresponds to the default brightness of the flash, the pulse width (in unit of μs) is adjusted to generate a variation in the flash generated by the flash and each of the flashed images are then captured. The aforementioned method that is used for obtaining the flash color temperature CT 2 is utilized for obtaining the RGB value of the G CT2 value in each of the images which captured with using flash for representing the light brightness of each flash, which is further referred to as the default brightness R.

Thereafter, a relationship table of the pulse width (P) and the default brightness (R) is generated based on the test result for the test, testing the pulse width of the flash corresponding to the brightness, as shown in FIG. 2 , and stored in the relationship table in the image capture device. In the relationship table, P field represents the pulse width while the R field represents the default brightness of the flash for each pulse width.

Definition for a specific area of the image and calculation step for averaging the summation of the color strengths of the RGB colors are detailed below.

Step 2 : a first image 300 is captured in an environment without any flash and an average summation of the RGB value for each pixel within a specific area of the first image are calculated and the calculation result represents the RGB value of an environment color temperature CT 1 , represented by R CT1 , G CT1 , B CT1 , under the environment in which the RGB value of the G CT1 value of the environment color temperature CT 1 represents a first environment brightness Y 1 that is brightness provided by the environment and the specific area comprises partial areas or entire areas of the first image.

The calculation method for obtaining the environment color temperature CT 1 is illustrated in FIG. 3 . FIG. 3 is a schematic diagram illustrating an embodiment for forming the first image of the invention. The first image 300 is formed by a pixel array that has H columns and V rows of pixels and a center area (H/2)×(V/2) of the first image 300 is set to be the specific area 301 . Then, the average of the summation of the RGB value for each pixel within the specific area 301 of the first image 300 are calculated and the calculation result represents the environment color temperature CT 1 in which the RGB value of the G CT1 value of the environment color temperature CT 1 is defined as the first environment brightness Y 1 .

For example, assume that the first image 300 has 1600×1200 pixels, and coordinates of each pixel within the image is defined as pix(x, y), i.e. x is between 0 and 1599 and y is between 0 and 1199, wherein the x coordinate axis represents the column of the pixel while the y coordinate axis represents the row of the pixel. Each pixel represents one unit of the coordinates in which the farthest left and top point is set to be the original point pix(0,0) and the values of the x axis and the y axis are sequentially increased from left to right and from top to bottom, respectively. Particularly, pixels within the area that is the x coordinate between 400 and 1200 and y coordinate between 300 and −900 in the first image 300 are selected to be the specific area 301 . It is understood that the specific area 301 can be selected based on the center of the first image 300 , but it is not limited thereto.

It is assumed that the RGB value for each pixel within each position of the specific area 301 can be represented as following:

pix(400,300)=(200,150,100);

pix(401,300)=(199,150, 101);

pix(402,300)=(201,150,99);

pix(1200,898)=(201,150,99);

pix(1200,899)=(199,150,101); and

pix(1200,900)=(200,150,100).

Due to the specific area 301 having a total of (H/2)×(V/2) pixels, i.e. 800×600=4800 pixels, the calculation results of the average summation of the RGB value for each pixel within the specific area 301 can be represented as following:

where 200,150,100 represents the environment color temperature CT 1 in which: 200 represents the RGB value of the R CT1 value of the environment color temperature CT 1 ; 150 represents the RGB value of the G CT1 value of the environment color temperature CT 1 ; and 100 represents the RGB value of the B CT1 value of the environment color temperature CT 1 . Since the G CT1 value of the RGB value represents the first environment brightness Y 1 , the first environment brightness Y 1 is set to be 150.

Step 3 : in the same situation as in step 2 , i.e. in an environment without any flash, the flash device is driven to flash a light by a first pulse width P 1 for capturing a second image and a second environment brightness Y 2 is obtained from the second image, wherein the first pulse width P 1 may be any natural number in unit of μs.

Step 4 : a first flash brightness F 1 that is the flash brightness generated under the first pulse width P 1 is obtained by subtracting the first environment brightness Y 1 from the second environment brightness Y 2 .

Step 5 : the image capture device drive the flash device with a second pulse width P 2 to flash a light and to capture a third image that is the image of the target which is expected to be captured.

