Gradation converting device, image processing apparatus, image processing method, and computer program

An image processing apparatus includes: a filter-coefficient storing unit that stores filter coefficients respectively associated with spatial frequencies, which are the numbers of strips displayed per unit angle with respect to an angle of field of a display apparatus; a viewing-condition determining unit that determines, as viewing conditions, a viewing distance between a viewer and the display apparatus and pixel density of the display apparatus; a filter-coefficient setting unit that sets a filter coefficient selected on the basis of a spatial frequency calculated from the viewing conditions among the stored filter coefficients; and a gradation modulating unit including a quantizing unit that quantizes a pixel value in a predetermined coordinate position in an image signal and outputs the pixel value as a quantized pixel value in the predetermined coordinate position, the gradation modulating unit gradation-modulating the image signal by multiply-accumulating a set filter coefficient with respect to quantization errors caused by the quantizing unit to feedback the quantization errors to an input side of the quantizing unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. JP 2008-028470, filed in the Japanese Patent Office on Feb. 8, 2008, the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus, and, more particularly to an image processing apparatus that quantizes pixel values of respective pixels of an image signal, a gradation converting device for the quantization, a processing method for the image processing apparatus and the gradation converting device, and a computer program for causing a computer to execute the method.

2. Description of the Related Art

In digital video display in digital camcorders, computer graphics, animations, and the like, the number of bits of a gradation of a material and the number of bits of a display apparatus or the number of bits on a digital transmission interface such as an HDMI (High-Definition Multimedia Interface) or a DVI (Digital Visual Interface) do not always coincide with each other. In signal processing in an apparatus that treats a digital video signal, a calculation process of the processing and the number of transmitted bits of video signal data in the apparatus may be different.

FIG. 20is a block diagram of the numbers of bits of respective components and the number of bits on a bus until a digital image is displayed on a display apparatus. InFIG. 20, an image processing unit811, a pixel-density converting unit812, a color-mode converting unit813, a panel control unit814, and a display unit815are shown. An image signal inputted to the image processing unit811is sequentially processed and finally displayed on the display unit815. In this case, it is seen that the numbers of bits of processing in the respective components and the numbers of bits of signal lines of the bus connecting the respective components are different. For example, whereas calculation in the panel control unit814is performed with 10 bits, an input signal to and an output signal from the panel control unit814are 8-bit RGB signals. The number of input and output signals and the number of bits of the internal calculation are different. In such a case, conversion of the number of bits is necessary as gradation conversion. As methods generally used for the conversion of the number of bits, there are bit shift for simply shifting the number of bits by a necessary number of bits and a method of, for example, once dividing the number of bits by the number of bits before conversion to normalize the number of bits to a value between 0 to 1 in order to expand quantization steps to equal interval and then multiplying the number of bits with a necessary number of bits.

FIG. 21is a diagram of gradation conversion from 10 bits to 8 bits by the bit shift. InFIG. 21, 10-bit gradation representation820and 8-bit gradation representation830after the bit shift are shown. In this case, the number of bits is converted from 10 bits into 8 bits by moving 8 bits (higher order 8 bits) on the left to the right by 2 bits (omitting lower order 2 bits).

However, when the lower order 2 bits are omitted in this way, in an image with smooth gradation and a flat image with a little change of a gray scale such as an image of the blue sky in a sunny day, steps called banding or Mach band may appear because of the influence of the human visual characteristic.

Such quantization errors due to a reduction in the number of bits cause deterioration in an image quality. As measures against the quantization errors, in general, methods called a dither method and an error diffusion method are used. These methods are methods of adding PDM (Pulse Depth Modulation) noise to a boundary of the banding to thereby making the steps less conspicuous.

FIGS. 22A to 22Care graphs of a change in a pixel value that occurs when the PDM noise is added to the bit shift from 10 bits to 8 bits. InFIGS. 22A to 22C, the abscissa indicates coordinates in the horizontal direction in an image and the ordinate indicates pixel values in the respective coordinates. A level of the pixel value on the ordinate is set to 0 to 8 for convenience of illustration.FIG. 22Ais a graph of a pixel value of a gray scale image quantized to 10 bits. InFIG. 22A, the level of the pixel value gradually increases by one level at a time from the left to the right in the horizontal direction.FIG. 22Bis a graph of an example of a pixel value of an image quantized to 8 bits by omitting lower order 2 bits of the 10-bit gray scale image shown inFIG. 22A. In this case, a state of the pixel value substantially changing stepwise is seen.FIG. 22Cis a graph of a change in a pixel value of an image obtained by adding the PDM noise to the image, the number of bits of which is converted into 8 bits, shown inFIG. 22B. In this case, it is seen that noise, a pixel value of which changes in a pulse-like manner, is added and intervals among the pieces of noise are narrowed in coordinates closer to steps. The steps are made less conspicuous by changing the pixel value in a pulse-like manner and changing the pulse intervals. The influence due to the addition of the PDM noise is explained with reference to the following diagrams in an example of an actual gray scale image.

FIGS. 23A to 23Dare diagrams of images formed when the PDM noise is added to the bit shift from 10 bits to 8 bits.FIG. 23Ais a diagram of an image of a 10-bit gray scale. Although a pixel value does not change in the vertical direction, a pixel value gradually changes in the horizontal direction.FIG. 23Bis a diagram of an image formed by converting the 10-bit gray scale image into 8 bits by omitting lower order 2 bits. In this case, a state of the pixel value steeply changing is clearly seen.FIGS. 23C and 23Dare diagrams of images formed by adding the PDM noise to the gray scale image quantized to 8 bits shown inFIG. 23B. In bothFIGS. 23C and 23D, it is seen that steps are inconspicuous. The image shown inFIG. 23Cis formed by the dither method and the image shown inFIG. 23Dis formed by the error diffusion method. The dither method and the error diffusion methods are substantially different in that, whereas noise is added regardless of the human visual characteristic in the dither method, noise is added taking into account the human visual characteristic in the error diffusion method. As a representative two-dimensional filter used for the error diffusion method, a Jarvis, Judice & Ninke's filter (hereinafter referred to as Jarvis filter) and a Floyd & Steinberg's filter (hereinafter referred to as Floyd filter) are known (see, for example, Hitoshi Kiya, “Yokuwakaru Digital Image Processing”, Sixth edition, CQ publishing Co., Ltd., January 2000, p. 196 to 213).

