Patent Abstract:
Methods and related computer program products, systems, and devices for auto-focusing in an image-capturing system includes sampling output signals from an auto-focusing circuit in a first interval of lens distances and determining a first lens distance and a second lens distance corresponding to the two highest values of the sampled output signals in the first interval of lens distances.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from Japanese Application Serial No. 2005-73339, filed on Mar. 15, 2005, Japanese Application Serial No. 2005-73340, filed on Mar. 15, 2005, and Japanese Application Serial No. 2005-73338, filed on Mar. 15, 2005, the entire contents of each of which are herein incorporated by reference.  
       BACKGROUND  
       [0002]     Image capturing systems such as video camera systems and still camera systems often include circuitry to enable auto-focusing of an image. Auto-focusing systems often include a combination of photo-sensors and signal processing circuits. Based on signals received from the photo-sensors, the signal processing circuits can determine various settings for the image capturing system.  
       SUMMARY  
       [0003]     According to an aspect of the present invention, a method for auto-focusing in an image-capturing system includes sampling output signals from an auto-focusing circuit in a first interval of lens distances and determining a first lens distance and a second lens distance corresponding to the two highest values of the sampled output signals in the first interval of lens distances. The method also includes sampling output signals from the auto-focusing circuit in a second interval between the first lens distance and the second lens distance.  
         [0004]     Embodiments can include one or more of the following.  
         [0005]     The method can include determining a lens distance corresponding to the maximum value of the sampled output signals in the second interval and setting the focus position to the determined lens distance. The second interval can be smaller than the first interval. The first interval can include a first set of lens positions having a first distance between each of the lens positions and the second interval includes a second set of lens positions having a second distance between each of the lens positions. The second distance can be smaller than the first distance.  
         [0006]     The method can also include performing signal processing can include performing pre-gamma correction according to predetermined function type, spatially filtering the image signals, weighting and integrating the filtered signals, and outputting the weighted and integrated signals as auto-focusing signals. The method can also include determining a third lens distance and a fourth lens distance corresponding to the two highest values of the sampled output signals in the second interval of lens distances, sampling output signals from the auto-focusing circuit in a third interval between the third lens distance and the fourth lens distance, determining a lens distance corresponding to the maximum value of the sampled output signals in the third interval, and setting the focus position to the determined lens distance.  
         [0007]     According to an aspect of the present invention, an image-capturing system can include a focus lens and an auto-focusing circuit. The auto-focusing circuit can be configured to sample output signals from in a first interval of lens distances and determine a first lens distance and a second lens distance corresponding to the two highest values of the sampled output signals in the first interval of lens distances. The auto-focusing circuit can also be configured to sample output signals in a second interval between the first lens distance and the second lens distance, the second interval being smaller than the first interval.  
         [0008]     Embodiments can include one or more of the following.  
         [0009]     The auto-focusing circuit can be further configured to determine a lens distance corresponding to the maximum value of the sampled output signals in the second interval and set the focus position to the determined lens distance. The image-capturing can also include a signal processing circuit. The signal processing circuit can be configured to perform pre-gamma correction according to predetermined function type, spatially filter the image signals, weight and integrate the filtered signals, and output the weighted and integrated signals as auto-focusing signals to the auto-focusing circuit.  
         [0010]     According to an aspect of the present invention, a method for determining an exposure parameter based on an illumination condition includes calculating a red divided by green (R/G) value, calculating a blue divided by green (B/G) value, and comparing the R/G and B/G values to a predetermined auto-white balancing map to determine an exposure parameter.  
         [0011]     Embodiments can include one or more of the following.  
         [0012]     The method can also include selecting a red signal, a blue signal, a first green signal, and a second green signal in a predetermined area of an imaging device. The predetermined area can include four adjacent pixels. Calculating the R/G value can include determining a first 1/G value based on the first green signal and multiplying the red signal by the first 1/G value. Calculating the B/G value can include determining a second 1/G value based on the second green signal and multiplying the blue signal by the second 1/G value.  