Step 6 : please refer to FIG. 2 , which is a relationship table of the pulse width and the default flash brightness of an embodiment of the invention. The pulse width field is compared with the first pulse width PI and the second pulse width P 2 to obtain a first default flash brightness R 1 and a second default flash brightness R 2 of the flash corresponding to the default brightness from the values of the corresponding default brightness R field, respectively.

For example, if the first pulse width P 1 is 20 μs and the second pulse width P 2 is 19 μs, it can be obtained from the relationship shown in the relationship table of the FIG. 2 that the first default flash brightness R 1 of the first pulse width P 1 is 200 while the second default flash brightness R 2 of the second pulse width P 2 is 190.

Step 7 : when the first pulse width P 1 and the second pulse width P 2 are applied in the darkroom, default flash brightness R 1 and R 2 will be generated so that the default brightness rate of the flash is R 1 /R 2 . Under the same pulse widths P 1 and P 2 , the flash brightness for the flash while capturing the second image and the third image are F 1 and F 2 respectively, and thus the brightness rate of the flash is F 1 /F 2 . Since the default brightness R 1 and R 2 and the flash brightness F 1 and F 2 are generated by driven the flash device with the first pulse width P 1 and the second pulse width P 2 , the relationship between default brightness rate and the flash brightness rate of the flash should be theoretically equaled. That is,

A second flash brightness formula is then derived from the formula (1):

where R 1 , R 2 and F 3 are predefined numbers.

Step 8 : the brightness rate of the image is defined by the light intensity of environment and the flash providing in capturing the third image in step 5 and are utilized to represent the weight of the image effect for the color temperatures of the two light sources, and thus the rate of the first environment brightness Y 1 and the second flash brightness F 2 is the brightness rate provided by the two light sources within the image. It is therefore known that the weight of the color temperature effect for the environment occupied in the image is

CT ⁢ ⁢ 1 × Y ⁢ ⁢ 1 Y ⁢ ⁢ 1 + F ⁢ ⁢ 2

and the weight of the color temperature effect for the flash occupied in the image is

To balance the effect for the two color temperatures to the image color, the occupied weightings of the two color temperature effects are added together. The calculation result is the balanced result for the effect of the color temperatures of the two light sources and is represented as a white balance correction value WB. Therefore, a white balance correction value formula can be obtained as below:

Since the color temperatures of the light sources can be represented by the RGB value, the white balance correction value WB may comprise a white balance correction value of a red color WB R , a white balance correction value of a green color WB G and a white balance correction value of a blue color WB B . The white balance correction value of the red color WB R is used for the correction of the RGB value of the red color for the pixel, the white balance correction value of the green color WB G is used for the correction of the RGB value of the green color for the pixel and the white balance correction value of the blue color WB B is used for the correction of the RGB value of the blue color for the pixel. With three correction values, colors displayed by the pixels can be corrected such that the displayed color is closest to the actual color.

According to the RGB value of the environment color temperature CT 1 and the default flash color temperature CT 2 calculated by the formula (3), a white balance correction value calculation formula for the white balance correction value of the blue color WB R , a white balance correction value calculation formula for the white balance correction value of the green color WB G and a white balance correction value calculation formula for the white balance correction value of the blue color WB B can be derived therefrom. The calculation formulas can be represented by following formulas:

Since the white balance correction value WB may comprise WB R , WB G and WB B , it can be represented as WB=(WB R , WB G , WB B ). That is,

Step 9 : the image capture device multiplies the RGB value for each pixel within the third image by the white balance correction value WB and the multiplication result is then divided by a number N for performing a normalization process in which different image capture devices have different number N. The result of the normalization process is utilized to replace the original RGB value of the image and then the image is outputted as the image which is a corrected image after the white balance correction has been completed.

The objective of the step 1 is to obtain the default information of the auxiliary light of the image capture device so as to provide the default information for white balance calculation for the same image capture device when it performs subsequent image capturing. Therefore, only steps 2 to 9 should be performed to correct the white balance for the captured image for the same image capture device to perform subsequent image capturing.

In summary, according to the calculation method of the invention, the weightings of the brightness provided by the environment light and the auxiliary light are utilized to calculate the white balance parameters for balancing the color deviation for the image caused by the color temperature of the auxiliary light, and thus the deviation effect caused by the color temperature can be improved due to the correction for the image colors by the white balance parameters under a darker environment with low brightness even if more color deviation effect may be caused by the auxiliary light that provides strong brightness compensation, thereby achieving a goal for white balancing of the image.

While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.