In order to represent the human visual characteristic, a contrast sensitivity curve representing a spatial frequency f [unit: cpd (cycle/degree)] on the abscissa and representing contrast sensitivity on the ordinate is used. The spatial frequency represents the number of stripes that can be displayed per unit angle (1 degree in angle of field) with respect to the angle of field. A maximum frequency in the spatial frequency depends on pixel density (the number of pixels per unit length) of a display apparatus and a viewing distance.

FIGS. 24A and 24Bare diagrams concerning calculation of the maximum frequency in the spatial frequency in the display apparatus. InFIGS. 24A and 24B, an angle θ represents 1 degree in angle of field and a viewing distance D represents a distance between the display apparatus and a viewer as shown inFIG. 24B. Width “d” on a display screen with respect to 1 degree in angle of field is calculated from the angle θ and the viewing distance D by using the following relational expression:
tan(θ/2)=(d/2)/D

The maximum frequency in the spatial frequency as the number of stripes on the display screen per 1 degree in angle of field can be calculated by dividing the width “d” on the display screen by length per two pixels (the two pixels form one set of stripes) calculated from the pixel density of the display screen.

When, for example, a high-resolution printer having the maximum frequency of about 120 cpd is assumed as the display apparatus, as shown inFIG. 25A, it is possible to modulate quantization errors to a frequency band that is less easily sensed in a human vision characteristic840even by the Jarvis filter851and the Floyd filter852. Amplitude characteristics of these representative filters are different. In general, the Jarvis filter is used when importance is attached to a low frequency band and the Floyd filter is used when a higher frequency is treated.

However, when a high-definition display having 1920 pixels×1080 pixels in the horizontal and vertical directions is assumed as the display apparatus, the maximum frequency per unit angle with respect to the angle of field is about 30 cpd. As shown inFIG. 25B, it is seen that it is difficult to modulate the quantization errors to a band with sufficiently low sensitivity with respect to the human visual characteristic840using the Jarvis Filter851and the Floyd filter852. Such a situation is caused because, whereas a sampling frequency depends on pixel density of the display apparatus, the human visual characteristic has a peculiar value.

SUMMARY OF THE INVENTION

It is possible to modulate, using the error diffusion method, quantization errors due to gradation conversion involved in processing in an image processing apparatus or digital transmission to a frequency band less easily sensed by the human visual characteristic. However, a filter characteristic used for the error diffusion method is uniquely decided. Therefore, if viewing conditions such as performance of a display apparatus for viewing and a viewing distance between a viewer and the display apparatus change, a maximum frequency in a spatial frequency in the display apparatus also changes. As a result, error diffusion processing suitable for the display apparatus is not obtained by the uniquely-decided filter characteristic. For a display apparatus that displays an image signal, it is difficult to modulate the quantization errors to a frequency band with sufficiently low sensitivity with respect to the human visual characteristic using the Jarvis filter and the Floyd filter.

Therefore, it is desirable to modulate the quantization errors to a band with sufficiently low sensitivity with respect to the human visual characteristic by setting an optimum filter coefficient according to viewing conditions.

According to an embodiment of the present invention, there is provided an image processing apparatus including: filter-coefficient storing means for storing filter coefficients respectively associated with spatial frequencies, which are the numbers of strips displayed per unit angle with respect to an angle of field of a display apparatus; viewing-condition determining means for determining, as viewing conditions, a viewing distance between a viewer and the display apparatus and pixel density of the display apparatus; filter-coefficient setting means for setting a filter coefficient selected on the basis of a spatial frequency calculated from the viewing conditions among the stored filter coefficients; and gradation modulating means including quantizing means for quantizing a pixel value in a predetermined coordinate position in an image signal and outputting the pixel value as a quantized pixel value in the predetermined coordinate position, the gradation modulating means gradation-modulating the image signal by multiply-accumulating a set filter coefficient with respect to quantization errors caused by the quantizing means to feed back the quantization errors to an input side of the quantizing means. Therefore, there is an effect that the quantization errors are modulated to a band with sufficiently low sensitivity with respect to the human visual characteristic by setting an optimum filter coefficient according to viewing conditions.

Preferably, the viewing-condition determining means receives the number of pixels and a screen size of the display apparatus from the display apparatus and determines the viewing conditions on the basis of the number of pixels and the screen size. Therefore, there is an effect that the number of pixels and the screen size are received from the display apparatus and the viewing conditions are calculated on the basis of the number of pixels and the screen size.

Preferably, the viewing-condition determining means receives the pixel density and a screen size of the display apparatus from the display apparatus and determines the viewing conditions on the basis of the pixel density and the screen size. Therefore, there is an effect that the pixel density and the screen size are received from the display apparatus and the viewing conditions are calculated on the basis of the pixel density and the screen size.

Preferably, the filter coefficient is set to reduce the quantized error of frequency components lower than a predetermined spatial frequency. Therefore, there is an effect that quantization noise of the frequency components lower than the predetermined spatial frequency is reduced. In this case, the predetermined spatial frequency is set to about two third of a maximum frequency in the spatial frequency. Therefore, there is an effect that quantization noise of frequency components lower than about two third of the maximum frequency in the spatial frequency is reduced.

Preferably, the gradation modulating means further includes: inverse quantization means for inversely quantizing the quantized pixel value in the predetermined coordinate position and outputting the quantized pixel value as an inversely quantized pixel value in the predetermined coordinate position; differential generating means for generating, as quantization errors in the predetermined coordinate position, a difference value between the quantized pixel value in the predetermined coordinate position and the inversely quantized pixel value in the predetermined coordinate position; arithmetic means for calculating, as a feedback value in the predetermined coordinate position, a value obtained by multiplying the respective quantization errors in a predetermined area corresponding to the predetermined coordinate position with the set filter coefficient and adding up the quantization errors; and adding means for adding the feedback value in the predetermined coordinate position to the corrected pixel value in the predetermined coordinate position. Therefore, there is an effect that the quantization errors are modulated to a band with sufficiently low sensitivity with respect to the human visual characteristic by setting an optimum filter coefficient according to viewing conditions.

