Abstract:
A video signal processing device includes: a first video signal processing section performing a first process on an input video signal for displaying a composite image that is a combination of a natural image and an artificial image, the first process being performed on pixels in a region larger than an artificial image combining region and being a process on which pixels within the combining region have influence; a second video signal processing section performing a second process on the input video signal, the second process being performed on pixels in a region larger than the combining region and being a process on which pixels within the combining region have no influence; and 
     a process restricting section restricting the first process in a first region overlapping and encompassing the combining region and restricting the second process in a second region which is identical to the artificial image combining region.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a video signal processing apparatus and a method and a program for processing video signals. More particularly, the invention relates to a video signal processing apparatus which performs image quality improving processes on video signals for displaying a composite image that is a combination of a natural image and an artificial image such as graphics or characters. 
         [0003]    2. Description of the Related Art 
         [0004]    It has been known that a television receiver performs image quality improving processes on video signals, including a sharpness improving process, a contrast improving process, and a color improving process. In the case of a video signal for displaying a composite image that is a combination of a natural image and an artificial image such as graphics or characters, the problem of variation of luminance or color has occurred even in the region of the artificial image under the influence of the image quality improving processes. 
         [0005]    For example, a solution to the above problem is proposed in JP-A-2007-228167 (Patent Document 1), the solution including the steps of setting a mask area extending the same range as a region having an artificial image (OSD region), outputting a video signal for the mask area without performing any image quality improving process on the signal, and outputting a video signal for the remaining region with image quality improving processes performed thereon. In this case, no variation of luminance or color occurs in the region having an artificial image because the region is not affected by image quality improving processes. 
       SUMMARY OF THE INVENTION 
       [0006]    For example, when a sharpness improving process is performed as an image quality improving process, the technique disclosed in Patent Document 1 has the following problem. The sharpness improving process can leave noticeable pre-shooting and over-shooting effects as shown in  FIG. 14  in a natural image region (in a part of the region adjoining an artificial image) when the entire image to be processed is as shown in  FIG. 13 . In  FIG. 14 , the rectangular window shown in a broken line represents a mask area extending the same range as an artificial image region.  FIGS. 15A to 15C  schematically illustrate the sharpness improving process. The sharpness improving process includes the steps of extracting a high frequency component ( FIG. 15B ) from an input video signal ( FIG. 15A ) and adding the extracted high frequency component to the input video signal to obtain an output video signal having a pre-shoot and an over-shoot added thereon ( FIG. 15C ). 
         [0007]    An alternative approach includes the steps of setting a mask area overlapping and encompassing a region having an artificial image therein, outputting a video signal for the mask area with no image quality improving process performed thereon, and outputting a video signal for the remaining region with image quality improving processes performed thereon. For example, when a contrast improving process and a color improving process are performed as image quality improving processes, a problem arises in that a boundary between the processed and unprocessed regions are visually noticeable as shown in  FIG. 16 . In  FIG. 16 , the rectangular window shown in a broken line represents the mask area overlapping and encompassing the artificial image region. 
         [0008]    Under the circumstance, it is desirable to prevent any reduction in image quality from being caused by a mask area which is set to keep a region including an artificial image therein unaffected by an image quality improving process. 
         [0009]    According to an embodiment of the invention, there is provided a video signal processing device including: 
         [0010]    a first video signal processing section performing a first process on an input video signal for displaying a composite image that is a combination of a natural image and an artificial image, the first process being performed on pixels in a region larger than an artificial image combining region and being a process on which pixels within the artificial image combining region have influence; 
         [0011]    a second video signal processing section performing a second process on the input video signal, the second process being performed on pixels in a region larger than the artificial image combining region and being a process on which pixels within the artificial image combining region have no influence; and 
         [0012]    a process restricting section restricting the first process performed by the first video signal processing section in a first region overlapping and encompassing the artificial image combining region and restricting the second process performed by the second video signal processing section in a second region which is identical to the artificial image combining region. 
         [0013]    According to the embodiment of the invention, the first video signal processing section performs a first process on an input video signal. The first process may be such a process that pixels within the artificial image combining region have influence on pixels in the region larger than the artificial image combining region. For example, the first process may be a sharpness improving process for extracting a high frequency component from the input video signal and adding the extracted high frequency component to the input video signal. 
         [0014]    When a region identical to the artificial image combining region is used as a mask area to restrict the process such that a video signal corresponding to the region is output without being subjected to the sharpness improving process and such that a video signal corresponding the other regions is output with being subjected to the sharpness improving process, a problem arises in that pre-shoot and over-shoot effects attributable to sharpness improvement can be visually noticeable in a region where a natural image is displayed. 
         [0015]    Under the circumstance, according to the embodiment of the invention, the process restricting section restricts the first process performed by the first video signal processing section in the first region which overlaps and encompasses the region where the artificial image is combined. For example, when the first process is a sharpness improving process, the problem of visually noticeable pre-shoot and over-shoot effects attributable to sharpness improvement can be eliminated in the natural image region. 
         [0016]    According to the embodiment of the invention, the second video signal processing section performs the second process on the input video signal. The second process may be such a process that pixels within the artificial image combining region have no influence on pixels in the region larger than the artificial image combining region. For example, the second process may be a contrast improving process performed on a luminance signal constituting the input video signal or a color improving process performed on a chrominance signal constituting the input video signal. 
         [0017]    When a region overlapping and encompassing the artificial image combining region is used as a mask area to restrict the process such that a video signal corresponding to the region is output without being subjected to the contrast improving process or color improving process and such that a video signal corresponding to the other region is output with being subjected to the sharpness improving process or color improving process, a problem arises in that the process is likely to leave a visually noticeable boundary between processed and unprocessed areas. 
         [0018]    Under the circumstance, according to the embodiment of the invention, the process restricting section restricts the second process performed by the second video signal processing section in the second region. For example, when the second process is a contrast improving process or color improving process, the problem of a visually noticeable boundary between processed and unprocessed areas can be eliminated in the natural image region. 