         [0013]     The auto-white balancing map can include a plurality of regions corresponding the different illumination conditions. The auto-white balancing map can include a region corresponding to fluorescent lamp illumination. The method can also include generating a flicker correction signal based on the determined exposure parameter if the R/G and B/G values correspond to the region corresponding to fluorescent lamp illumination. The exposure parameter can be a shutter speed.  
         [0014]     Calculating the R/G value and calculating the B/G value can include selecting a red signal, a blue signal, a first green signal, and a second green signal in a plurality of predetermined areas of an imaging device, calculating a plurality of intermediate R/G values and intermediate B/G values based on the selected red signal, the selected blue signal, the selected first green signal, and the selected second green signal in the plurality of predetermined areas, averaging the calculated intermediate R/G values to generate the R/G value, and averaging the calculated intermediate B/G values to generate the B/G value.  
         [0015]     According to an aspect of the present invention, an image-capturing system can include a circuit configured to calculate a red divided by green (R/G) value, calculate a blue divided by green (B/G) value, and compare the R/G and B/G values to a predetermined auto-white balancing map to determine an exposure parameter.  
         [0016]     Embodiments can include one or more of the following.  
         [0017]     The auto-white balancing map can include a plurality of regions corresponding the different illumination conditions. The auto-white balancing map can include a region corresponding to fluorescent lamp illumination and the circuit if further configured to generate a flicker correction signal based on the determined exposure parameter if the R/G and B/G values correspond to the region corresponding to fluorescent lamp illumination.  
         [0018]     According to an aspect of the present invention, a method includes providing a pre-gamma function having a first region, a second region, and a third region, the derivative of the function in the second region being greater than the derivative of the function in the first and third regions. The method also includes receiving image signals from a predetermined number of locations on an imaging device and performing pre-gamma correction on the received image signals using the pre-gamma function to generate a pre-gamma corrected image signal.  
         [0019]     Embodiments can include one or more of the following.  
         [0020]     Performing pre-gamma correction on the received image signals can include multiplying the received image signals by the derivative of the pre-gamma function in a region of the pre-gamma function corresponding to an illumination level of the received signal. The he image signals can correspond to the image signals for a plurality of green pixels. The pre-gamma function can be an approximately s-shaped function. The method can also include performing signal processing on the pre-gamma corrected signal. Performing signal processing on the pre-gamma corrected signal can include spatially filtering the image signals, weighting and integrating the filtered signals, and outputting the weighted and integrated signals as auto-focusing signals. Spatially filtering the image signals can include spatially filtering the image signals using Lapracian filtering or differential filtering.  
         [0021]     According to an aspect of the present invention, an image-capturing system includes a circuit configured to provide a pre-gamma function having a first region, a second region, and a third region, the derivative of the function in the second region being greater than the derivative of the function in the first and third regions, receive image signals from a predetermined number of locations on an imaging device, and perform pre-gamma correction on the received image signals by multiplying the received image signals by the derivative of the pre-gamma function in a region of the pre-gamma function corresponding to an illumination level of the received signal using the pre-gamma function to generate a pre-gamma corrected image signal.  
         [0022]     Embodiments can include one or more of the following.  
         [0023]     The pre-gamma function can be an approximately s-shaped function.  
         [0024]     In some embodiments, performing multi-sampling can provide higher auto-focusing accuracy and/or can reduce the probability of selecting an undesired signal peak during the auto focus process.  
         [0025]     In some embodiments, performing multi-sampling can reduce the total time for the auto-focus process.  
         [0026]     In some embodiments, the auto-focusing system can realize both higher focusing accuracy and shorter focusing time due to the use of a multi-sampling process.  
         [0027]     In some embodiments, a flicker correction can be implemented using existing hardware and software resources for auto-white balancing, thus providing a low cost flicker correction system and method. In addition, in some embodiments, the flicker correction time can be reduced, because the flicker correction requires no additional process time except existing AWB process time.  
         [0028]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below.  
