JP4720088B2 - Gradation correction circuit, image display device, and image processing method - Google Patents

Description

  The present invention relates to tone correction processing of image data, and more particularly to tone correction processing such as color correction and gamma (γ) correction using a lookup table (LUT).

  In an image display apparatus that displays image data, a gradation correction process such as a gamma correction process is known as a process for adjusting the display characteristics of image data in accordance with the characteristics of a display device such as a CRT or LCD. Generally, gradation correction processing is performed using an LUT or the like that stores gradation correction characteristic (gamma characteristic) data created based on display characteristics of a display device. Gradation correction characteristics such as gamma characteristics are characteristics that define the relationship between the input gradation value and the output gradation value, and the image display device corresponds to the input gradation value of the input image data by referring to the gradation correction characteristic. The output gradation value to be acquired is acquired, and image data is displayed on the display device with the output gradation value.

  Also, when the input image data is subjected to color correction in consideration of desired color characteristics and displayed on the image display device, an LUT that stores color conversion characteristics prepared in advance is used. An example of the color correction and gamma correction methods as described above is described in Patent Document 1.

  In the conventional gradation correction circuit, gradation correction is performed by calculation on input gradation data and interpolation of reference gradation data according to the input gradation data. A method for performing gamma correction by calculation is described in Patent Document 2. When performing gradation correction by calculation, it is possible to obtain gradation characteristics constituted by a smooth curve, but there is a problem that it is difficult to change the gradation characteristics. On the other hand, when performing gradation correction by interpolation, it is easy to change the gradation characteristics, but on the other hand, in the section where the curvature of the curve portion of the gradation characteristics is large, the error from the intended gradation becomes large, There has been a problem that the quality of the displayed image is degraded. From this point of view, there has been proposed a technique for reducing errors by adding an LUT for error correction data and performing gradation conversion (see Patent Document 3), but for reference data and error correction data. Therefore, there remains a problem that the circuit scale is increased and the cost is increased.

Japanese Patent Laid-Open No. 9-271036 JP 2003-224740 A Special table 2002-534007 gazette

  The present invention has been made in view of the above points, and is capable of performing accurate gradation correction even on a portion having a high curvature of gradation characteristics without causing an increase in circuit scale. It is an object to provide a correction circuit, an image display device, and an image processing method.

  In one aspect of the present invention, the gradation correction circuit includes a gradation reference data holding unit that holds a gradation reference data group for the number of gradations smaller than the number of gradations of the input image data, and input of the input image data. A gradation reference data selection unit that selects two adjacent gradation reference data from the gradation reference data group based on the gradation value, and an interpolation that holds a plurality of interpolation reference data groups according to the input gradation value A reference data holding unit; an interpolation reference data group selection unit that selects one of the plurality of interpolation reference data groups based on an input gradation value of the input image data; and the selected interpolation reference data group A calculation unit that interpolates between the two adjacent gradation reference data using the interpolation reference data therein, and calculates and outputs an output gradation value corresponding to the input image data.

  The tone correction circuit is used for tone correction of input image data such as gamma correction and color correction. A group of gradation reference data having a smaller number of gradations than the number of gradations of the input image data is held, and two adjacent gradation reference data are selected based on the gradation value of the input image data. Also, a plurality of interpolation reference data groups corresponding to the input gradation values are held, and one of the plurality of interpolation reference data groups is selected based on the gradation values of the input image data. The interpolation reference data is data necessary for interpolating between two adjacent gradation reference data, and can include, for example, interpolation reference data for linear interpolation and interpolation reference data for nonlinear interpolation. Then, using the interpolation reference data in the selected interpolation reference data group, interpolation between the two gradation reference data is performed, and an output gradation value is output. In this way, by preparing a plurality of interpolation reference data groups, selecting an appropriate interpolation reference data group according to the gradation value of the input image data, and executing the interpolation process, it is possible to accurately perform according to the desired gradation characteristics. Gradation correction is possible.