         [0019]    According to the embodiment of the invention, a mask signal generating section may be provided to generate a first mask signal representing the first region and a second mask signal representing the second region. The process restricting section may restrict the first process performed by the first video signal processing section based on the first mask signal generated by the mask signal generating section and may restrict the second process performed by the second video signal processing section based on the second mask signal generated by the mask signal generating section. 
         [0020]    According to the embodiment of the invention, a combining region detecting section may be provided to acquire information on the artificial image combining region based on an input video signal. The process restricting section may restrict the first process performed by the first video signal processing section to the first region overlapping and encompassing the artificial image combining region and may restrict the second process performed by the second video signal processing section to the second region which is identical to the artificial image combining region, based on the information on the artificial image combining region obtained by the combining region detecting section. 
         [0021]    According to the embodiment of the invention, a video signal combining section may be provided to obtain an input video signal by combining a first video signal for displaying a natural image with a second video signal for displaying an artificial image. The process restricting section may restrict the first process performed by the first video signal processing section to the first region overlapping and encompassing the artificial image combining region and may restrict the second process performed by the second video signal processing section to the second region which is identical to the artificial image combining region, based on information on the artificial image combining region in the video signal combining section. 
         [0022]    According to the embodiment of the invention, the video signal processing section may start relaxing the degree of the restriction on the first process by the first video signal processing section at the outline of the artificial image combining region and may gradually relax the restriction toward the outline of the first region. In this case, since the outline of the first region does not constitute a processing boundary of the first process, the first region can be prevented from becoming visually noticeable as a boundary between processed and unprocessed areas. 
         [0023]    According to the embodiment of the invention, processes on a composite image is restricted by providing two regions in which the processes are to be restricted (mask areas), i.e., the first region overlapping and encompassing the artificial image combining region and the second region identical to the artificial image combining region. It is therefore possible to prevent degradation of image quality which can otherwise occur when the mask area is set to prevent an image quality improving process from affecting the artificial image combining region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a block diagram showing an exemplary configuration of a television receiver as an embodiment of the invention; 
           [0025]      FIG. 2  is a block diagram showing an exemplary configuration of an image quality improving process section forming part of the television receiver; 
           [0026]      FIG. 3  is an illustration for explaining an A-region (a region overlapping and encompassing an artificial image combining region) and a B-region (a region identical to the artificial image combining region); 
           [0027]      FIG. 4  is a block diagram showing an exemplary configuration of a mask signal generating portion forming part of the image quality improving process section; 
           [0028]      FIGS. 5A to 5C  show examples of changes that occur in an A-region horizontal control signal h_mask_A and a B-region horizontal control signal h_mask_B relative to the horizontal synchronization signal HD; 
           [0029]      FIGS. 6A to 6C  show examples of changes that occur in an A-region vertical control signal v_mask_A and a B-region vertical control signal v_mask_B relative to the vertical synchronization signal VD; 
           [0030]      FIGS. 7A to 7C  are diagrams for explaining a sharpness improving process which is restricted in an A-region overlapping and encompassing an artificial image combining region; 
           [0031]      FIG. 8  shows an example of an image displayed using the image quality improving process section with restriction placed on the process in two regions, i.e., A- and B-regions; 
           [0032]      FIGS. 9A to 9D  show examples of signals associated with the sharpness improving process performed by the image quality improving process section; 
           [0033]      FIG. 10  is a block diagram showing another exemplary configuration of the image quality improving process section forming part of the television receiver; 
           [0034]      FIGS. 11A to 11D  show examples of signals associated with the sharpness improving process performed by the image quality improving process section; 
           [0035]      FIG. 12  is a graph showing an example of a change that occurs in a marginal area of a mask signal mask_A; 
           [0036]      FIG. 13  is an illustration showing an example of a composite image obtained by combining a natural image with an artificial image; 
           [0037]      FIG. 14  is an illustration showing an example of a composite image obtained by combining a natural image and an artificial image, in which pre-shoot and over-shoot effects attributable to sharpness improvement is visually noticeable in the region of the natural image; 
           [0038]      FIGS. 15A to 15C  are diagrams for explaining the sharpness improving process; and 
           [0039]      FIG. 16  is an illustration showing an example of a composite image obtained by combining a natural image and an artificial image, in which a processing boundary attributable to a contrast improving process and a color improving process is visually noticeable in the region of the natural image. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0040]    Modes for implementing the invention (hereinafter referred to as embodiment) will now be described in the following order. 
         [0041]    1. Embodiment 
         [0042]    2. Modification 
       1. Embodiment 
     Exemplary Configuration of Television Receiver 
       [0043]    An exemplary configuration of a television receiver  100  as an embodiment of the invention will now be described.  FIG. 1  shows the exemplary configuration of the television receiver  100 . The television receiver  100  includes an antenna terminal  101 , a digital tuner  102 , a demultiplexer  103 , a video decoder  104 , a BML (broadcast markup language) browser  105 , and a video signal processing circuit  106 . The video signal processing circuit  106  includes a combining process section  107 , a switch section  108 , and an image quality improving process section  109 . 
         [0044]    The television receiver  100  also includes an HDMI (High-Definition Multimedia Interface) terminal  110 , an HDMI receiving section  111 , a panel driving circuit  112 , and a display panel  113 . The television receiver  100  further includes an audio decoder  114 , a switch section  115 , an audio signal processing circuit  116 , an audio amplifier circuit  117 , and a speaker  118 . 
         [0045]    The television receiver  100  also includes an internal bus  120  and a CPU (central processing unit)  121 . The television receiver  100  also includes a flash ROM (read only memory)  122  and a DRAM (dynamic random access memory)  123 . The television receiver  100  further includes a remote control receiving section  125 , and a remote control transmitter  118 . 
         [0046]    The antenna terminal  101  is a terminal to which television broadcast signals received by a receiving antenna are input. The digital tuner  102  processes television broadcast signals input to the antenna terminal  101  to output a predetermined stream (bit stream data) associated with a channel selected by a user. 