     
    
     DESCRIPTION OF DRAWINGS  
       [0029]      FIG. 1  is a block diagram of an auto-focusing system.  
         [0030]      FIG. 2  is a graph of an auto-focusing signal output from auto-focusing circuit.  
         [0031]      FIG. 3  is a flow chart of an auto-focusing process.  
         [0032]      FIG. 4  is a flow chart of a flicker correction process.  
         [0033]      FIG. 5  is a diagram of pixel color pattern on an imaging device.  
         [0034]      FIG. 6  is a diagram of pixel color pattern on an imaging device.  
         [0035]      FIG. 7  is a diagram of pixel color pattern on an imaging device.  
         [0036]      FIG. 8  is a graph representative of an auto-white balancing chart.  
         [0037]      FIG. 9  is a diagram of G pixel pattern on an imaging device.  
         [0038]      FIG. 10  is a graph representative of a pre-gamma correction function.  
         [0039]      FIG. 11  is a graphical representation of a filter.  
         [0040]      FIG. 12  is a graphical representation of a filter.  
         [0041]      FIG. 13  is a graphical representation of a filter.  
         [0042]      FIG. 14  is a block diagram of an auto focus system. 
     
    
     DETAILED DESCRIPTION  
       [0043]     Referring first to  FIG. 1 , a block diagram of an auto-focusing system  12  that includes a focus lens  10  and zoom lens  20  is shown. Through the focus lens  10  and the zoom lens  20 , an optical image is projected on a plurality of pixels on an imaging device  30 . The pixels convert the optical image into electrical analog image signals. The electrical analog image signals are converted into digital image signals by an analog digital (A/D) converter (not shown). The digital image signals are fed to an optical black (OB) clamping circuit  40  and clamped to predetermined levels. The clamped digital image signals are fed to a defect correction circuit  50  which electrically corrects the signals. The corrected image signals are fed to a lens shading correction circuit  60  and electrically corrected to image signals without lens shading.  
         [0044]     An output signal line of the lens shading correction circuit  60  is divided into multiple lines (e.g., four lines). The first signal line is connected to offset gain circuit  70  which is connected to a video signal processing circuit (not shown). The second signal line is connected to an auto-exposing (AE) circuit  80 . The third signal line is connected to an auto-white-balancing (AWB) circuit  90 . The fourth signal line is connected to an auto-focusing (AF) circuit  100 .  
         [0045]     The auto-focusing (AF) circuit  100  includes a pre-gamma correction circuit  110 , a spatial filtering circuit  120 , and a weighting and integrating circuit  130 . The weighting and integrating circuit  130  outputs an auto-focusing signal. The auto-focusing (AF) output signal from the auto-focusing (AF) circuit  100  is fed to CPU  140 . The CPU  140  supplies driving signals to drive a focus motor  150  and a zoom motor  160  which move the focus lens  10  and the zoom lens  20  to a focus position.  
         [0046]     During an auto-focusing operation, the auto-focusing circuit  100  analyzes signals from the imaging device  30 . The auto-focusing circuit changes the distance of focus lens  10  by using motor  150  and observes characteristics of the images at the various focus distances to determine if the image is in focus.  
         [0047]     Referring to  FIG. 2 , a graph  200  of image samples measured by the auto-focusing circuit  100  as the distance of the focus lens  10  is changed is shown. The horizontal axis  204  is lens moving distance of focus lens  10  and vertical axis  202  is the amplitude of the auto-focus signal output. The amplitude of the auto-focus signal output changes as the distance of the lens changes as the distance of the lens  10  is changed. A larger amplitude signal indicates that the image is more focused than a lower amplitude signal. The auto-focus signal output is maximized at focus position (X AF )  232  to focus amplitude (A AF )  231 .  