According to another embodiment of the present invention, there is provided a filter coefficient setting processing method for an image processing apparatus including a display apparatus, filter-coefficient storing means for storing filter coefficients respectively associated with spatial frequencies, which are the numbers of strips displayed per unit angle with respect to an angle of field of a display apparatus, and gradation modulating means including quantizing means for quantizing a pixel value in a predetermined coordinate position in a pixel signal and outputting the pixel value as a quantized pixel value in the predetermined coordinate position, the gradation modulating means gradation-modulating the image signal by multiply-accumulating a set filter coefficient with respect to quantization errors caused by the quantizing means to feed back the quantization errors to an input side of the quantizing means, the filter coefficient setting processing method including: a viewing-condition determining step of determining, as viewing conditions, a viewing distance between a viewer and the display apparatus and pixel density of the display apparatus; and a filter-coefficient setting step of setting, in the gradation modulating means, a filter coefficient selected on the basis of a spatial frequency calculated from the viewing conditions among the filter coefficients stored in the filter-coefficient storing means. There is also provided a computer program for causing a computer to execute these steps. Therefore, there is an effect that the gradation modulating means is caused to set an optimum filter coefficient according to viewing conditions.

According to the embodiments of the present invention, it is possible to realize an excellent effect that quantization errors can be modulated to a band with sufficiently low sensitivity with respect to the human visual characteristic by setting an optimum filter coefficient according to viewing conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is explained in detail below with reference to the accompanying drawings.

FIG. 1is a diagram of a configuration example of a reproduction and display system according to the embodiment of the present invention. The reproduction and display system includes a reproducing apparatus10that reproduces a data signal recorded in a recording medium and a broadcast signal digitally broadcasted and a display apparatus30that displays the reproduced signals. The reproducing apparatus10and the display apparatus30are connected by a digital transmission signal line50. An image signal and a sound signal processed by the reproducing apparatus10are transmitted to the display apparatus30via the digital transmission signal line50and displayed on a display screen of the display apparatus30.

FIG. 2is a diagram of a configuration example of the reproducing apparatus10according to this embodiment.

The reproducing apparatus10includes a tuner11, a decoder12, a processor15, a ROM (Read-Only Memory)16, a RAM (Random Access Memory)17, a digital transmission interface (I/F)18, a network interface (I/F)19, a recording control unit21, a recording medium22, an operation receiving unit23, and a bus24. The reproducing apparatus10transmits the processed image signal and sound signal to the display apparatus30via the digital transmission interface18.

The tuner11receives a radio wave of a digital broadcast and demodulates a modulated wave of a channel designated by the operation receiving unit23. The tuner11supplies demodulated image data and sound data to the decoder12.

The decoder12decodes the image data and the sound data demodulated by the tuner11. The decoder12supplies the decoded image signal and sound signal to the processor15.

The ROM16is a memory that stores various control programs and the like. The RAM17is a memory that has a work area for the processor15.

The digital transmission interface18performs data communication between the reproducing apparatus10and the display apparatus30connected to the digital transmission signal line50. The digital transmission interface18can be realized by a digital transmission interface such as an HDMI (High-Definition Multimedia Interface) or a DVI (Digital Visual Interface). The digital transmission interface18transmits the image signal and the sound signal processed by the processor15to the display apparatus30. Specifically, the digital transmission interface18acquires screen information concerning viewing conditions from the display apparatus30and supplies the screen information to the processor15. The viewing conditions are a viewing distance between a viewer and the display apparatus30and pixel density of the display apparatus30. As shown inFIGS. 24A and 24B, the viewing conditions are parameters for calculating a maximum frequency in a spatial frequency in the display apparatus30. The screen information concerning the viewing conditions is a parameter necessary for calculating the viewing conditions. In this embodiment, for calculation of the pixel density, the digital transmission interface18acquires the vertical length of the screen and the number of pixels in the vertical direction of the screen from the display apparatus30as the screen information. Concerning the viewing distance, in general, in a display screen with an aspect ratio of 16:9, a viewing distance 2.5 to 3.0 times as large as the vertical length of the screen is an optimum viewing distance. Therefore, a viewing distance obtained by multiplying the vertical length of the screen with 2.5 or 3.0 is set as the viewing distance. This makes it possible to calculate the vertical length of the screen included in the screen information.

As an example, the pixel density, which is one of the viewing conditions, is calculated from the vertical length of the screen and the number of pixels in the vertical direction of the screen. However, the pixel density may be calculated from the horizontal width of the screen and the number of pixels in the horizontal direction of the screen. The viewing distance, which is the other of the viewing conditions, is calculated on the basis of the vertical length of the screen. However, the viewing distance may be calculated by using the horizontal width of the screen instead of the vertical length of the screen. In this case, for example, the viewing distance can be calculated by using the following relational expression of the horizontal width of the screen in the display screen with the aspect ratio of 16:9 and the viewing distance:
Horizontal width of the screen=viewing distance×0.650

The network interface19performs data communication with an external apparatus connected to the Internet, a LAN (Local Area Network), or the like.

The recording control unit21records image data in the recording medium22in a predetermined format on the basis of the control by the processor15or reads out image data recorded in the recording medium22.

The recording medium22stores video data. The recording medium22is, for example, a hard disk driver or a Blu-ray disk.

The operation receiving unit23receives channel selection or operation inputs such as reproduction and stop of the reproduction of the image data stored in the recording medium22from a user of the reproducing apparatus10.

The processor15controls the respective components of the reproducing apparatus10on the basis of the control programs stored in the ROM16. For example, the processor15calculates the viewing conditions (the pixel density and the viewing distance) from the screen information (the vertical length of the screen and the number of pixels in the vertical direction of the screen) supplied from the digital transmission interface18and calculates a maximum frequency in a spatial frequency in the display apparatus30from the viewing conditions as shown inFIGS. 24A and 24B. The processor15applies, on the basis of the spatial frequency, gradation modulation for modulating quantization errors (quantization noise) to a high-frequency region to the image signal supplied from the recording control unit21or the decoder12and controls the reproducing apparatus10to transmit the image signal to the display apparatus30via the digital transmission interface18.

