Apparatus and method for processing image

An image processing apparatus includes a decoding unit configured to decode coded image data; a filtering unit configured to filter the image data; an analog-image output unit configured to convert the image data into analog signals and output the analog signals in an analog manner; a digital-image output unit configured to output the image data in a digital manner; an output determination unit configured to select the analog-image output unit or the digital-image output unit for outputting the image data to an external device; and a filter control unit configured to switch a characteristic of a filter used in a filtering unit in accordance with the selection made by the output determination unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to apparatuses and methods for processing images for decoding coded video information and outputting the information as analog baseband signals or digital baseband signals.

2. Description of the Related Art

Image pickup apparatuses such as digital video cameras mainly adopt Moving Picture Experts Group (MPEG) 2 as a coding method. Moreover, a large number of image recording apparatuses and image reproducing apparatuses support the MPEG-2 coding.

In the MPEG-2 coding, in general, quantization errors and the like cause more coding noise in reproduced images as the compression rate of the images is increased, resulting in degradation in image quality. Typical coding noise includes mosquito noise and block noise as high-frequency noise generated in high-frequency regions. In order to remove such coding noise and improve the quality of reproduced images, decoded images are filtered using low-pass filters (coding-noise reduction filter) in some technologies (for example, see Japanese Patent Laid-Open No. 2002-232889).

On the other hand, when image pickup apparatuses, Digital Versatile Disc (DVD) players, or the like are used as image output apparatuses and connected to image display apparatuses such that images are reproduced and displayed, the image output apparatuses and the image display apparatuses are connected to each other via S-terminal connectors, D-terminal connectors, or the like. In general, the image output apparatuses convert digital video signals into analog video signals, and output analog baseband signals to the image display apparatuses.

Recently, connection interfaces typified by High-Definition Multimedia Interface (HDMI) capable of transmitting digital baseband signals in addition to the analog baseband signals have been in widespread use.

FIG. 6is a block diagram schematically illustrating a configuration of an image processing apparatus that decodes coded video information and outputs analog baseband signals or digital baseband signals. The structure and operations of an image processing apparatus110will now be described with reference toFIG. 6.

Video signals (a so-called MPEG stream) compressed and coded using the MPEG-2 coding method are input to an MPEG decoding unit114via an input terminal112. The MPEG decoding unit114reconstructs image signals in each frame or each field by decoding the compressed and coded video signals from the input terminal112.

A filter-strength determination section118in a noise reduction unit116determines the strength of a noise filter120. The filter strength indicates, for example, the cutoff frequency of the noise filter120. The noise filter120is, for example, a spatial low-pass filter (LPF) used for digital processing, and removes or reduces coding noise in the video signals output from the MPEG decoding unit114using the fixed filter strength specified by the filter-strength determination section118.

An output-format determination unit124operates a switch122in response to, for example, a selection instruction issued by a user.

The switch122outputs the video signals passed through the noise filter120to a digital encoder126or a National Television System Committee (NTSC) encoder130.

When digital baseband signals are output, the digital encoder126converts the digital image data output from the noise filter120into digital baseband signals, and outputs the signals to an output terminal128. The digital baseband signals are output from the output terminal128to an external device.

On the other hand, when analog baseband signals are output, the NTSC encoder130converts the digital image data output from the noise filter120into digital baseband signals in the NTSC format. Subsequently, a D/A converter132converts the digital signals output from the NTSC encoder130into analog signals, and supplies the signals to an anti-aliasing filter (AAF)134. The AAF134performs anti-aliasing on the analog signals output from the D/A converter132so as to remove high-frequency components. The analog baseband signals processed by the AAF134are output from an output terminal136to an external device.

In this manner, high-frequency components need to be removed from the analog baseband signals such that aliasing is prevented. The application of the anti-aliasing filter causes differences in the degree of the coding noise generated in the digital baseband signals and the analog baseband signals. When the AAF134has an ideal frequency response whose gain at a frequency lower than or equal to the cutoff frequency is 1.0 and whose gain at a frequency higher than the cutoff frequency is zero with respect to a sampling frequency fs, the band width of the output digital baseband signals corresponds to that of the output analog baseband signals. However, it is impossible to realize an anti-aliasing filter having such an ideal frequency response.