         [0047]    The demultiplexer  103  extracts a video stream, an audio stream, and a data stream from a transport stream (TS). The demultiplexer  103  extracts required streams based on the value of a PID (packet identification) stored in a header portion of each TS packet included in the transport stream. The video decoder  104  performs a decoding process on a video stream extracted by the demultiplexer  103  to obtain a baseband (uncompressed) image data (video signals). Such image data are image data for displaying a natural image. 
         [0048]    The BML browser  105  obtains BML data from a data stream extracted by the demultiplexer  103 , analyzes the structure of the data, and generates image data (video signals) for a data broadcast. Such image data are image data for displaying an artificial image such as graphics or characters. The combining process section  107  combines image data obtained by the video decoder  104  with image data for a data broadcast generated by the BML browser  105  according to an operation performed by a user. 
         [0049]    The HDMI terminal  110  is a terminal for connecting an HDMI source apparatus to the television receiver  100  serving as an HDMI sink apparatus. For example, the HDMI source apparatus may be a DVD (digital versatile disc) recorder, a BD (Blu-ray disc) recorder, or a set top box. The HDMI source apparatus is connected to the HDMI terminal  110  through an HDMI cable which is not shown. 
         [0050]    The HDMI receiving section  111  performs communication according to the HDMI standard to receive baseband (uncompressed) image and audio data supplied from the HDMI source apparatus to the HDMI terminal  110  through the HDMI cable. Such image data received by the HDMI receiving section  111  are image data for displaying a natural image or image data for displaying a composite image that is a combination of a natural image and an artificial image. For example, such an artificial image may be an image of a menu displayed on the HDMI source apparatus. 
         [0051]    The switch section  108  selectively acquires image data output by the combining process section  107  or image data received at the HDMI receiving section  111  according to an operation performed by a user. Image data output by the combining process section  107  are acquired when a television program is watched, and image data received at the HDMI receiving section  111  are acquired when there is an input from outside. 
         [0052]    The image quality improving process section  109  performs image quality improving processes such as a sharpness improving process, a contrast improving process, and a color improving process on image data acquired by the switch section  108  according to an operation performed by a user. The image quality improving process section  109  restricts an image quality improving process performed on image data associated with a region where an artificial image is combined with a natural image such that the region will not be adversely affected by the process. As a result, any variation of luminance or color attributable to the image quality improving process can be prevented in the region having the artificial image. Details of the image quality improving process portion  109  will be described later. 
         [0053]    The panel driving circuit  112  drives the display panel  113  based on image data output from the image quality improving process section  109  or the video signal processing circuit  106 . The display panel  113  is constituted by, for example, an LCD (liquid crystal display) or PDP (plasma display panel). 
         [0054]    The audio decoder  114  performs a decoding process on an audio stream extracted by the demultiplexer  103  to obtain baseband (uncompressed) audio data. The switch section  115  selectively acquires audio data output by the audio decoder  114  or audio data received at the HDMI receiving section  111  according to an operation performed by a user. Audio data output by the audio decoder  114  are acquired when a television program is watched, and audio data received at the HDMI receiving section  111  are acquired when there is an input from outside. 
         [0055]    The audio signal processing circuit  116  performs required processes such as a sound quality adjusting process and DA conversion on audio data acquired by the switch section  115 . The sound quality adjusting process is performed, for example, according to an operation of a user. The audio amplifier circuit  117  amplifies an audio signal output by the audio signal processing circuit  116  and supplies the signal to the speaker  118 . 
         [0056]    The CPU  121  controls operations of various parts of the television receiver  100 . The flash ROM  122  is provided for storing control programs and saving data. The DRAM  123  serves as a work area for the CPU  121 . The CPU  121  deploys programs and data read from the flash ROM  122  on the DRAM  123  and activates the programs to control various parts of the television receiver  100 . 
         [0057]    The remote control receiving section  125  receives remote control signals (remote control codes) transmitted from the remote control transmitter  126  and supplies the signals to the CPU  121 . Based on the remote control codes, the CPU  121  controls various parts of the television receiver  100 . The CPU  121 , the flash ROM  122 , and the DRAM  123  are connected to the internal bus  120 . 
         [0058]    Operations of the television receiver  100  shown in  FIG. 1  will now be briefly described. A television broadcast signal input to the antenna terminal  101  is supplied to the digital tuner  102 . At the digital tuner  102 , the television broadcast signal is processed to obtain a predetermined transport stream (TS) associated with a channel selected by a user. The transport stream is supplied to the demultiplexer  103 . 
         [0059]    The demultiplexer  103  extracts required streams such as a video stream, an audio stream, and a data stream from the transport stream. A video stream extracted by the demultiplexer  103  is supplied to the video decoder  104 . The video decoder  104  performs a decoding process on the video stream to obtain baseband (uncompressed) image data. The image data are supplied to the combining process section  107 . 
         [0060]    A data stream extracted by the demultiplexer  103  is supplied to the BML browser  105 . The BML browser  105  acquires BML data from the data stream and analyzes the structure of the data to generate image data for a data broadcast. The image data are image data for displaying an artificial image such as graphics or characters, and the data are supplied to the combining process section  107 . At the combining process section  107 , the image data obtained by the video decoder  104  is combined with the image data for a data broadcast generated by the BML browser  105  according to an operation performed by the user. Image data output by the combining process section  107  are supplied to the switch section  108 . 
         [0061]    The HDMI receiving section  111  performs communication according to the HDMI standard to receive baseband (uncompressed) image and audio data from the HDMI source apparatus. The image data received by the HDMI receiving section  111  are supplied to the switch section  108 . The switch section  108  selectively acquires the image data output by the combining process section  107  or the image data received at the HDMI receiving section  111  according to an operation performed by a user. The image data output by the combining process section  107  are acquired when a television program is watched, and the image data received at the HDMI receiving section  111  are acquired when there is an input from outside. Image data output by the switch section  108  are supplied to the image quality improving process section  109 . 