The bus24is a system bus of the reproducing apparatus10that connects the processor15and the respective components to each other.

The processor15calculates the pixel density, which is one of the viewing conditions, from the vertical length of the screen and the number of pixels in the vertical direction of the screen acquired from the display apparatus30and calculates the viewing distance, which is the other of the viewing conditions, on the basis of the vertical length of the screen. However, the processor15may acquire the pixel density and the vertical length of the screen and calculate only the viewing distance from the vertical length of the screen. Further, the processor15calculates the viewing distance on the basis of the vertical length of the screen. However, the processor15may measure a distance between a remote controller of the display apparatus30and the display screen of the display apparatus30using a technique such as UWB (Ultra Wide Band) and causes the display apparatus30to transmit the distance to the reproducing apparatus10as a viewing distance. The processor15acquires, for calculation of the viewing conditions, the screen information of the display apparatus30via the digital transmission interface18. However, the processor15may directly set the viewing conditions (the pixel density and the viewing distance) in the operation receiving unit23.

FIG. 3is a diagram of a configuration example of the display apparatus30according to the embodiment.

The display apparatus30includes a tuner31, a decoder32, a display control unit33, a display unit34, a processor35, a ROM36, a RAM37, a digital transmission interface (I/F)38, a network interface (I/F)39, an operation receiving unit43, and a bus44. The display apparatus30receives, via the digital transmission interface38, an image signal subjected to image processing by the reproducing apparatus10and displays the image signal on the display screen. Functions of the components other than the display control unit33, the display unit34, the processor35, and the digital transmission interface38are the same as those of the reproducing apparatus10. Therefore, explanation of the functions is omitted.

The display control unit33causes the display unit34to display the image signal on the basis of the control by the processor35.

The display unit34displays the image signal on the basis of the control by the display control unit33. The digital transmission interface38performs data communication between the display apparatus30and the reproducing apparatus10connected to the digital transmission signal line50. Specifically, the digital transmission interface38transmits the screen information (the vertical length of the screen and the number of pixels in the vertical direction of the screen) on the basis of the control by the processor35. The digital transmission interface38receives an image signal and a sound signal processed by the reproducing apparatus10.

The processor35controls the respective components of the display apparatus30on the basis of the control programs stored in the ROM36. Specifically, for example, the processor35controls the display apparatus30to transmit screen information of the display apparatus30to the reproducing apparatus10via the digital transmission interface38. The processor35controls the display apparatus30to display an image signal supplied from the digital transmission interface38or the decoder32on the display unit34.

FIG. 4is a diagram of a functional configuration example of the reproducing apparatus10according to this embodiment. The reproducing apparatus10includes a gradation modulator200, a filter-coefficient setting unit260, a filter-coefficient storing unit270, and a viewing-condition determining unit280.

The gradation modulator200receives a two-dimensional image signal from a signal line201as an input signal IN(x,y) and outputs an output signal OUT (x,y) from a signal line209. The gradation modulator200configures a ΔΣ modulator and has a noise shaping effect for modulating quantization errors to a high-frequency region.

The quantizing unit210is a quantizer that quantizes an output of an adder250. For example, when data having 12-bit width is inputted from the adder250, the quantizing unit210omits lower order 4 bits and outputs higher order 8 bits as an output signal OUT(x,y).

The inverse quantization unit220is an inverse quantizer that inversely quantizes the output signal OUT(x,y) quantized by the quantizing unit210. For example, when the quantized output signal OUT(x,y) has 8-bit width, the inverse quantization unit220embeds “0000” in the lower order 4 bits (padding) and outputs 12-bit width data.

A subtracter230is a subtracter that calculates a difference between the output of the adder250and the output of the inverse quantization unit220. The subtracter230subtracts the output of the inverse quantization unit220from the output of the adder250to thereby output quantized errors Q(x,y) omitted by the quantizing unit210to a signal line239.

A feedback arithmetic unit240multiplies the quantization errors Q(x,y) in the past outputted from the subtracter230with a filter coefficient set by the filter-coefficient setting unit260and adds up the quantization errors Q(x,y). A value calculated by multiply-accumulate by the feedback arithmetic unit240is supplied to the adder250as a feedback value.

The adder250is an adder for feeding back the feedback value calculated by the feedback arithmetic unit240to a correction signal F(x,y) inputted to the gradation modulator200. The adder250adds up the correction signal F(x,y) inputted to the gradation modulator200and the feedback value calculated by the feedback arithmetic unit240and outputs a result of the addition to the quantizing unit210and the subtracter230.

In the image processing apparatus, the gradation modulator200has an input and output relation explained below.
OUT(x,y)=F(x,y)+(1−G)×Q(x,y)
It is seen that the quantization errors Q(x,y) is modulated to a high frequency by the noise shaping of “1−G”.

The filter-coefficient setting unit260selects, on the basis of the viewing conditions supplied from the viewing-condition determining unit280, a filter coefficient associated with a spatial frequency determined on the basis of the viewing conditions from the filter-coefficient storing unit270. The filter-coefficient setting unit260sets the selected filter coefficient in the feedback arithmetic unit240. The filter-coefficient setting unit260can be realized by the processor15.

The filter-coefficient storing unit270stores filter coefficients associated with spatial frequencies, respectively. The filter-coefficient storing unit270can be realized by the ROM16.

The viewing-condition determining unit280receives screen information from the display apparatus30and calculates viewing conditions. When it is difficult for the viewing-condition determining unit280to receive the screen information, the viewing-condition determining unit280may calculate the viewing conditions using a value decided in advance. The viewing-condition determining unit280supplies the calculated viewing conditions to the filter-coefficient setting unit260. The viewing-condition determining unit280can be realized by the digital transmission interface18and the processor15.

FIG. 5is a diagram of processing order for respective pixels of an image signal according to this embodiment. As an arrangement of the pixels of the image signal, a reference coordinate (0,0) is set at the upper left and the horizontal direction X and the vertical direction Y are indicated by the abscissa and the ordinate, respectively.

Image processing according to this embodiment is performed to sequentially raster-scan the pixels from the left to the right and from the top to the bottom as indicated by arrows in the figure. Input signals are inputted in order of IN(0,0), IN(1,0), IN(2,0), . . . , IN(0,1), IN(1,1), IN(2,1), . . . . .