         [0048]     Referring to  FIG. 3 , a flow chart of an auto-focusing operation that uses a multi-sampling process  170  is shown. Process  170  includes measuring a first set of auto-focus signal outputs from the auto-focus circuit  100  within a first interval of lens moving distances (step  172 ). The first sampling uses a relatively large step size to generate a rough sampling of the AF signal outputs over a wide range of lens positions. For example, as shown in  FIG. 2 , the first sampling is performed using a step size indicated by arrow  226  and generates AF amplitudes for the auto-focus signal at lens distances  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218 , and  219 .  
         [0049]     Based on the determined amplitudes of the AF signal at the samples lens distances, process  170  determines the two samples have the greatest amplitude of the AF signal (step  174 ). These two samples provide a narrowed range of lens distances in which the focus amplitude (A AF )  231  and focus position (X AF )  232  is expected to lie. For example, in  FIG. 2 , the sampled values  230  and  234  have the greatest amplitudes, therefore, the focus amplitude (A AF )  231  is expected to lie between lens distances  210  and  212 .  
         [0050]     After the first sampling, a second sampling is performed using a smaller step size for the lens using the lens position associated with the selected first maximum value as the starting lens position and using the lens position associated with the selected second maximum value as the end location of the sampled range (step  176 ). For example, in  FIG. 2 , the step size for the second sampling is a quarter of the step size for the first sampling (indicated by arrow  228 ) and begins at location  210  and ends at location  212 .  
         [0051]     Based on the second sampling, process  170  determines the position where maximum value of the auto-focus signal is present (step  178 ). Process  170  sets this lens distance as the focus position (step  180 ). For example, in  FIG. 2 , point  232  has the highest measured AF value. Therefore, the lens distance  222  corresponding to the AF value  232  will be set as the lens auto focus distance based on the auto-focus process  170 .  
         [0052]     It is believed that performing multi-sampling can provide higher auto-focusing accuracy and/or can reduce the probability of selecting an undesired signal peak during the auto focus process.  
         [0053]     It is also believed that performing multi-sampling can reduce the total time for the auto-focus process. In order to determine the correct distance for the lens during the auto focus process, the lens must be moved to various positions and samples must be taken at the various positions. For example, if an auto focus procedure uses 16 lens positions (as shown in  FIG. 2 ) and 100 ms is needed to move the lens and measure the AF signal at each lens location, then a total of 1.6 seconds would be required to measure the 16 locations. However, using the multi-sampling process described, the first sampling has 8 steps amounting to a time of 800 ms and the second sampling uses four steps amounting to a time of 400 ms. Therefore, the total time for the multi-sampling process is 1.2 seconds as compared to 1.6 seconds for the process that samples all 16 locations. The multi-sampling process maintains the accuracy (e.g., results in the same step size) of the auto-focus while reducing total amount of time needed for the auto focusing.  
         [0054]     As described above, the auto-focusing circuit  100  can realize both higher focusing accuracy and shorter focusing time, due to detailed investigation near the focus position by the double sampling.  
         [0055]     Although the multi-sampling process described above has been shown using a double-sampling process, the auto-focusing process is not limited to a double-sampling process. Rather, any multi-sampling, such as triple-sampling, quadruple-sampling, or more, that is capable of auto-focusing can be used.  
         [0056]     In some embodiments, the auto-focusing system can use one sample having highest amplitude of auto-focus signal instead of two samples, and a second sampling can be performed around this sample.  
         [0057]     Referring back to  FIG. 1 , imaging system  12  can be used in a variety of different lighting conditions. These differing lighting conditions can cause various changes in the imaging. For example, when the imaging system  12  is used outside the sun or natural lighting provides the illumination. In contrast, when the imaging system  12  is used indoors a fluorescent lamp may provide the indoor illumination. The light intensity resulting from illumination provided by a fluorescent lamp periodically changes (e.g., 50 Hz or 60 Hz). This phenomenon is referred to as flicker noise. In some cases, flicker noise can generate undesirable effects in the resulting image such as spatially varying luminance change. Conventional auto-exposure circuits often determine exposure parameters by measuring an average value of light intensity and do not correct the exposure parameters in the flicker noise environment. In system  12 , auto-white-balancing (AWB) circuit  90  corrects for flicker noise based on a comparison of the color intensity and a predetermined mapping of the color intensity for various lighting conditions.  