         [0062]    The image quality improving process section  109  performs image quality improving processes such as a sharpness improving process, a contrast improving process, and a color improving process on the image data acquired by the switch section  108  according to an operation performed by a user. The image quality improving process section  109  restricts an image quality improving process performed on image data associated with a region where an artificial image is combined with a natural image such that the region will not be adversely affected by the process. Image data output by the image quality improving process section  109  or the video signal processing circuit  106  are supplied to the panel driving circuit  112 . Therefore, an image associated with the channel selected by the user is displayed on the display panel  113  when a television program is watched, and an image received from the HDMI source apparatus is displayed when there is an input from outside. 
         [0063]    An audio stream extracted by the demultiplexer  103  us supplied to the audio decoder  114 . The audio decoder  114  performs a decoding process on the audio stream to obtain baseband (uncompressed) audio data. The audio data is supplied to the switch section  115 . 
         [0064]    Audio data received at the HDMI receiving section  111  are also supplied to the switch section  115 . The switch section  115  selectively acquires the audio data output by the audio decoder  114  or the audio data received at the HDMI receiving section  111  according to an operation of the user. The audio data output by the audio decoder  114  is acquired when a television program is watched, and the audio data received at the HDMI receiving section  111  are acquired when there is an input from outside. The audio data output by the switch section  115  are supplied to the audio signal processing circuit  116 . 
         [0065]    At the audio signal processing circuit  116 , required processes such as a sound quality adjusting process and DA conversion are performed on the audio data acquired by the switch section  115 . An audio signal output from the audio signal processing circuit  116  is amplified by the audio amplifier circuit  117  and supplied to the speaker  118 . Therefore, the speaker outputs sounds associated with the channel selected by the user when a television program is watched and outputs sounds received from the HDMI source apparatus when there is an input from outside. 
       [Details of Image Quality Improving Process Section] 
       [0066]    Details of the image quality improving process section  109  will now be described.  FIG. 2  shows an exemplary configuration of the image quality improving process section  109 . The image quality improving process section  109  includes a combining region detecting portion  131 , a mask signal generating portion  132 , a contrast improving process portion  133 , a switch portion  134 , a sharpness improving process portion  135 , another switch portion  136 , and an adding portion  137 . The image quality improving process section  109  also includes a color improving process portion  138 , another switch portion  139 , another sharpness improving process portion  140 , another switch portion  141 , and another adding portion  142 . 
         [0067]    Luminance data Yin and chrominance data Cin are supplied to the image quality improving process section  109  as input image data (input video signals). The chrominance data Cin include red chrominance data and blue chrominance data. For simplicity of description, those items of data are collectively referred to as “chrominance data”. For example, the combining region detecting portion  131  detects a region of an artificial image combined with a natural image based on the luminance data Yin and transmits information of the region to the CPU  121 . 
         [0068]    The mask signal generating portion  132  generates a mask signal mask_A (first mask signal) and a mask signal mask_B (second mask signal) under control exercised by the CPU  121 . As shown in  FIG. 3 , a mask signal mask_A represents a region (A-region) overlapping and encompassing a region where an artificial image is combined. As shown in  FIG. 3 , a mask signal mask_B represents a region (B-region) which is identical to the region where an artificial image is combined.  FIG. 3  shows an example in which there is one region where an artificial image is combined. When artificial images are combined in a plurality of regions, the mask signal generating portion  132  generates mask signals mask_A and mask_B in association with all image combining regions. 
         [0069]    The mask signal generating portion  132  generates mask signals mask_A and mask_B based on information on a region where an artificial image is combined. As described above, the switch section  108  acquires image data output by the combining process section  107  when a television program is watched. The CPU  121  has information on a region where an artificial image (an image for a data broadcast) is combined by the combining process section  107 . Therefore, when a television program is watched, such information held by the CPU  121  is used as artificial image combining region information. 
         [0070]    As described above, when there is an input from outside, the switch section  108  acquires image data received at the HDMI receiving section  111 . The CPU  121  has no information on an artificial image combining region according to the received image data. Therefore, when there is an input from outside, information on the region having an artificial image detected by the combining region detecting portion  131  is used as artificial image combining region information. 
         [0071]      FIG. 4  shows an exemplary configuration of the mask signal generating portion  132 . The mask signal generating portion  132  includes an A-region horizontal mask generating part  161 , an A-region vertical mask generating part  162 , and an AND circuit  163 . The mask signal generating portion  132  also includes a B-region horizontal mask generating part  164 , a B-region vertical mask generating part  165 , and another AND circuit  166 . 
         [0072]    A pixel clock CK is input to the A-region horizontal mask generating part  161  and the B-region horizontal mask generating part  164 . A horizontal synchronization signal HD is input to the A-region horizontal mask generating part  161 , the A-region vertical mask generating part  162 , the B-region horizontal mask generating part  164 , and the B-region vertical mask generating part  165 . A vertical synchronization signal VD is input to the A-region vertical mask generating part  162  and the B-region vertical mask generating part  165 . 
         [0073]    The A-region horizontal mask generating part  161  and the B-region horizontal mask part  164  are constituted by counters which are reset by the horizontal synchronization signal HD and incremented by the pixel clock CK. The A-region horizontal mask generating part  161  generates an A-region horizontal control signal h_mask_A, and the B-region horizontal mask part  164  generates a B-region horizontal control signal h_mask_B. 
         [0074]      FIGS. 5A to 5C  show examples of changes that occur in the A-region horizontal control signal h_mask_A and the B-region horizontal control signal h_mask_B relative to the horizontal synchronization signal HD. The horizontal synchronization signal HD is indicated in  FIG. 5A . The A-region horizontal control signal h_mask_A is indicated in  FIG. 5B . The B-region horizontal control signal h_mask_B is indicated in  FIG. 5C . 
         [0075]    The A-region horizontal control signal h_mask_A has a value “1” in an A-region (horizontal direction) and has a value “0” in other regions. Similarly, the B-region horizontal control signal h_mask_B has the value “1” in a B-region (horizontal direction) and has the value “0” in other regions. In this case, the A-region horizontal control signal h_mask_A stays at “1” longer than the B-region horizontal control signal h_mask_B by horizontal margins Wh. 