FIG. 7illustrates a spectrum of digital image signals output from the MPEG decoding unit114. As shown inFIG. 7, the digital image signals output from the MPEG decoding unit114include components repeatedly appearing at each sampling frequency fs.FIG. 8illustrates an ideal frequency response of an anti-aliasing filter applied to analog signals into which digital signals including such cyclic components are converted. As shown inFIG. 8, the gain at frequencies from a direct-current frequency to the cutoff frequency fs/2 is 1.0, and the gain at frequencies higher than or equal to the cutoff frequency fs/2 is zero. That is, the frequency response completely removes components whose frequencies are higher than or equal to the cutoff frequency fs/2.

In order to achieve such an ideal cutoff characteristic, the number of taps needs to be infinite. However, that is impractical. As shown inFIG. 9, an anti-aliasing filter with a finite number of taps has a frequency response whose gain is gently reduced from a frequency immediately lower than the cutoff frequency fs/2. When the AAF134has the frequency response shown inFIG. 9, the passband of the analog baseband signals output from the AAF134corresponds to a portion indicated by a hatched area shown inFIG. 10. That is, the gain at a frequency immediately lower than the cutoff frequency fs/2 is smaller than the original value, and furthermore, the gain is reduced as the frequency approaches the cutoff frequency fs/2.

When the noise filter120for reducing the coding noise is disposed upstream of the AAF134as shown inFIG. 6, the spectral characteristic of the analog baseband signals output from the AAF134corresponds to that obtained by overlapping the frequency response of the noise filter120with that of the AAF134.FIG. 11illustrates the frequency response150of the noise filter120, the frequency response152of the AAF134, and a frequency response154obtained by synthesizing the frequency responses150and152. The abscissa represents the frequency, and the ordinate represents the gain.

The noise filter120and the AAF134are applied to the analog baseband signals. That is, the frequency response154obtained by synthesizing the frequency response150of the noise filter120and the frequency response152of the AAF134is applied to the analog baseband signals. A hatched area shown inFIG. 12(inner zone of the frequency response154shown inFIG. 11) corresponds to the passband of the output analog baseband signals.

On the other hand, a hatched area shown inFIG. 13(inner zone of the frequency response150shown inFIG. 11) corresponds to the passband of the output digital baseband signals since only the noise filter120is applied to the digital baseband signals. That is, the passband of the digital baseband signals is wider than that of the analog baseband signals. Herein, the frequency response of the transfer function of the digital encoder126is regarded as being flat.

As described above, the strength of the noise filter for reducing the coding noise is set to the same value when either analog or digital images are output in the known method for removing noise. Therefore, the output bands of the analog baseband signals and the digital baseband signals differ from each other due to the effects of the anti-aliasing filter. This leads to differences in the degree of noise in reproduced images.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus and a method for processing images in which the degree of coding noise generated in output digital images can be substantially the same level as that generated in output analog images.

According to an aspect of the present invention, an image processing apparatus includes a decoding unit configured to decode coded image data; a filtering unit configured to filter the image data; an analog-image output unit configured to convert the image data into analog signals and output the analog signals in an analog manner; a digital-image output unit configured to output the image data in a digital manner; an output determination unit configured to select the analog-image output unit or the digital-image output unit for outputting the image data to an external device; and a filter control unit configured to switch a characteristic of a filter used in a filtering unit in accordance with the selection made by the output determination unit.

According to another aspect of the present invention, a method for processing images includes decoding coded image data; selecting whether the image data is output as digital image data or analog image signals; coding-noise reducing during which the image data is filtered using a frequency response for digital output when the image data is output as the digital image data, and the image data is filtered using a frequency response for analog output when the image data is output as the analog image signals; analog-image outputting during which the image data is converted into analog signals and output in an analog manner when the image data is output as the analog image signals; and digital-image outputting during which the image data is output in a digital manner when the image data is output as the digital image data.

DESCRIPTION OF THE EMBODIMENTS

Numerous exemplary embodiments, features and aspects of the present invention will now be described with reference to the drawings.

First Exemplary Embodiment

FIG. 1is a block diagram illustrating an exemplary configuration of an image processing apparatus according to an exemplary embodiment of the present invention. An image processing apparatus10shown inFIG. 1decodes coded video information, and selectively outputs analog baseband signals and digital baseband signals of the decoded video information. The image processing apparatus10includes an input terminal12, an MPEG decoding unit14, a noise reduction unit16, a switch (or selector)22, an output-format determination unit24, a digital encoder26, an output terminal28such as HDMI for outputting digital baseband signals, an NTSC encoder30, a D/A converter32, an AAF34, and an output terminal36for outputting analog baseband signals. Moreover, the noise reduction unit16includes a noise filter20and a filter-strength determination section18that is controlled by the output-format determination unit24and sets the strength of the noise filter20.