  In one aspect of the gradation correction circuit, the interpolation reference data holding unit holds the interpolation reference data input from the outside in a rewritable manner. According to this aspect, since a plurality of interpolation reference data groups can be rewritten from the outside, accurate gradation correction suitable for desired gradation characteristics can be performed by changing to appropriate interpolation reference data as necessary. Is possible.

  In another aspect of the gradation correction circuit, the interpolation reference data group selection unit includes an input gradation value of the input image data and an interpolation reference data group to be selected corresponding to the input gradation value. And a means for changing the correspondence according to an input from the outside. According to this aspect, the correspondence between the gradation value of the input image data and the interpolation reference data group to be selected for the gradation value can be arbitrarily set and changed from the outside. Therefore, the degree of freedom in setting the gradation characteristics used for gradation correction increases.

  In another aspect of the gradation correction apparatus, the interpolation reference data group selection unit performs another predetermined operation on the interpolation reference data group corresponding to a predetermined range of input gradation values, thereby obtaining another predetermined value. Creation means for creating an interpolation reference data group corresponding to the input gradation value of the range, and when the input gradation value of the input image data is included in the other predetermined range, the created interpolation reference data group is selected Means. In this aspect, when there is a curved portion or a target portion similar to the gradation characteristics, another input gradation value is determined based on one interpolation reference data group corresponding to a certain input gradation value range. An interpolation reference data group corresponding to the range can be created. Therefore, it is possible to reduce the amount of data to be held as the interpolation reference data group.

  In one preferred embodiment, the interpolation reference data group selection unit outputs one interpolation reference data corresponding to an input gradation value of the input image data from the selected interpolation reference data group to the calculation unit, The calculation unit is configured to calculate a difference between the two adjacent gradation reference data, and multiply the difference by the one interpolation reference data supplied from the interpolation reference data group selection unit to obtain a multiplication result. Means for outputting, and means for calculating the output gradation value by adding the multiplication result to one of the two gradation reference data.

  In this example, interpolation reference data corresponding to the gradation value of the input image data is extracted from the selected interpolation reference data group. Also, a difference between two adjacent gradation reference data is calculated, and the result obtained by multiplying the difference by interpolation reference data is added to one of the two gradation reference data to calculate an output gradation value. Can do.

  In addition, an image display apparatus including the above-described gradation correction circuit and a display unit that displays the gradation-corrected image data can be configured.

  In another aspect of the present invention, there is provided a holding unit that holds a gradation reference data group corresponding to the number of gradations smaller than the number of gradations of the input image data and a plurality of interpolation reference data groups corresponding to the input gradation values. An image processing method executed by the image processing apparatus includes a gradation reference data selection step of selecting two adjacent gradation reference data from the gradation reference data group based on an input gradation value of the input image data. , An interpolation reference data selection step for selecting one of the plurality of interpolation reference data groups based on an input gradation value of the input image data, and interpolation reference data in the selected interpolation reference data group And an operation step of interpolating between the two adjacent gradation reference data and calculating and outputting an output gradation value corresponding to the input image data. Also in the above image processing method, as in the above image processing apparatus, a plurality of interpolation reference data groups are prepared, and an interpolation process is performed by selecting an appropriate interpolation reference data group according to the gradation value of the input image data. Since this is executed, accurate gradation correction can be performed according to desired gradation characteristics.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.

[Image display device]
FIG. 1 is a block diagram showing a schematic configuration of an image display apparatus to which the gradation correction circuit of the present invention is applied. As illustrated, the image display apparatus 100 includes an image processing circuit 101 and an image display unit 102. Examples of the image display device 100 include a mobile phone, a portable terminal, a PDA, and a digital camera.