The feedback arithmetic unit240takes into account the order of the raster scan as a predetermined area in referring to the other pixels. For example, when the feedback arithmetic unit240calculates a feedback value corresponding to the correction signal F(x,y), the feedback arithmetic unit240refers to twelve quantization errors Q(x−2,y−2), Q(x−1,y−2), Q(x,y−2), Q(x+1,y−2), Q(x+2,y−2), Q(x−2,y−1), Q(x−1,y−1), Q(x,y−1), Q(x+1,y−1), Q(x+2,y−1), Q(x−2, y), and Q(x−1,y) in an area surrounded by a dotted line, i.e., quantization errors in the past.

In the case of a color image signal including a luminance signal Y, color difference signals Cb and Cr, and the like, gradation conversion processing is applied to the respective signals. The luminance signal Y is independently subjected to the gradation conversion processing. The color difference signals Cb and Cr are also independently subjected to the gradation conversion processing.

FIG. 6is a diagram of a configuration example of the feedback arithmetic unit240according to this embodiment. The feedback arithmetic unit240includes a quantization-error supplying unit241, multipliers2461to2472, and an adder248.

The quantization-error supplying unit241supplies values in the past of the quantization errors Q(x,y). In this example, it is assumed that the twelve quantization errors Q(x−2,y−2), Q(x−1,y−2), Q(x,y−2), Q(x+1,y−2), Q(x+2,y−2), Q(x−2,y−1), Q(x−1,y−1), Q(x,y−1), Q(x+1,y−1), Q(x+2,y−1), Q(x−2, y), and Q(x−1,y) are supplied.

The multipliers2461to2472are multipliers that multiply the quantization errors Q supplied from the quantization-error supplying unit241and filter coefficients “g” together. In this example, assuming twelve filter coefficients, the multiplier2461multiplies the quantization error Q(x−2,y−2) and a filter coefficient g(1,1) together, the multiplier2462multiplies the quantization error Q(x−1,y−2) and a filter coefficient g(2,1) together, the multiplier2463multiplies the quantization error Q(x,y−2) and a filter coefficient g(3,1) together, the multiplier2464multiplies the quantization error Q(x+1,y−2) and a filter coefficient g(4,1) together, the multiplier2465multiplies the quantization error Q(x+2,y−2) and a filter coefficient g(5,1) together, the multiplier2466multiplies the quantization error Q(x−2,y−1) and a filter coefficient g(1,2) together, the multiplier2467multiplies the quantization error Q(x−1,y−1) and a filter coefficient g(2,2) together, the multiplier2468multiplies the quantization error Q(x,y−1) and a filter coefficient g(3,2) together, the multiplier2469multiplies the quantization error Q(x+1,y−1) and a filter coefficient g(4,2) together, the multiplier2470multiplies the quantization error Q(x+2,y−1) and a filter coefficient g(5,2) together, the multiplier2471multiplies the quantization error Q(x−2,y) and a filter coefficient g(1,3) together, and the multiplier2472multiplies the quantization error Q(x−1,y) and a filter coefficient g(2,3) together.

The adder248is an adder that adds up outputs of the multipliers2461to2472. A result of the addition by the adder248is supplied to one input of the adder250as a feedback value via a signal line249.

FIG. 7is a diagram of a configuration example of the quantization-error supplying unit241according to this embodiment. The quantization-error supplying unit241includes a memory2411, a write unit2414, read units2415and2416, and delay elements2421to2432.

The memory2411includes line memories #0(2412) and #1(2413). The line memory #0(2412) is a memory that stores the quantization errors Q of a line in the vertical direction Y=(y−2). The line memory #1(2413) is a memory that stores the quantization errors Q of a line in the vertical direction Y=(y−1).

The write unit2414writes the quantization errors Q(x,y) in the memory2411. The read unit2415reads out the quantization errors Q of the line in the vertical direction Y=(y−2) one by one from the line memory #0(2412). The quantization error Q(x+2,x−2) as an output of the read unit2415is inputted to the delay element2424and supplied as one input to the multiplier2465via a signal line2455. The read unit2416reads out the quantization errors Q of the line in the vertical direction Y=(y−1) one by one from the line memory #1(2413). The quantization error Q(x+2,y−1) as an output of the read unit2416is inputted to the delay element2429and supplied as one input to multiplier2470via a signal line2450.

The delay elements2421to2424configure a shift resistor that delays an output of the read unit2415. The quantization error Q(x+1,y−2) as an output of the delay element2424is inputted to the delay element2423and supplied as one input to the multiplier2464via a signal line2444. The quantization error Q(x,y−2) as an output of the delay element2423is inputted to the delay element2422and supplied as one input to the multiplier2463via a signal line2443. The quantization error Q(x−1,y−2) as an output of the delay element2422is inputted to the delay element2421and supplied as one input to the multiplier2462via a signal line2442. The quantization error Q(x−2,y−2) as an output of the delay element2421is supplied as one input to the multiplier2461via a signal line2441.

The delay elements2426to2429configure a shift register that delays an output of the read unit2416. The quantization error Q(x+1,y−1) as an output of the delay element2429is inputted to the delay element2428and supplied as one input to the multiplier2469via a signal line2449. The quantization error Q(x,y−1) as an output of the delay element2428is inputted to the delay element2427and supplied as one input to the multiplier2468via a signal line2448. The quantization error Q(x−1,y−1) as an output of the delay element2427is inputted to the delay element2426and supplied as one input to the multiplier2467via a signal line2447. The quantization error Q(x−2,y−1) as an output of the delay element2426is supplied as one input to the multiplier2466via a signal line2446.

The delay elements2431and2432configure a shift resister that delays the quantization errors Q(x,y). The quantization error Q(x−1,y) as an output of the delay element2432is inputted to the delay element2431and supplied as one input to the multiplier2472via a signal line2452. The quantization error Q(x−2,y) as an output of the delay element2431is supplied as one input to the multiplier2471via a signal line2451.