For example, video data compressed and coded using the MPEG-2 coding method (a so-called video stream) is input from an apparatus, a storage medium, or a transmission path (not shown) to the input terminal12. The MPEG decoding unit14reconstructs image signals in each frame or each field by decoding the compressed and coded video data from the input terminal12using MPEG decoding such as dequantizing and inverse DCT.

When a signal output from the output-format determination unit24indicates that analog output is selected, the filter-strength determination section18sets the strength for analog output to the noise filter20, the strength having a passband reaching high frequencies. On the other hand, when digital output is selected, the filter-strength determination section18sets the strength for digital output to the noise filter20, the strength having a passband not reaching high frequencies as compared with that for analog output. The noise filter20is, for example, a LPF used for digital processing, and removes or reduces coding noise in the video data output from the MPEG decoding unit14using the filter strength set by the filter-strength determination section18. Herein, setting the filter strength mainly corresponds to specifying the cutoff frequency of the noise filter20, and, for example, the cutoff frequency is reduced as the filter strength is increased. That is, setting the filter strength corresponds to setting the strength of coding-noise reduction. The filter-strength determination section18can be regarded as a controller for switching the frequency response of the noise filter20.

The output-format determination unit24includes, for example, a connection detecting mechanism that detects the presence of video cables (not shown) connected to the output terminal28for digital baseband signals and to the output terminal36for analog baseband signals. The output-format determination unit24connects the switch22to the side of the output terminal28or the side of the output terminal36in accordance with the detected connection state of the video cables. That is, the output-format determination unit24selects digital output or analog output. When both the output terminals28and36are connected to video cables, the switch22is connected to the one that is specified in advance. Herein, the switch22is connected to the side of the output terminal28on a priority basis. The output format can be selected in response to a selection instruction issued by a user regardless of the connection state of the video cables.

The switch22outputs the digital image data passed through the noise filter20to the digital encoder26or the NTSC encoder30in accordance with the signal indicating the output format output from the output-format determination unit24. The image data is output to the digital encoder26when digital output is selected, whereas the image data is output to the NTSC encoder30when analog output is selected.

In the case where the switch22is switched in accordance with the connection state of video cables, the output-format determination unit24connects the switch22to the digital encoder26when it is detected that a video cable is connected to the output terminal28. On the other hand, the output-format determination unit24connects the switch22to the NTSC encoder30when it is detected that a video cable is connected to the output terminal36and no video cable is connected to the output terminal28.

When digital baseband signals are output, the digital encoder26converts the digital image data output from the noise filter20into digital baseband signals, and outputs the signals to the output terminal28. The digital baseband signals are output from the output terminal28to an external device. On the other hand, when analog baseband signals are output, the NTSC encoder30converts the digital image data output from the noise filter20into digital baseband signals in the NTSC format. Subsequently, the D/A converter32converts the digital signals output from the NTSC encoder30into analog signals, and supplies the signals to the AAF34. The AAF34performs anti-aliasing on the analog signals output from the D/A converter32so as to remove high-frequency components. The analog baseband signals processed using the AAF34are output from the output terminal36to an external device.

FIG. 2illustrates the frequency responses of the noise filter20and the AAF34. Reference numbers40and42denote the frequency responses for analog output and digital output, respectively, of the noise filter20. Reference number44denotes the frequency response of the AAF34.

In this exemplary embodiment, the frequency responses40and42are set to the noise filter20by the output-format determination unit24and the filter-strength determination section18such that the frequency response42corresponds to a frequency response obtained from the product of the frequency response40and the frequency response44of the AAF34. Therefore, as shown inFIG. 2, the passband of the frequency response42for digital output is narrower than that of the frequency response40for analog output.

FIG. 3is a flow chart illustrating operations of the noise reduction unit16. First, an MPEG stream is input to the MPEG decoding unit14via the input terminal12(Step S1). The MPEG decoding unit14decodes the MPEG stream, and outputs uncompressed digital image data (Step S2). At this moment, analog output or digital output has been already selected by the output-format determination unit24using the above-described method.

