Patent Publication Number: US-2007110427-A1

Title: Method of exposure compensation

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates to exposure compensation, and in particular to method of exposure compensation in a camera.  
      2. Description of the Prior Art  
      Generally, color and brightness of a background may cause an incorrect exposure in photography. For example, when using the center weighted averaging metering function of a camera, a background of light color or high brightness may result in a captured image with under-exposed background due to the camera misjudging the exposure, for example, the original white background registering as gray. Similarly, a background of dark color or low brightness can result in a captured image with overexposed background, for example, an original black background registering as gray.  
      A conventional method of exposure compensation to address the above problem adjusts the overall brightness of the subject. Namely, the foreground and the background are adjusted at the same time, to be lighter or darker. This method, however, while correcting the exposure of the background, can inaccurately expose the foreground. Accordingly, some multi-featured cameras utilize flash compensation to overcome the problem, although this requires the two compensation methods to cooperate.  
      This cooperative solution can be complicated and difficult to use, causing inconvenience for users.  
     SUMMARY OF THE INVENTION  
      It is therefore a primary objective of the invention to provide a method of exposure compensation to solve the above-mentioned problems.  
      The invention discloses a method of exposure compensation, comprising capturing a first image without a flash, capturing a second image with the flash, generating a F-number, a shutter speed value, and a flash value according to brightness distribution of the first and second images, and capturing a third image according to the F-number, the shutter speed value, and the flash value.  
      These and other objectives of the invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.  
      These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram of a camera of the invention in use.  
       FIG. 2  is a schematic diagram of a first image captured by the camera of the invention without a flash.  
       FIG. 3  is a schematic diagram of a second image captured by the camera of the invention using a flash.  
       FIG. 4  is a flowchart of a method of exposure compensation of the invention.  
    
    
     DETAILED DESCRIPTION  
       FIG. 1  is a schematic diagram of a camera  100  of the invention during use.  FIG. 2  depicts a first image  200  captured by the camera  100  without a flash  120 .  FIG. 3  depicts a second image  300  captured by the camera  100  with the flash  120 . First, the camera  110  captures the first image  200  without the flash  120 . The first image  200  is divided into a plurality of areas, such as sixteen areas A 1 ˜A 16 , and the brightness of each area is calculated. Next, the camera  110  captures the second image  300  with the flash  120 . The second image  300  is also divided into a plurality of areas, such as sixteen areas B 1 ˜B 16 , and the brightness of each area is calculated. In this embodiment, the F-number and the shutter speed value are the same when capturing the first image  200  and the second image  300 . In another embodiment, the F-number and the shutter speed value may be different when capturing the first image  200  and the second image  300 . The brightness of each area A 1 ˜A 16  of first image  200  minus the brightness of each area B 1 ˜B 16  of first image  200  leaves flash contributing values L flash     —     n  of each area. A flash contributing value L flash  is the sum of each flash contributing values L flash     —     n . Here, n indicates each area. In this embodiment, n is 1 to 16. Calculation of flash contributing values L flash     —     n  is as follows:
   L   flash     —     n   =L   Bn   −L   An    formula (1) 
      When flash  120  of the camera  110  is used, exposure enhancement of the background  140  is less than that of the foreground  130  because the background  140  is further from the camera  110 . Therefore, when one of the flash contributing values L flash     —     n  is less than a threshold value L thd , the area indicates the background. Otherwise, when one of the flash contributing values L flash     —     n  exceeds a threshold value L thd , the area indicates the foreground. The threshold value L thd  is decided automatically by the camera  110  itself or by the user. Thus, the foreground and the background of the capturing environment  100  are distinguished according to the flash contributing values L flash     —     n . In this embodiment, foreground comprises areas with n of 6, 7, 10, 11, 14 and 15, and background comprises areas with n of 1˜5, 8˜9, 12˜13 and 16, shown in  FIGS. 2 and 3 . Although, the first image  200  and the second image  300  are divided into sixteen areas in this embodiment, more areas can be utilized to increase accuracy.  
      After the foreground and the background of the capturing environment  100  are distinguished, a compensation F-number Fn c  and a compensation shutter speed value Exp c  are generated according to the average brightness LA bg  of the background. More specifically, an environmental brightness LV calculated according to the average brightness LA bg  generates the compensation F-number Fn c  and compensation shutter speed value Exp c . However, another environmental brightness LV may be calculated according to the average brightness LB bg  to generate the compensation F-number Fn c  and compensation shutter speed value Exp c . Calculation of environmental brightness LV is as follows:
 