         [0076]    The A-region vertical mask generating part  162  and the B-region vertical mask part  165  are constituted by counters which are reset by the vertical synchronization signal VD and incremented by the horizontal synchronization signal HD. The A-region vertical mask generating part  162  generates an A-region vertical control signal v_mask_A, and the B-region vertical mask part  165  generates a B-region vertical control signal v_mask_B. 
         [0077]      FIGS. 6A to 6C  show examples of changes that occur in the A-region vertical control signal v_mask_A and the B-region vertical control signal v_mask_B relative to the vertical synchronization signal VD. The vertical synchronization signal VD is indicated in  FIG. 6A . The A-region vertical control signal v_mask_A is indicated in  FIG. 6C . The B-region vertical control signal v_mask_B is indicated in  FIG. 6C . 
         [0078]    The A-region vertical control signal v_mask_A has the value “1” in the A-region (vertical direction) and has the value “0” in other regions. Similarly, the B-region vertical control signal v_mask_B has the value “1” in the B region (vertical direction) and has the value “0” in other regions. In this case, the A-region vertical control signal v_mask_A stays at “1” longer than the B-region vertical control signal v_mask_B by vertical margins Wv. 
         [0079]    The A-region horizontal control signal h_mask_A generated by the A-region horizontal mask generating part  161  and the A-region vertical control signal v_mask_A generated by the A-region vertical mask generating part  162  are input to the AND circuit  163 . The A-region horizontal control signal h_mask_A and the A-region vertical control signal v_mask_A are ANDed by the AND circuit  163  to output a mask signal mask_A (first mask signal). The mask signal mask_A has the value “1” in the A-region and has the value “0” in other regions. 
         [0080]    The B-region horizontal control signal h_mask_B generated by the B-region horizontal mask generating part  164  and the B-region vertical control signal v_mask_B generated by the B-region vertical mask generating part  165  are input to the AND circuit  166 . The B-region horizontal control signal h_mask_B and the B-region vertical control signal v_mask_B are ANDed by the AND circuit  166  to output a mask signal mask_B (second mask signal). The mask signal mask_B has the value “1” in the B-region and has the value “0” in other regions. 
         [0081]    In the present embodiment, the mask signal mask_B is used as a mask signal for a contrast improving process and a color improving process as will be described later. Therefore, a B-region is preferably set as a region that is identical to an artificial image combining region. In the present embodiment, the mask signal mask_A is used as a mask signal for a sharpness improving process. Therefore, each of the horizontal margins Wh and the vertical margins Wv of an A-region extending beyond an artificial image combining region is preferably set at about two pixels. When the size of those margins is too large, an area subjected to image processing becomes too noticeable. When the margins are too small, edges are excessively enhanced, which results in degradation of image quality. 
         [0082]    Referring to  FIG. 2  again, the contrast improving process portion  133  performs a luminance improving process on input luminance data Yin according to the histogram equalization which is well-known in the related art. Histogram equalization is a method in which a level conversion function is adaptively changed according to the frequency distribution of pixel values of an input image. The method allows gray levels to be corrected by decreasing gray levels in regions where pixel values are distributed at low frequencies. 
         [0083]    The switch portion  134  selectively acquires input luminance data Yin or luminance data Ya output by the contrast improving process portion  133  based on a mask signal mask_B generated by the mask signal generating portion  132 . Specifically, the switch portion  134  selects the input luminance data Yin for the B-region (the region identical to the artificial image combining region) for which the mask signal mask_B has the value “1” and selects the output luminance data Ya for other regions for which the mask signal mask_B has the value “0”. At the image quality improving process section  109 , the contrast improving process is restricted for the B-region through the selective operation of the switch portion  134  as thus described. That is, the contrast improving process is not performed for the B-region in the present embodiment. 
         [0084]    The sharpness improving process portion  135  extracts high frequency components Yh from the input luminance data Yin. The high frequency components Yh include both of high frequency components in the horizontal direction and high frequency components in the vertical direction. The sharpness improving process portion  135  extracts high frequency components in the horizontal direction using a horizontal high-pass filter formed by pixel delay elements as known in the related art. The sharpness improving process portion  135  extracts high frequency components in the vertical direction using a vertical high-pass filter formed by line delay elements as known in the related art. 
         [0085]    The switch portion  141  selectively acquires the high frequency components Yh output from the sharpness improving process portion  135  or “0” based on the mask signal mask_A generated by the mask signal generating portion  132 . That is, the switch portion  141  selects “0” in the above-described A-region (the region overlapping and encompassing the artificial image combining region) where the mask signal mask_A has the value “1” and selects the output high frequency components Yh in other regions where the mask signal mask_A has the value “0”. 
         [0086]    The adding portion  137  adds data output by the switch portion  136  to luminance data Yb output by the switch portion  134  to obtain output luminance data Yout. At the image quality improving process section  109 , the sharpness improving process on the input luminance data Yin is restricted in the A-region through the selective operation of the switch portion  136  as described above. That is, the sharpness improving process is not performed on the input luminance data Yin in the A-region in the present embodiment. 
         [0087]    The color improving process portion  138  performs a color improving process on the input chrominance data Cin, for example, by increasing the color gain beyond 1 to display a vivid image. The switch portion  139  selectively acquires the input chrominance data Cin or chrominance data Ca output by the color improving process portion  138  based on the mask signal mask_B generated by the mask signal generating portion  132 . Specifically, the switch portion  139  selects the input chrominance data Cin in a period associated with the above-described B-region (the region identical to the artificial image combining region) where the mask_signal_maskB has the value “1” and selects the output chrominance data Ca where the mask signal mask_B has the value “0”. At the image quality improving process section  109 , the color improving process is restricted in the B-region through the selective operation of the switch portion  139  as thus described. That is, the color improving process is not performed in the B-region in the present embodiment. 