  The image processing circuit 101 performs color conversion processing, gradation characteristic correction processing including gamma correction, and the like on the image data D1 input from the outside, and supplies the corrected image data D10 to the image display unit 102. Note that the image processing circuit 101 also receives a clock signal CLK synchronized with the image data D1. The image display unit 102 includes a display device such as a CRT or LCD (Liquid Crystal Display), for example, and displays the corrected image data D10.

[Image processing circuit]
FIG. 2 is a block diagram showing an internal configuration of the image processing circuit 101 shown in FIG. As illustrated, the image processing circuit 101 includes a color conversion calculation unit 10, a gradation correction unit 20, and a color reduction processing unit 30.

  The color conversion calculation unit 10 performs color conversion processing to desired color characteristics on the image data D1 input from the outside, and supplies the image data D2 after color conversion to the gradation correction unit 20. The input image data D1 is RGB 8-bit digital data, and the color conversion calculation unit 10 performs color conversion processing by 3 × 3 matrix calculation. In addition to the image data D1, a register control signal Sc is input to the color conversion calculation unit 10.

  The gradation correction unit 20 performs gradation correction such as gamma correction on the color-converted image data D2, and supplies the corrected image data D3 to the color reduction processing unit 30. The corrected image data D3 is also RGB 8-bit data for each color. Note that the register control signal Sc is input to the gradation correction unit 20.

  The color reduction processing unit 30 performs color reduction processing on the image data D3 after gamma correction. As described above, the image data D3 after gamma correction is 8-bit data for each RGB color, and the color reduction processing unit 30 converts the upper 6 bits into, for example, 6-bit data for each RGB color, and the lower 2-bit data. Based on the above, dither processing is applied to supply 6-bit RGB image data D10 (corresponding to 8 bits for each color by dither processing) to the image display apparatus 102.

  Depending on the display capability of the image display unit 102, the color reduction processing unit 30 can also supply 8-bit image data of each color to the image display unit 102 without performing the color reduction process. For example, when the image display unit 102 has an 8-bit display capability for each color, the color reduction processing unit 30 may supply 8-bit image data D10 for each color to the image display unit 102 without performing the color reduction processing. On the other hand, if the image display unit 102 has only 6-bit display capability for each color, the color reduction processing unit 30 can create 6-bit image data for each color by the color reduction processing and supply the image data to the image display unit 102. In addition to the image data D3 after the gamma correction, the color reduction processing unit 30 is supplied with a register control signal Sc, and a horizontal synchronization signal Hsync and a vertical synchronization signal Vsync synchronized with the image data D1.

[Color conversion operation part]
Next, the color conversion calculation unit 10 will be described in detail. FIG. 3A shows a configuration example of the color conversion calculation unit 10. The color conversion calculation unit 10 includes three multipliers 11 to 13, an adder 14, and a register value control unit 15, and performs a 3 × 3 matrix calculation shown in FIG. Coefficients a1 to a3, b1 to b3, and c1 to c3 to be multiplied by the multipliers 11 to 13 are determined by the register value control unit 15 based on the register control signal Sc and set to the multipliers 11 to 13, respectively. .

  Specifically, the multiplier 11 multiplies R (red) data Rin in the image data D1 by coefficients a1, b1, and c1 and outputs the result to the adder 14. Multiplier 12 multiplies G (green) data Gin in image data D1 by coefficients a2, b2, and c2, and outputs the result to adder 14. The multiplier 13 multiplies the B (blue) data Bin in the image data D1 by coefficients a3, b3, and c3 and outputs the result to the adder 14. The adder 14 adds the outputs of the multipliers 11 to 13 to generate Rout, Gout, and Bout, and outputs these as image data D2.

  The color characteristics of the output image data D2 (that is, Rout, Gout, and Bout) change according to the coefficients a1 to a3, b1 to b3, and c1 to c3 set by the register value control unit 15. If the coefficients a1, b2 and c3 are set to “1” and other count values are set to “0”, the input image data D1 and the output image data D2 have the same color characteristics. For example, when the output image data D2 is desired to have a color characteristic in which red is stronger, the coefficients a1 to a3 that affect Rout may be set larger.