         [0058]     Referring to  FIG. 4 , a process  250  for AWB mapping to remove flicker noise based on the color intensity of pixels in image is shown.  FIG. 5  shows exemplary mappings of color pixels on an imaging device.  FIGS. 6 and 7  show sub-sampled pixels for AWB processing. The auto-white-balancing circuit can use all or part of these color pixels on an imaging device. The color mappings include four color signals that are derived from four color pixels adjacent to each other. For example, a set of red, green, blue, and green (RGBG) pixels in a predetermined area on the imaging device can be used.  
         [0059]     Process  250  calculates a value for R/G and B/G (step  254 ). In some embodiments, the green (G) signal is converted to l/G signal using predetermined conversion table. The R signal and the B signal are multiplied with the 1/G signal to obtain the R/G signal and B/G signal. In other embodiments, the system divides the red signal by the green signal and the blue signal by the green signal without first calculating 1/G.  
         [0060]     The calculated R/G can B/G values are compared to a predetermined auto-white balancing (AWB) map to determine the lighting type (step  256 ).  FIG. 8  shows an exemplary AWB map  260  in which the B/G value is graphed on the x-axis  262  and the R/G value is graphed on the y-axis  264 . AWB map  260  is divided into multiple regions (e.g., regions  266 ,  268 ,  272 ,  274 ,  276 ,  278 , and  280 ) corresponding to various lighting conditions. The regions are determined based on scene information for images taken in various different lighting conditions. For example, region  280  is common scene, region  284  is indoor scene, regions  266 ,  268 ,  272 ,  274 , and  278  are fluorescent lamp scenes and region  276  is an outdoor scene. The number of scenes and/or the shape of the corresponding AWB regions can vary as desired. The AWB map can be determined experimentally or can be calculated based on previously obtained image information. By comparing the R/G signal (x-axis  262 ) and B/G signal (y-axis  264 ) with the AWB map, the system determines whether fluorescent lamp illumination is used. If so, system  90  generates a flicker correction signal and modifies the exposure parameters according to the flicker correction signal (step  258 ). For example, if the analyzed image corresponds to a fluorescent lamp illumination the shutter speed can be increased to be greater than 10 ms to reduce or eliminate the flicker from the resulting image.  
         [0061]     While in the above embodiment, a single calculation of R/G and B/G was used to determine the lighting conditions based on the AWB map, multiple calculations can be used. In some embodiments, the system calculates multiple R/G and B/G values from various portions of the image. These values are averaged to determine an average R/G and an average B/G value to be used to determine the lighting condition from the AWB mapping.  
         [0062]     In some embodiments, the flicker correction described above can be implemented using existing hardware and software resources, thus providing a low cost flicker correction system and method. Furthermore, the flicker correction time can be significantly reduced, because it requires no additional process time except existing AWB process time.  
         [0063]     Referring back to  FIG. 1 , the image signals derived from the pixels on the imaging device  30  may include noise which influences the accuracy of the auto focus operation for the imaging device  30 . For example, if there is a low luminosity in the image there may be low auto focus accuracy in comparison to a high luminosity image. Therefore, noise included in the low luminosity image may have a greater effect on the focusing of the system. In order to reduce the effect of the luminosity level on the auto-focusing operation, the input signal can be modified by a pre-gamma correction function. Auto focus circuit  100  provides pre-gamma correction to the image signal to reduce the influence of the noise in low level lighting on the auto focus operation.  
         [0064]     Auto-focusing circuit  100  in image-capturing system  12  performs pre-gamma correction to image signals derived from predetermined pixels on the imaging device  30  and performs signal processing on the pre-gamma-corrected image signals. The signal processing of the pre-gamma-corrected image signals can include spatial filtering of the image signals, weighting and integrating the filtered signals, and outputting the weighted and integrated signals as auto-focusing signals. The pre-gamma correction is performed to multiple signals sampled at predetermined intervals.  