         [0088]    The sharpness improving process portion  140  extracts high frequency components Ch from the input chrominance data Cin. The high frequency components Ch include both of high frequency components in the horizontal direction and high frequency components in the vertical direction. The sharpness improving process portion  140  extracts high frequency components in the horizontal direction using a horizontal high-pass filter formed by pixel delay elements as known in the related art. The sharpness improving process portion  140  extracts high frequency components in the vertical direction using a vertical high-pass filter formed by line delay elements as known in the related art. 
         [0089]    The switch portion  136  selectively acquires the high frequency components Ch output from the sharpness improving process portion  140  or “0” based on the mask signal mask_A generated by the mask signal generating portion  132 . That is, the switch portion  136  selects “0” in the above-described A-region (the region overlapping and encompassing the artificial image combining region) where the mask signal mask_A has the value “1” and selects the high frequency components Ch in other regions where the mask signal mask_A has the value “0”. 
         [0090]    The adding portion  142  adds data output by the switch portion  141  to chrominance data Cb output by the switch portion  139  to obtain output chrominance data Cout. At the image quality improving process section  109 , the sharpness improving process on the input chrominance data Cin is restricted in the A-region through the selective operation of the switch portion  141  as described above. That is, the sharpness improving process is not performed on the input chrominance data Cin in the A-region in the present embodiment. 
         [0091]    Operations of the image quality improving process section  109  shown in  FIG. 2  will now be described. Luminance data Yin constituting input image data are supplied to the combining region detecting portion  131 . The combining region detecting portion  131  detects a region where an artificial image is combined with a natural image based on the luminance data Yin. Information on the combining region detected by the combining region detecting portion  131  is transmitted to the CPU  121 . 
         [0092]    The mask signal generating portion  132  generates a mask signal mask_A (first mask signal) and a mask signal mask_B (second mask signal) under control exercised by the CPU  121 . A mask signal mask_A represents a region (A-region) overlapping and encompassing a region where an artificial image is combined. A mask signal mask_B represents a region (B-region) which is identical to the artificial image combining region (see  FIG. 3 ). 
         [0093]    Luminance data Yin constituting input image data are supplied to the contrast improving process portion  133 . The contrast improving process portion  133  performs a luminance improving process such as histogram equalization on the input luminance data Yin. 
         [0094]    Luminance data Ya output by the luminance improving process portion  133  are supplied to the switch portion  134 . The input luminance data Yin are also supplied to the switch portion  134 . Further, the mask signal mask_B generated by the mask signal generating portion  132  is supplied to the switch portion  134  as a switch control signal. 
         [0095]    The switch portion  134  selectively acquires the input luminance data Yin or the luminance data Ya output by the contrast improving process portion  133  based on the mask signal mask_B. Specifically, the switch portion  134  acquires the input luminance data Yin in the above-described B-region (the region identical to the artificial image combining region) where the mask signal mask_B has the value “1” and acquires the luminance data Ya in other regions where the mask signal mask_B has the value “0”. 
         [0096]    The luminance data Yin constituting the input image data are also supplied to the sharpness improving process portion  135 . The sharpness improving process portion  135  extracts high frequency components Yh from the input luminance data Yin. The high frequency components Yh include both of high frequency components in the horizontal direction and high frequency components in the vertical direction. The high frequency components Yh output from the sharpness improving process portion  135  are supplied to the switch portion  136 . Data “0” is also supplied to the switch portion  136 . Further, the mask signal mask_A generated by the mask signal generating portion  132  is supplied to the switch portion  136  as a switch control signal. 
         [0097]    The switch portion  136  selectively acquires the high frequency components Yh output from the sharpness improving process portion  135  or “0” based on the mask signal mask_A. That is, the switch portion  136  acquires in the above-described A-region (the region overlapping and encompassing the artificial image combining region) where the mask signal mask_A has the value “1” and acquires the output high frequency components Yh in other regions where the mask signal mask_A has the value “0”. 
         [0098]    Data output by the switch  136  are supplied to the adding portion  137 . Luminance data Yb output by the switch portion  134  are also supplied to the adding portion  137 . The adding portion  137  adds the data output by the switch portion  136  to the luminance data Yb output by the switch portion  134  to obtain output luminance data Yout. 
         [0099]    As described above, the switch portion  134  is controlled by the mask signal mask_B such that it acquires the input luminance data Yin in a period associated with the B-region and acquires the luminance data Ya in other periods. Therefore, the output luminance data Yout reflect a limited or no contrast improving effect in the B-region (the region identical to the artificial image combining region). In other words, the effect of the contrast improving process is reflected in the luminance data Yout only in the regions other than the B-region. 
         [0100]    The switch portion  134  is controlled by the mask signal mask_A such that it acquires “0” in the A-region and acquires the output high frequency components Yh in other regions. Therefore, the adding portion  137  does not add the high frequency components Yh to the luminance data Yb output from the switch portion  134  in the A-region. Therefore, the output luminance data Yout reflect a limited or no sharpness improving effect in the A-region (the region overlapping and encompassing the artificial image combining region). In other words, the effect of the sharpness improving process is reflected in the output luminance data Yout only in the regions other than the region A. 
         [0101]    Chrominance data Cin constituting the input image data are supplied to the color improving process portion  138 . The color improving process portion  138  performs a color improving process on the input chrominance data Cin, for example, by increasing the color gain beyond 1 to display a vivid image. Chrominance data Ca output by the color improving process portion  138  are supplied to the switch portion  139 . The input chrominance data Cin is also supplied to the switch portion  139 . Further, the mask signal mask_B generated by the mask signal generating portion  132  is supplied to the switch portion  139  as a switch control signal. 
         [0102]    The switch portion  139  selectively acquires the input chrominance data Cin or chrominance data Ca output by the color improving process portion  138  based on the mask signal mask_B. Specifically, the switch portion  139  acquires the input chrominance data Cin in the above-described B-region (the region identical to the artificial image combining region) where the mask signal mask_B has the value “1” and acquires the output chrominance data Ca in other regions where the mask signal mask_B has the value “0”. 