The quantization errors Q(x,y) of the signal line239are stored in an address “x” of the line memory #0(2412). When processing for one line is finished in the order of the raster scan, the line memory #0(2412) and the line memory #1(2413) are interchanged. Therefore, quantization errors stored in the line memory #0(2412) correspond to the lines in the vertical direction Y=(y−2) and quantization errors stored in the line memory #1(2413) correspond to the lines in the vertical direction Y=(y−1).

FIG. 8is a graph of the human visual characteristic and amplitude characteristics of filters at the time when a maximum frequency in a spatial frequency is set to 30 cpd. The abscissa represents a spatial frequency “f” [cpd]. Concerning the human visual characteristic840, the ordinate represents contrast sensitivity. Concerning the amplitude characteristics (851,852, and860) of the filters, the ordinate represents gains of the filters.

The human visual characteristic840reaches a peak value near the spatial frequency “f” of 7 cpd and is attenuated to near 60 cpd. On the other hand, the amplitude characteristic860by the reproducing apparatus according to this embodiment is a curve that is attenuated in a minus direction to near the spatial frequency “f” of 12 cpd and, thereafter, steeply rises. In the amplitude characteristic860, a quantization error of low frequency components is attenuated to about two third of the maximum frequency in the spatial frequency. The quantization error is modulated to a band with sufficiently low sensitivity with respect to the human visual characteristic840.

In the Jarvis filter851and the Floyd filter852in the past, it is difficult to modulate quantization errors to a band with a sufficiently low sensitivity with respect to the human visual characteristic840.

FIG. 9is a diagram of a conceptual configuration example concerning screen information acquisition by the digital transmission interface18according to this embodiment. As an example, an interface conforming to the HDMI standard is explained. In the HDMI standard, a transmission direction by a high-speed transmission line as a basis is set in one direction. An apparatus on a transmission side is referred to as source apparatus and an apparatus on a reception side is referred to as sync apparatus. In the example shown inFIG. 1, the reproducing apparatus10corresponds to the source apparatus and the display apparatus30corresponds to the sync apparatus. In this example, the source apparatus310and the sync apparatus320are connected by an HDMI cable330. The source apparatus310includes a transmitter311that performs a transmission operation. The sync apparatus320includes a receiver321that performs a reception operation. The transmitter311corresponds to the digital transmission interface18and the receiver321corresponds to the digital transmission interface38.

A TMDS serial transmission system is used for the transmission between the transmitter311and the receiver321. In the HDMI standard, an image signal and a sound signal are transmitted by using three TMDS channels331to333. In a valid image section, which is a section obtained by excluding a horizontal blanking section and a vertical blanking section in a section from a certain vertical synchronization signal to the next vertical synchronization signal, a differential signal corresponding to pixel data of an image for uncompressed one screen is transmitted in one direction to the sync apparatus320by the TMDS channels331to333. In the horizontal blanking section and the vertical blanking section, a differential signal corresponding to sound data, control data, other auxiliary data, or the like is transmitted in one direction to the sync apparatus320by the TMDS channels331to333.

In the HDMI standard, a clock signal is transmitted by a TMDS clock channel334. In each of the TMDS channels331to333, pixel data for 10 bits can be transmitted during one clock transmitted by the TMDS clock channel334.

In the HDMI standard, a display data channel (DDC)335is provided. The display data channel335is used by the source apparatus310to read out EDID (Extended Display Identification Data) in the sync apparatus320. The EDID information indicates, when the sync apparatus320is a display apparatus, information concerning a model, setting of a screen size, timing, and the like, and performance of the sync apparatus320. The EDID information is stored in an EDID ROM322of the sync apparatus320. Further, in the HDMI standard, a CEC (Consumer Electronics Control) line336is provided. The CEC line336is a line for performing bidirectional communication of an apparatus control signal. Whereas the display data channel335connects apparatuses in a one to one relation, the CEC line336directly connects all apparatuses connected to the HDMI.

FIG. 10is a schematic diagram of a storage format400for EDID information. The storage format400for EDID information includes items such as Vender/Product Identification410, Basic Display Parameters420, and Standard Timing Identification430. The Vendor/Product Identification410includes information such as an ID Manufacturer Name411, an ID Product Code412, and an ID Serial Number413. The Basic Display Parameters420includes information such as a Max. Horizontal Image Size421and a Max. Vertical Image Size422. The Standard timing Identification430includes information such as the number of pixels in horizontal direction431, the number of pixels in vertical direction432, and a scanning frequency433.

Consequently, in this embodiment, the viewing-condition determining unit280receives, as screen information, the Max. Vertical Image Size422and the number of pixels in vertical direction432among the EDID information via the display data channel. The viewing-condition determining unit280calculates, as a viewing condition, a viewing distance from the Max. Vertical Image Size422and calculates, as a viewing condition, pixel density from the Max. Vertical Image Size422and the number of pixels in vertical direction432. The viewing-condition determining unit280acquires screen information via the display data channel335. However, when the Max. Vertical Image Size422and the number of pixels in vertical direction432are not stored in the EDID ROM322, the viewing-condition determining unit280acquires screen information via the CEC line336. When it is still difficult to acquire one of both of these kinds of screen information, the viewing-condition determining unit280calculates viewing conditions using values decided in advance.

FIGS. 11A and 11Bare tables of a storage format example of the filter-coefficient storing unit270according to this embodiment.FIG. 11Ais a correspondence table of spatial frequencies corresponding to viewing conditions (pixel density and a viewing distance) in a 40-inch display screen with an aspect ratio of 16:9.FIG. 11Bis a correspondence table of filter coefficients corresponding to the spatial frequencies decided from the correspondence table shown inFIG. 11A. InFIG. 11A, spatial frequencies calculated from relations between pixel densities511to513and viewing distances521to522are stored. The vertical length of the screen is represented by V. The pixel densities511to513are calculated by dividing the vertical length V of the screen by the number of pixels in the vertical direction. For example, the pixel density511is represented by V/1080 because the number of pixels of the display apparatus30is 1920×1080 (in the horizontal and vertical directions). The viewing distances521and522are obtained by multiplying the vertical length V of the screen with 2.5 and 3.0 and are represented by 2.5V and 3V, respectively. InFIG. 11B, filter coefficients G corresponding to spatial frequencies531to533decided from the correspondence table shown inFIG. 11Aare stored.