         [0065]     Referring to  FIG. 9  which shows an example of image signals derived from predetermined pixels on the imaging device  30 , the pixels on imaging device  30  represent a plurality of green (G) pixels at predetermined positions on the imaging device  30 .  
         [0066]     Referring to  FIG. 10 , an exemplary pre-gamma correction function  310  for the auto-focusing system is shown. The pre-gamma correction is performed on multiple signals sampled at predetermined intervals using function  310 . In the graph of the pre-gamma function  310 , the x-axis represents the input signal luminosity and the y-axis represents the output signal that is based on a mathematical transformation of the input signal. The pre-gamma function includes multiple regions  316 ,  318 ,  320 ,  322 ,  324 ,  326 ,  328 , and  330  having differing slopes resulting in an “S-shaped” function. The slope of each region is determined based on the slope of a line formed between two endpoints for the region. The slope can also be determines based on a derivative of the “s-shaped” function at a particular location. The slope of the regions having a relatively low luminosity (e.g., regions  316  and  318 ) and the regions having a relatively high luminosity (e.g., regions  328  and  330 ) is less than the slope of the regions having a moderate luminosity (e.g., regions  322 ,  324 , and  326 ).  
         [0067]     In operation, a value of an input signal (e.g., the input shown in  FIG. 9 ) is multiplied by the slope of the pre-gamma function for the associated luminosity level. Since the slope of the pre-gamma signal is lower for signal inputs having relatively low or relatively high luminosities, high frequency component of the low and high luminosity signals in the image are reduced. For example, if the signal is at a low luminosity level at input, differential of the signal will be lower relative to the other signals after calculating the pre-gamma correction using function  310 .  
         [0068]     After performing the pre-gamma correction, additional filtering may be performed in the auto focusing circuit  100 .  FIGS. 11, 12 , and  13  show graphical representations of Lapracian filtering, vertical differential filtering, and lateral differential filtering respectively. The filtering can be used to emphasize edge components of the image.  
         [0069]      FIG. 14  shows a weighting and integrating circuit  130  in the auto-focusing circuit  100 . The image signals (e.g., represented by arrow  109 ) derived from predetermined pixels on imaging device are input into the pre-gamma correction circuit  110 . The pre-gamma correction on the image signals  109 . The pre-gamma correction is based on the S-shaped function  310 . The pre-gamma correction circuit  110  multiplies the input signals by the slope of the pre-gamma function  310  in the appropriate luminosity range. The pre-gamma-corrected image signals (e.g., represented by arrow  134 ) are fed to the spatial filtering circuit  120  which emphasizes the edge components of the image using a filter such as those shown in  FIGS. 11, 12 , and  13 . The filtered image signals are input into the weighting and integrating circuit  130  which averages the filtered image signals to generate an average weighted signal. The weighted and integrated image signals are fed to the CPU  140 .  
         [0070]     The CPU  140  receives the weighted and integrated image signals (also referred to as the auto-focusing signal) and produces the driving signal to drive the focus motor  150  based on the received signal. The focus motor  150  moves the focus lens  10  to the focus position. For example, if the auto focus value is high, the value indicates a high frequency component. In general, the high frequency component will be greater if the image is in focus; and if the image is not in focus then the high frequency component will be small. The magnitude of the high-frequency component indicates to the drive motor the type of correction that should be made to the lens distance to correct the focusing of the image.  
         [0071]     As described above, the auto-focusing circuit  100  can realize both higher focusing accuracy and shorter focusing time.  
         [0072]     Finally, although the present invention has been particularly shown and described above, the present invention is not so limited. For instance, the present invention is not only limited to the signal processing to the pre-gamma-corrected image signals shown and described. Rather, any signal processing that is capable of auto-focusing can be used. Therefore, these and other changes in form and details may be made to the preferred embodiments without departing from the true spirit and scope of the invention as defined by the appended claims.  
         [0073]     Accordingly, other embodiments are within the scope of the following claims:

Technology Classification (CPC): 6