         [0103]    The chrominance data Cin constituting the input image data are also supplied to the sharpness improving process portion  140 . The sharpness improving process portion  140  extracts high frequency components Ch from the input chrominance data Cin. The high frequency components Ch include both of high frequency components in the horizontal direction and high frequency components in the vertical direction. The high frequency components Ch output from the sharpness improving process portion  140  are supplied to the switch portion  141 . Data “0” is also supplied to the switch portion  141 . Further, the mask signal mask_A generated by the mask signal generating portion  132  is supplied to the switch portion  141  as a switch control signal. 
         [0104]    The switch portion  141  selectively acquires the high frequency components Ch output from the sharpness improving process portion  140  or “0” based on the mask signal mask_A. That is, the switch portion  141  acquires in the above-described A-region (the region overlapping and encompassing the artificial image combining region) where the mask signal mask_A has the value “1” and acquires the output high frequency components Ch in other regions where the mask signal mask_A has the value “0”. 
         [0105]    Data output by the switch portion  141  is supplied to the adding portion  142 . Chrominance data Cb output by the switch portion  139  are also supplied to the adding portion  142 . The adding portion  142  adds the data output by the switch portion  141  to the chrominance data Cb output by the switch portion  139  to obtain output chrominance data Cout. 
         [0106]    As described above, the switch portion  138  is controlled by the mask signal mask_B such that it acquires the input lchrominance data Cin in a period associated with the B-region and acquires the chrominance data Ca in other periods. Therefore, the output chrominance data Cout reflect a limited or no color improving effect in the B-region (the region identical to the artificial image combining region). In other words, the effect of the color improving process is reflected in the chrominance data Cout only in the regions other than the B-region. 
         [0107]    The switch portion  141  is controlled by the mask signal mask_A such that it acquires “0” in a period associated with the A-region and acquires the output high frequency components Ch in other periods. Therefore, the adding portion  142  does not add the output high frequency components Ch to the chrominance data Cb output from the switch portion  139  in the period associated with the A-region. Thus, the output chrominance data Cout reflect a limited or no sharpness improving effect in the A-region (the region overlapping and encompassing the artificial image combining region). In other words, the effect of the sharpness improving process is reflected in the output chrominance data Cout only in the regions other than the region A. 
         [0108]    The contrast improving process and the color improving process at the image quality improving process section  109  shown in  FIG. 2  are restricted in the B-region which is identical to the artificial image combining region. Therefore, the contrast improving process and the color improving process result in no change in the luminance and color of the artificial image. The approach also eliminates the problem of a visually noticeable boundary which can appear between the processed region and the region of the natural image. 
         [0109]    The sharpness improving process at the image quality improving process section  109  shown in  FIG. 2  is restricted in the A-region overlapping and encompassing the artificial image combining region. The approach eliminates the problem of a visually noticeable trace of pre-shooting and over-shooting which can remain in the region of the natural image as a result of improved sharpness. The sharpness improving process results in no change in the luminance of the artificial image. For example,  FIGS. 7A to 7C  show an example of an original signal which is indicated in  FIG. 7A  and a high frequency component extracted from the original signal which is indicated in  FIG. 7B . In an A-region which overlaps and encompasses an artificial image combining region, the sharpness improving process is not performed, and the high frequency component is not added to the original signal. Thus, a signal as indicated in  FIG. 7C  is output. Therefore, no visually noticeable trace of pre-shooting and over-shooting attributable to sharpness improvement appears in the region of the natural image. 
         [0110]    As thus described, the image improving process section  109  shown in  FIG. 2  leaves no visually noticeable boundary between a region subjected to a contrast improving process and a color improving process and a region of a natural image. Further, no visually noticeable trace of pre-shooting and over-shooting attributable to sharpness improvement appears in the region of the natural image.  FIG. 8  shows an example of an image displayed using the image quality improving process section  109  shown in  FIG. 2 . 
       2. Modification 
       [0111]    The sharpness improving process at the image quality improving process section  109  shown in  FIG. 2  is restricted in an A-region overlapping and encompassing an artificial image combining region. That is, the image quality improving process section  109  performs no sharpness improving process in the A-region. 
         [0112]      FIGS. 9A to 9D  show examples of signals associated with the sharpness improving process performed by the image quality improving process section  109  shown in  FIG. 2 . An original signal is indicated in  FIG. 9B , and high frequency components extracted from the original signal are indicated in  FIG. 9A . A mask signal mask_A is indicated in  FIG. 9C , and an output signal is indicated in  FIG. 9D . 
         [0113]    As apparent from  FIGS. 9A to 9D , no sharpness improving process is performed at all in a marginal area W (corresponding to the margin Wh or Wv) of a natural image region located between a line representing an artificial image combining region and a line representing an A-region. The sharpness improving process is performed only outside the line representing the A-region. Therefore, when the marginal area W is large, the sharpness improving process may leave a visually noticeable boundary between the processed and unprocessed areas. 
         [0114]      FIG. 10  shows an image quality improving process section  109 A as a modification of the image quality improving process section  109  shown in  FIG. 2 . Elements corresponding between  FIGS. 2 and 10  are indicated by like reference numerals, and detailed description will be omitted for such elements when appropriate. 
         [0115]    A mask signal generating portion  132 A generates mask signals mask_A′ and mask_B based on information on an artificial image combining region. The mask signal mask_B is similar to the mask signal mask_B generated by the mask signal generating portion  132  of the image quality improving process section  109  in  FIG. 2 . The mask signal mask_B has a value “1” in a B-region (a region identical to the artificial image combining region) and has a value “0” in other regions. 
         [0116]    The mask signal mask_A′ is different from the mask signal mask_A generated by the mask signal generating portion  132  of the image quality improving process section  109  in  FIG. 2 . The mask signal mask_A′ has the value “0” in the artificial image combining region and has the value “1” in an A-region (a region overlapping and encompassing the artificial image combining region). Further, in a marginal area W between the line representing the artificial image combining region and the line representing the A-region, the value of the signal changes from “0” to “1”, as indicated in  FIG. 11C . The change may proceed in a parabolic form as represented by a solid line b in  FIG. 12  instead of a linear form as represented by a solid line a in  FIG. 12 . 