As explained above, the filter-coefficient storing unit270is configured to store the correspondence table between the viewing conditions and the spatial frequencies shown inFIG. 11Aand the correspondence table between the spatial frequencies and the filter coefficients shown inFIG. 11B. Therefore, the filter-coefficient setting unit260acquires a filter coefficient stored in the filter-coefficient storing unit270according to viewing conditions supplied from the viewing-condition determining unit280and sets the filter coefficient in the feedback arithmetic unit240. As an example, the filter-coefficient storing unit270is configured to acquire, after specifying a spatial frequency from the viewing conditions, a filter coefficient corresponding to the spatial frequency. However, the filter-coefficient storing unit270may be configured to directly acquire a filter coefficient from the viewing conditions. A specific storage format for filter coefficients is explained with reference to subsequent drawings.

FIG. 12is a diagram of a modification of the storage format of the filter-coefficient storing unit270according to this embodiment. InFIG. 12, instead of spatial frequencies calculated from relations between pixel densities541to543and viewing distances551and552in the 40-inch display screen with an aspect ratio of 16:9, filter coefficients G corresponding to the spatial frequencies are directly stored. Items of the pixel densities541to543and the viewing distances551and552are the same as those shown inFIG. 11A. Therefore, explanation of the items is omitted.

As explained above, the filter-coefficient storing unit270may be configured to store a correspondence table shown inFIG. 12. However, in such a configuration, when there are plural viewing conditions in which spatial frequencies are the same, same filter coefficients are redundantly stored.

FIG. 13is a flowchart of a processing procedure example of an image processing method according to this embodiment. In this embodiment, first, the reproducing apparatus10performs filter coefficient setting processing on the basis of the viewing conditions supplied from the viewing-condition determining unit280(step S910). Subsequently, the reproducing apparatus10applies processing to the respective pixels in the directions from the left to the right and from the top to the bottom of the image signal (step S932). The reproducing apparatus10performs gradation modulation processing by the gradation modulator200(step S950). The reproducing apparatus10applies this processing to the pixels one by one. When the processing for the last pixel of the image signal is finished, the reproducing apparatus10finishes the processing for the image signal (step S934).

FIG. 14is a flowchart of a processing procedure example of the filter coefficient setting processing (step S910) according to this embodiment. The reproducing apparatus10establishes communication between the reproducing apparatus10and the display apparatus30via the digital transmission interface18and determines whether EDID information of the display apparatus30has been successfully acquired in the display data channel335(step S911). The reproducing apparatus10repeats the processing in step S911until the EDID information is received. On the other hand, when the EDID information has been successfully acquired, the reproducing apparatus10determines whether information indicating the vertical length of the screen has been successfully acquired (step S912). When the information has not been successfully acquired, the reproducing apparatus10establishes communication through the CEC line336and determines whether the information indicating the vertical length of the screen has been successfully acquired through the CEC line336(step S913). When the information has not been successfully acquired through the CEC line336either, the reproducing apparatus10calculates, with the viewing-condition determining unit280, a viewing distance (step S915) using a default value of the vertical length of the screen (step S914). On the other hand, when the information indicating the vertical length of the screen has been successfully acquired in step S912or S913, the reproducing apparatus10calculates, with the viewing-condition determining unit280, a viewing distance using the information indicating the vertical length of the screen.

Subsequently, the reproducing apparatus10determines whether information indicating the number of pixels in the vertical direction of the screen has been successfully acquired (step S916). When the information has not been successfully acquired, the reproducing apparatus10establishes communication through the CEC line336and determines whether the information indicating the number of pixels in the vertical direction of the screen has been successfully acquired through the CEC line336(step S917). When the information has not been successfully acquired through the CEC line336either, the reproducing apparatus10calculates, with the viewing-condition determining unit280, pixel density from a default value of the number of pixels in the vertical direction of the screen (step S918) and the information indicating the vertical length of the screen used in step S915(step S919). On the other hand, when the information indicating the number of pixels in the vertical direction of the screen has been successfully acquired in step S916or S917, the reproducing apparatus10calculates, with the viewing-condition determining unit280, pixel density from the information indicating the number of pixels in the vertical direction of the screen and the information indicating the vertical length of the screen used in step S915(step S919).

The reproducing apparatus10acquires, with the filter-coefficient setting unit260, a filter coefficient corresponding to the calculated viewing distance and pixel density among the filter coefficient stored in the filter-coefficient storing unit270(step S921). The reproducing apparatus10sets, with the filter-coefficient setting unit260, the filter coefficient acquired in this way in the feedback arithmetic unit240(step S922).

FIG. 15is a flowchart of a processing procedure example of the gradation modulation processing (step S950) according to this embodiment. The reproducing apparatus10quantizes, with the quantizing unit210, the output of the adder250(step S951) and outputs the quantized output as an output signal OUT(x,y). The reproducing apparatus10inversely quantizes, with the inverse quantization unit220, the quantized output signal OUT (x,y) (step S952).

The reproducing apparatus10calculates a quantized error Q(x,y) by calculating, with the subtracter230, a difference between a signal before the quantization by the quantizing unit210and a signal inversely quantized by the inverse quantization unit220(step S953).

The reproducing apparatus10accumulates the quantized error Q(x,y) calculated in this way and uses, with the feedback arithmetic unit240, the quantized error Q(x,y) for calculation of a feedback value (step S954). The reproducing apparatus10feeds back the feedback value calculated in this way to the adder250(step S955).

A first modification of the embodiment of the present invention is explained with reference to the drawings. In the example explained with reference toFIG. 2, the screen information concerning the viewing conditions is acquired via the digital transmission interface18. However, in an explanation explained below, the screen information is acquired via the network interface19.

FIG. 16is a diagram of a configuration example of a content provision system according to the first modification. It is assumed that a content viewing apparatus750accesses a content providing apparatus700and views contents through an external network. The content providing apparatus700includes a management server710, content servers731to734, and a communication unit741. The content viewing apparatus750includes a communication unit742and a display apparatus760.