[Tone correction part]
Next, a gradation correction unit to which the gradation correction circuit of the present invention is applied will be described.

(First embodiment)
First, the gradation correction unit 20a according to the first embodiment will be described with reference to FIGS. FIG. 4 shows the configuration of the gradation correction unit 20a. FIG. 4 shows only the portion corresponding to R in the RGB three-color input image data. Since the portions corresponding to G and B are also configured in the same manner, illustration and overlapping description are omitted.

  As shown in the figure, the gradation correction unit 20 a is roughly divided into a gradation reference data unit 21, an interpolation reference data unit 22, and a calculation unit 23. The gradation reference data unit 21 stores gradation reference data indicating gradation characteristics. Here, the gradation characteristic is a characteristic indicating the relationship between the input gradation value of the input image data D2 and the corresponding output gradation value, as illustrated in FIG. 5A. The interpolation reference data unit 22 stores interpolation reference data for interpolating gradation characteristics.

  However, if the gradation characteristics stored in the gradation reference data section 21 have output gradation values for all input gradation values of the input image data D2, the storage required for the gradation reference data section 21 is stored. The capacity becomes enormous. Therefore, in the present invention, the gradation characteristics stored in the gradation reference data unit 21 are set to the number of gradations smaller than the number of gradations of the input image data D2, and the insufficient amount is compensated by interpolation processing to perform gradation correction. . The gradation reference data unit 21 outputs two adjacent output gradation values close to the gradation value (referred to as “input gradation value”) of the input image data D <b> 2 to the calculation unit 23.

  As described above, the interpolation reference data unit 22 stores data for interpolation used when performing the interpolation operation of the gradation characteristics stored in the gradation reference data unit 21. The interpolation reference data unit 22 acquires interpolation reference data used for interpolation processing based on a predetermined number (4 bits in this example) of lower-order bit data of the input image data D2, and outputs the acquired interpolation reference data to the calculation unit 23.

  The calculation unit 23 performs an interpolation calculation based on the two output gradation values received from the gradation reference data unit 21 and the interpolation reference data received from the interpolation reference data unit 22, and outputs the output level after gradation correction. The tone value is output to the color reduction processing unit 30 as output image data D3.

  Next, each of the above elements will be described in detail.

  The gradation reference data unit 21 includes an SRAM 211 and a selector 212. The SRAM 211 stores gradation reference data indicating gradation characteristics. An example of the gradation characteristic is shown in FIG. As shown in the figure, the gradation characteristic 71 includes 1024 pieces of input gradation data corresponding to 10-bit input image data Rin [9: 0] and 8-bit output gradation data Rout [7: 0] corresponding thereto. ] Is shown. In FIG. 5A, the gradation characteristic 71 is continuously indicated by a solid line, but the gradation reference data does not have output gradation data for all 1024 input gradation data. , There is one output gradation data for every 16 input gradation data (corresponding to 4 bits). This is shown in FIG. 5 (b). In FIG. 5B, each gradation characteristic data 73 is on the gradation characteristic 71, but is prepared for every 16 input gradation values.

  The selector 212 receives Rin [9: 4] which is the upper 6 bits of the input image data D2 (hereinafter also referred to as Rin [9: 0]), and refers to the gradation reference data stored in the SRAM 211. The output gradation data Rout1 [7: 0] and Rout2 [7: 0] corresponding to the two adjacent input gradation data closest to the input image data Rin [9: 0] are output to the computing unit 23. The output gradation data Rout1 [7: 0] and Rout2 [7: 0] indicate both ends of the region to be interpolated on the gradation characteristic 71 in the interpolation calculation executed by the calculation unit 23.