Next, it is determined whether or not analog output is selected by the output-format determination unit24(Step S3). When analog output is selected (Yes in Step S3), the filter-strength determination section18sets the strength for analog output (frequency response40shown inFIG. 2) to the noise filter20(Step S4). On the other hand, when digital output is selected (No in Step S3), the filter-strength determination section18sets the strength for digital output (frequency response42shown inFIG. 2) to the noise filter20(Step S5).

The noise filter20operates using the frequency response corresponding to the set filter strength, and filters the digital image data output from the MPEG decoding unit14. That is, the noise filter20reduces coding noise (Step S6).

In the case of analog output, the frequency response40of the noise filter20and the frequency response44of the AAF34are applied to the image data decoded by the MPEG decoding unit14. The hatched area (inner zone of the frequency response obtained by applying both the frequency responses40and44) shown inFIG. 4is the resultant passband. On the other hand, in the case of digital output, the frequency response42of the noise filter20is applied to the image data decoded by the MPEG decoding unit14. The hatched area (inner zone of the frequency response42) shown inFIG. 5is the resultant passband. This passband is substantially equal to that shown inFIG. 4. Since the frequency response42for digital output is set so as to correspond to the frequency response obtained by applying both the frequency responses40and44, the passband for outputting analog baseband signals and that for outputting digital baseband signals become substantially the same. Thus, the degree of coding noise generated in both cases becomes substantially the same.

In this exemplary embodiment, MPEG coding is described as an example. However, the coding method is not limited to this, and other coding methods such as Digital Video (DV) coding and H.264 (Advanced Video Coding; AVC) coding can be used. H.264 (AVC) coding is described in ITU-T H.264 standard and MPEG-4 Part 10 standard (ISO/IEC 14496-10) in detail.

In this exemplary embodiment, the output-format determination unit24includes a mechanism that detects the presence of video cables connected to the output terminals. In addition, the output-format determination unit24can include components such as menus and buttons with which users can manually select output signals. Furthermore, the output-format determination unit24can be controlled by a remote controller. In the case of a structure where digital baseband signals and analog baseband signals can be simultaneously output, the switch22can be connected to both output terminals, and at the same time, a plurality of noise filters20can be used for parallel processing. At this moment, the filter-strength determination section18sets the frequency response42to the noise filter for digital output, and sets the frequency response40to the noise filter for analog output. With this, the same decoded images can be simultaneously output in the analog manner and in the digital manner, and at the same time, the degree of noise generated in both cases can be substantially the same level.

Other Exemplary Embodiments

Program codes of software for achieving the functions according to the above-described exemplary embodiment can be supplied to computers in apparatuses or systems connected to various devices so that the devices can operate so as to achieve the functions according to the above-described exemplary embodiment. The program codes executed by operating the devices in accordance with programs stored in the computers (CPUs or MPUs) in the apparatuses or the systems are also included in the scope of the present invention.

In this case, the program codes of the software achieve the functions according to the above-described exemplary embodiment. Moreover, the program codes and units for supplying the program codes to the computers, for example, storage media in which the program codes are stored are included in the present invention. The storage media for storing the program codes include, for example, flexible disks, hard disks, optical disks, magneto-optical disks, CD-ROMs, magnetic tapes, nonvolatile memory cards, and ROMs.

The functions according to the above-described exemplary embodiment can be achieved by executing the program codes supplied to the computer. In addition, the functions according to the above-described exemplary embodiment can also be achieved by executing the program codes in conjunction with operating systems (OSs), other application software, or the like working on the computers. In such a case, the program codes are also included in the exemplary embodiments of the present invention.

After the program codes are stored in memories provided for function expansion boards in the computers, CPUs or the like provided for the function expansion boards execute a part of or all the actual processes in response to instructions of the program codes. The processes that achieve the functions according to the above-described exemplary embodiment are also included in the present invention. Furthermore, after the program codes are stored in memories provided for function expansion boards connected to the computers, CPUs or the like provided for the function expansion boards execute a part of or all the actual processes in response to instructions of the program codes. The processes that achieve the functions according to the above-described exemplary embodiment are also included in the present invention.

This application claims the priority of Japanese Application No. 2006-319624 filed Nov. 28, 2006, which is hereby incorporated by reference herein in its entirety.