The management server710unitarily manages the content servers731to734. The management server710acquires content data from the content servers731to734in response to a request from the content viewing apparatus750and transmits the content data to the content viewing apparatus750. Specifically, the management server710acquires screen information concerning viewing conditions from the display apparatus760and, as explained with reference toFIG. 4, sets, in the gradation modulator200, the filter coefficient selected on the basis of the viewing conditions calculated from the screen information and performs the gradation modulation processing. The management server710transmits an image signal subjected to other predetermined image processing to the content viewing apparatus750. The content servers731to734store content data and supply the stored content data to the management server710in response to a request from the management server710.

The communication units741and742perform communication between the viewing apparatus750and the content providing apparatus700via a network such as the Internet.

The display apparatus760displays the image signal transmitted from the content providing apparatus700on a display screen.

FIG. 17is a flowchart of a processing procedure example of filter coefficient setting processing according to the first modification of the embodiment. Processing except steps S961and S962is the same as that shown inFIG. 14. Therefore, explanation of the processing is omitted. In this case, since the processing procedure is a processing procedure for acquiring screen information through the network interface, the processing by the CEC line336(steps S913to S917) is excluded. The reproducing apparatus10determines whether communication has been successfully established between the reproducing apparatus10and the display apparatus760via the network interface. When the communication has been successfully established (step S961), the reproducing apparatus10proceeds to step S912.

Thereafter, in step S922, after setting the acquired filter coefficient in the feedback arithmetic unit240, the reproducing apparatus10performs the gradation modulation processing and transmits an image signal subjected to other predetermined image processing to the display apparatus760(step S962).

Consequently, the management server710can transmit the image signal, which is subjected to the gradation modulation processing on the basis of the screen information concerning the viewing conditions from the display apparatus760, to the display apparatus760connected to the network such as the Internet.

A second modification of the embodiment is explained with reference to the drawings. In the example explained with reference toFIG. 16, the screen information concerning the viewing conditions is acquired via the network interface. In an example explained below, screen information is acquired according to a manufacturing number of a display apparatus on the assumption that it is difficult to acquire the screen information.

FIG. 18is a diagram of a configuration example of a content provision system according to the second modification of the embodiment. In the content providing apparatus700of the content provision system, an apparatus-information storing unit720is added to the content providing apparatus700shown inFIG. 16. Functional components other than the management server710and the apparatus-information storing unit720are the same as those shown inFIG. 16. Therefore, explanation of the components is omitted.

The apparatus-information storing unit720stores a manufacturing number of a display apparatus and screen information concerning viewing conditions in association with each other.

When it is difficult to acquire one or both of the pieces of screen information concerning the viewing conditions from the display apparatus760, the management server710acquires a manufacturing number from the display apparatus760and acquires screen information corresponding to the manufacturing number from the apparatus-information storing unit720. Functions other than this function are the same as those of the management server710explained with reference toFIG. 16. Therefore, explanation of the functions is omitted.

FIG. 19is a diagram of a storage form example of the apparatus-information storing unit720according to the second modification of the embodiment. The apparatus-information storing unit720stores fields for a manufacturer name781, a manufacturing number782, the number of pixels783, and a screen size784. The number of pixels783and the screen size784correspond to the screen information. In the example explained above, the screen information concerning the viewing conditions is stored in association with the manufacturing number. However, viewing conditions calculated from the screen information may be stored in association with the manufacturing number.

Since the apparatus-information storing unit720is provided in this way, when it is difficult to obtain the screen information concerning the viewing conditions from the display apparatus760, the management server710acquires the manufacturing number from the display apparatus760. Therefore, the management server710can acquire screen information from the manufacturing number and perform gradation modulation processing suitable for the display apparatus760.

As explained above, according to this embodiment, when the gradation modulation processing is performed, a filter coefficient is selected on the basis of the viewing conditions, which are calculated according to the screen information from the display apparatus30that displays an image signal, and set in the feedback arithmetic unit240. This makes it possible to modulate quantization errors to a band with sufficiently low sensitivity with respect to the human visual characteristic.

Consequently, for example, even if bit widths of respective pixel values of a liquid crystal panel of a television are 8 bits, an image quality equivalent to 12 bits can be represented. Even if an input signal to the television is an 8-bit signal, bit length can be expanded to 8 bits or more by various kinds of image processing. For example, 8-bit image is expanded to 12 bits by noise reduction. When the bit widths of the respective pixel values of the liquid crystal panel are 8 bits, 12-bit data needs to be quantized to 8 bits. In this case, an image quality equivalent to 12 bits can be represented by the 8-bit liquid crystal panel by applying the present invention. The present invention can be applied in a transmission line in the same manner. For example, when a transmission line from a video apparatus to a television has 8-bit width, if a 12-bit image signal in the video apparatus is converted into 8 bits according to the present invention and transferred to the television, an image quality equivalent to 12 bits can be viewed on the television side.

The embodiment of the present invention indicates an example for embodying the present invention. The embodiment has correspondence relations with the respective elements explained above in the section of the summary of the invention. However, the present invention is not limited to this. Various modifications are possible without departing from the spirit of the present invention.

The filter-coefficient storing means corresponds to, for example, the filter-coefficient storing unit270. The viewing-condition determining means corresponds to, for example, the viewing-condition determining unit280. The filter-coefficient setting means corresponds to, for example, the filter-coefficient setting unit260. The gradation modulating means corresponds to, for example, the gradation modulator200. The quantizing means corresponds to, for example, the quantizing unit210. The filter coefficient corresponds to, for example, the filter coefficient G of the filter-coefficient storing unit270.

The number of pixels corresponds to, for example, the number of pixels432or the number of pixels783in the vertical direction. The screen size corresponds to, for example, the vertical length422or the screen size784.

The inverse quantization means corresponds to, for example, the inverse quantization unit220. The difference generating means corresponds to, for example, the subtracter230. The arithmetic means corresponds to, for example, the feedback arithmetic unit240. The adding means corresponds to, for example, the adder250.

The viewing condition determining step corresponds to, for example, steps S912to S919. The filter coefficient setting step corresponds to, for example, steps S921and S922.

The processing procedures explained in the embodiment may be grasped as a method having the series of procedures or may be grasped as a computer program for causing a computer to execute these series of procedures or a storage medium that stores the computer program.