  The interpolation reference data unit 22 includes a ROM 220 and a selector 223. The ROM 220 stores interpolation reference data 1 and 2. Data 1 is interpolation reference data for linear interpolation, and data 2 is interpolation reference data for nonlinear interpolation. In the present embodiment, two interpolation reference data of data 1 and data 2 are stored in the ROM 220 of the interpolation reference data unit 22, but other interpolation reference data may be stored. Hereinafter, a plurality of interpolation reference data may be collectively referred to as an “interpolation reference data group”. An example of the interpolation reference data is shown in FIG. The interpolation reference data is set for the difference between the two output gradation data Rout1 [7: 0] and Rout2 [7: 0] output from the gradation reference data unit 21. The interpolation reference data 1 for linear interpolation is located on a linear graph as indicated by a black dot, and the interpolation reference data 2 for nonlinear interpolation is located on a nonlinear (curve in this example) as indicated by a white point. . A numerical example of the interpolation reference data 2 for linear interpolation is shown in FIG. 6A, and a numerical example of the interpolation reference data 2 for nonlinear interpolation is shown in FIG. 6B.

  The selector 223 receives the input image data Rin [9: 0], and outputs an interpolation value Rintp [7: 0] corresponding to the input gradation value from the ROM 220 to the calculation unit 23 based on the input gradation value. Here, the selector 223 selects one of the interpolation reference data 1 and 2 based on the input gradation value of the input image data Rin [9: 0], and the interpolation value included in the selected interpolation reference data. Is output to the calculation unit 23.

  Now, it is assumed that the gradation characteristics stored in the gradation reference data unit 21 are as shown in FIGS. 5 (a) and 5 (b). FIG. 5B is an enlarged view of a region where the input gradation value of the gradation characteristic shown in FIG. In the gradation characteristic 71, when the input gradation value is small, specifically, in the region 72, the curvature of the gradation characteristic 71 is large, so that error is increased when linear interpolation is performed. Therefore, when the input gradation value belongs to the region 72, interpolation processing is performed using nonlinear (curved) interpolation reference data. On the other hand, when the input gradation value does not belong to the region 72, the curvature of the gradation characteristic is not so large, and linear interpolation may be performed. Therefore, the selector 223 selects the interpolation reference data 2 for nonlinear interpolation when the input gradation value belongs to the area 72 shown in FIG. 5A, and selects an interpolation value corresponding to the input gradation value from the selection. Output as Rintp [7: 0]. On the other hand, when the input tone value belongs to a region other than the region 72 shown in FIG. 5A, the selector 223 selects the interpolation reference data 1 for linear interpolation and corresponds to the input tone value from among them. The interpolation value is output as Rintp [7: 0].

  The calculation unit 23 includes a subtracter 231, a multiplier 232, and an adder 233. The subtractor 231 calculates a difference Dout between the output gradation data Rout1 [7: 0] and Rout2 [7: 0] received from the gradation reference data unit 21 and supplies the difference Dout to the multiplier 232. The multiplier 232 multiplies the difference Dout by the interpolation value received from the interpolation reference data unit 223, and outputs the multiplication result Rmul [15: 8] to the adder 233. The adder 233 adds the multiplication result Rmul [15: 8] to the output gradation data Rout1 [7: 0] input from the gradation reference data unit 21, and the image data Rout [7] after gradation correction. : 0] (corresponding to the output image data D3 in FIG. 2).

  As described above, the calculation unit 23 outputs the two output gradation data Rout1 [7: 0] and Rout2 corresponding to the two input gradation data close to the input gradation value of the input image data from the gradation reference data unit 21. [7: 0] is received, and the interval is interpolated according to either interpolation reference data 1 or 2 stored in the interpolation reference data unit 22. Specifically, the input image is interpolated between the two output gradation data Rout1 [7: 0] and Rout2 [7: 0] according to the data 1 or 2 indicating the interpolation characteristic illustrated in FIG. An interpolation value corresponding to the input gradation value of data is determined. The interpolation value is determined by multiplying the difference between the two output gradation data Rout1 [7: 0] and Rout2 [7: 0] by the interpolation value according to the interpolation characteristics defined by the interpolation reference data 1 or 2. Desired.