 LV= Log 2 [( Fn   2   ×LA   bg )/(Exp× L   target )]  formula (2)
 
      In formula (2), Fn and Exp indicate the F-number and the shutter speed value when capturing the first image  200 . L target  indicates the normal exposure recognized by the camera  110 , varying with circumstance. Next, the compensation F-number Fn c  and compensation shutter speed value Exp c  are generated according to environmental brightness LV. Calculation of compensation F-number Fn c  and compensation shutter speed value Exp c  is as follows:
 
 LV+EV   bias =log 2 ( Fn   c   2 /Exp c )   formula (3)
 
      In formula (3), EV bias  indicates that exposure of the background requires adjustment, and may vary with circumstance. Thus, the generated compensation F-number Fn c  and compensation shutter speed value Exp c  compensate the exposure of the background to achieve expected results. Multiple sets of compensation F-number Fn c  and compensation shutter speed value Exp c  can be calculated from formula (3). Camera  110  or the user chooses a compensation F-number Fn c , and the corresponding compensation shutter speed value Exp c  is calculated, or a compensation shutter speed value Exp c  is chosen to calculate the corresponding compensation F-number Fn c .  
      When the compensation F-number Fn c  and the compensation shutter speed value Exp c  are generated, a compensation flash value S c  is generated according to the average brightness of the foreground, the compensation F-number Fn c , the compensation shutter speed value Exp c , and the flash contributing value L flash . More specifically, a new average brightness of the foreground L fg     —     new  and a new flash contributing value L flash     —     new  are calculated, and the compensation flash value S c  is then calculated accordingly. Calculation of the new average brightness of the foreground L fg     —     new  is as follows:
 
 L   fg     —     new   =LA   fg ×[( Fn   2 ×Exp c )/( Fn   c   2 ×Exp)]  formula (4)
 
      In formula (4), Fn and Exp indicate the used F-number and shutter speed value when capturing the first image  200 . The LA fg  indicates the average brightness of foreground in the first image  200 . In this embodiment, the L fg     —     new  is calculated according to the LA fg , however, it can also be calculated according to a LB fg  which indicates an average brightness of background in the first image  200 .  
      Otherwise, calculation of a new flash contributing value L flash     —     new  is as follows:
 
 L   flash     —     new   =L   flash ×( Fn   2   ×Fn   c   2 )   formula (5)
 
      After the new average brightness of the foreground L fg     —     new  and the new flash contributing value L flash     —     new  are calculated, the compensation flash value S c  is generated. Calculation of the compensation flash value S c  is as follows:
 
 S   c   =S ×[( L   target   −L   fg     —     new )/ L   flash     —     new ]  formula (6)
 
      In formula (6), S indicates the flash value when capturing the second image  300 .  
      With formulae (1) to (6), the camera  110  of the invention utilizes the generated compensation F-number Fn c , the compensation shutter speed value Exp c , and the compensation flash value S c  to shoot a third image with correct exposure in foreground and background.  
       FIG. 4  is a flowchart  400  of the method of the invention.  
      In step  410 , a first image is generated without using a flash.  
      In step  420 , a second image is generated with a flash.  
      In step  430 , a foreground area, a background area and a flash contributing value L flash  are generated according to brightness of each area of the first image  200  and the second image  300 .  
      In step  440 , a compensation F-number Fn c  and a compensation shutter speed value Exp c  are generated according to the average brightness of the background.  
      In step  450 , a compensation flash value S c  is generated according to the compensation F-number Fn c , the compensation shutter speed value Exp c , and the flash contributing value L flash .  
      In step  460 , a third image is generated according to the compensation F-number Fn c , the compensation shutter speed value Exp c , and the compensation flash value S c .  
      The steps may follow the above sequence, or alternatively, may vary with circumstance. Other steps may be inserted into the process. The method of the invention can be executed by software, firmware, hardware, or combinations thereof.  
      As mentioned, a first image  200  and a second image  300  are captured to generate the compensation F-number Fn c , the compensation shutter speed value Exp c , and the compensation flash value S c  so that a third image is captured with accurate foreground and background exposure. The procedure may executed by a camera (such as a digital camera)  110  during a predetermined period. The exposure compensation of the foreground and the background is automatically adjusted according to the exposure of the background (via choosing a EV bias ) such that the method of the exposure compensation is easily employed.  
      The invention provides a method of exposure compensation utilized for capturing an image by a camera with a flash, such that the captured picture has accurate foreground and background exposure, and the conventional flash compensation and method of exposure compensation are not required. Thus, the invention is more convenient and easily used than conventional methods.  
      While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.  
      Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.