         [0117]    A multiplying portion  151  multiplies high frequency components Yh output by a sharpness improving process portion  135  by the mask signal mask_A′ generated by the mask signal generating portion  132 A. At this time, the multiplying portion  151  outputs “0” in the artificial image combining region. That is, none of the output high frequency components Yh of the sharpness improving process portion  135  is output from the multiplying portion  151  in the artificial image combining region. 
         [0118]    In the region beyond the outline of the A-region, the high frequency components Yh from the sharpness improving process portion  135  are output as they are as the output of the multiplying portion  151 . Further, in the area between the outline of the artificial image combining region and the outline of the A-region, the magnitude of the high frequency components output from the multiplier  151  gradually increases from 0 to Yh toward the outline of the A-region. 
         [0119]    An adding portion  137  adds the data output by the multiplying portion  151  to luminance data Yb output by a switch portion  134  to obtain an output luminance data Yout. In the case of the image quality improving process section  109 A, the above-described multiplying operation of the multiplying portion  151  allows the sharpness improving process to be performed on the input luminance data Yin even in the marginal area W (corresponding to the margin Wh or Wv) between the outline of the artificial image combining region and the outline of the A-region. The restriction placed on the sharpness improving process on the input luminance data Yin starts becoming weak beyond the outline of the artificial image combining region and becomes weaker toward the outline of the A-region. 
         [0120]    A multiplying portion  152  multiplies high frequency components Ch output by a sharpness improving process portion  140  by the mask signal mask_A′ generated by the mask signal generating portion  132 A. At this time, the multiplying portion  152  outputs “0” in the artificial image combining region. That is, none of the output high frequency components Ch of the sharpness improving process portion  140  is output from the multiplying portion  152  in the artificial image combining region. 
         [0121]    In the region beyond the outline of the A-region, the output high frequency components Ch from the sharpness improving process portion  140  are output as they are as the output of the multiplying portion  152 . Further, in the area between the outline of the artificial image combining region and the outline of the A-region, the magnitude of the high frequency components output from the multiplier  152  gradually increases from 0 to Ch toward the outline of the A-region. 
         [0122]    An adding portion  142  adds the data output by the multiplying portion  152  to chrominance data Cb output by a switch portion  139  to obtain an output chrominance data Cout. In the case of the image quality improving process section  109 A, the above-described multiplying operation of the multiplying portion  152  allows the sharpness improving process to be performed on the input chrominance data Cin even in the marginal area W (corresponding to the margin Wh or Wv) between the outline of the artificial image combining region and the outline of the A-region. The restriction placed on the sharpness improving process on the input chrominance data Cin starts becoming weak beyond the outline of the artificial image combining region and becomes weaker toward the outline of the A-region. 
         [0123]    As described above, the image quality improving process section  109 A performs the sharpness improving process not only in the are beyond the A-region but also in the marginal area inside the A-region. In addition, in the case of the image quality improving process section  109 A, the restriction placed on the sharpness improving process on the input chrominance data Cin starts becoming weak beyond the outline of the artificial image combining region and becomes weaker toward the outline of the A-region. It is therefore possible to prevent the sharpness improving process from leaving a visually noticeable boundary between the processed and unprocessed areas even when the marginal area W is large. 
         [0124]      FIGS. 11A to 11D  show examples of signals associated with the sharpness improving process performed by the image quality improving process section  109 A shown in  FIG. 10 . An original signal is indicated in  FIG. 11C , and high frequency components extracted from the original signal are indicated in  FIG. 11A . A mask signal mask_A′ as described above is indicated in  FIG. 11C , and an output signal is indicated in  FIG. 11D . 
         [0125]    Although not described in detail, the configuration of the image quality improving process section  109 A shown in FIG.  10  is otherwise the same as that of the image quality improving process section  109  shown in  FIG. 2 , and the section  109 A can provide the same advantages as described above. 
         [0126]    In the above description of the embodiment, the A-region has been described as having a fixed size. The size of the A-region may alternatively be varied depending on the quality of the natural image of interest. In this case, for example, the image quality improving process section  102  shown in  FIG. 2  may be provided with a high frequency component extracting portion for extracting high frequency components of a natural image region based on input luminance data Yin, and level information of the components may be transmitted to the CPU  121 . 
         [0127]    The CPU  121  may control the size of the margin W (Wh or Wv) based on the level information of the high frequency components. For example, an area which has received the sharpness improving process is more visually noticeable against an area which has not received the process, the greater the amount of high frequency components. Therefore, in the case of a natural image including a great amount of high frequency components, the size of the margin W (Wh or Wv) is set small. 
         [0128]    In the image quality improving process sections  109  and  109 A shown in  FIGS. 2 and 10 , the respective mask signal generating portions  132  and  132 A generate mask signals under control exercised by the CPU  121  to restrict the processes using the mask signals. Instead of restricting the processes using mask signals as thus described, for example, the CPU  121  may directly restrict each process depending on the region where the process is performed. As a result, the hardware configuration of the image quality improving process sections can be simplified. 
         [0129]    In the above-described embodiment, the contrast improving process and the color improving process are restricted in a B-region (a region identical to an artificial image combining region). However, the invention is not limited to such a configuration. In general, a process to be restricted in a B-region is such a process that the pixels in the combining region will not affect the pixels outside the combining region. 
         [0130]    In the above-described embodiment, the sharpness improving process is restricted in an A-region (a region overlapping and encompassing an artificial image combining region). However, the invention is not limited to such a configuration. In general, a process is to be restricted in an A-region when the pixels in the combining region will affect the pixels outside the artificial image combining region. 
         [0131]    The embodiment of the invention may be applied to television receivers or the like in which image quality improving processes are restricted in a region having an artificial image such as a data broadcast image or characters by setting a mask area in such a region such that the processes will not affect the artificial image. 
         [0132]    The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-195007 filed in the Japan Patent Office on Aug. 26, 2009, the entire contents of which is hereby incorporated by reference. 
         [0133]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.