  In the present invention, interpolation reference data 1 for linear interpolation and interpolation reference data 2 for nonlinear interpolation are prepared as interpolation characteristics, and either one is selected and calculated according to the input gradation value of the input image data. Interpolation processing in the unit 23 is performed. Therefore, for a region where the predetermined gradation characteristic is linear, interpolation processing is performed using the data 1 for linear interpolation, and a region where the gradation characteristic is nonlinear (region 72 shown in FIG. 5A). Etc.) is interpolated using the interpolation reference data 2 for nonlinear interpolation. As a result, it is possible to accurately interpolate a portion having a predetermined curvature characteristic having a large curvature or a portion having a complicated curved shape.

(Second embodiment)
Next, a second embodiment of the gradation correction unit will be described with reference to FIGS. Although the second embodiment is premised on the first embodiment, the second embodiment is characterized in that the interpolation reference data stored in the interpolation reference data section 223 can be set from the outside.

  FIG. 7 shows the configuration of the gradation correction unit 20b according to the second embodiment. In FIG. 7, the configuration and operation of the gradation reference data unit 21 are the same as those in the first embodiment, and a description thereof is omitted.

  The interpolation reference data unit 22 includes a register 225 and a selector 223. The register 225 receives external interpolation reference data INTPRset [7: 0] and a write signal INTPRwr. Specifically, the interpolation reference data INTPRset [7: 0] is set to the interpolation reference data 1 or 2 at the timing when the write signal INTPRwr is input. With this configuration, it is possible to change the interpolation reference data indicating the interpolation characteristics used for the interpolation processing at an arbitrary timing.

  Further, in this embodiment, one interpolation reference data can be used for interpolation processing for a plurality of areas in the gradation characteristics. FIG. 9A shows another example of gradation characteristics. In this example, the gradation characteristic 81 has a substantially S shape. FIG. 9B shows an enlarged view of the vicinity of the region 82 in FIG. Now, it is assumed that, in the gradation characteristic 81, the region 82 is interpolated using the interpolation reference data 2 for nonlinear interpolation, and the other regions are interpolated using the interpolation reference data 1 for linear interpolation. In this case, basically, the interpolation reference data 2 corresponding to the entire area 82 may be prepared and stored in the register 225 in the interpolation reference data unit 22. However, as shown in the example of FIG. When the gradation characteristic 81 of the second pixel is point-symmetric, it is not necessary to store the interpolation reference data for the entire region 82, store the interpolation reference data for the half of the region 82, and obtain the other half by calculation. be able to.

  Now, as shown in FIG. 9B, it is assumed that a region 82 to be subjected to nonlinear interpolation processing is point-symmetric at the center, and the region 82 is divided into two regions 82a and 82b at the center. The parts of the gradation characteristic 81 are 81a and 81b. In the interpolation reference data unit 22, interpolation reference data according to the gradation characteristic 81b shown in FIG. On the other hand, the gradation characteristic 81a in the region 82a is obtained by subtracting the gradation characteristic 81b from “1” to generate a characteristic 81b ′ shown in FIG. It can be obtained by reading from 225.

  A configuration example of the interpolation reference data unit 22 for realizing this is shown in FIG. 8, the interpolation reference data unit 22 includes a register 225 and selectors 223a to 223c. The selector 223a first determines whether Rin [9: 4], which is the upper 6 bits of the input image data Rin [9: 0], is “31”, “32”, or otherwise. If the upper 6 bits Rin [9: 4] = 31 of the input image data, the input gradation value belongs to the area 82a, and if Rin [9: 4] = 32, the input gradation value belongs to the area 82b. Otherwise, the input tone value belongs outside the area 82. Therefore, the selector 223a selects data 2 (dat2 [127: 0]) when the upper 6 bits Rin [9: 4] of the input image data is “31” or “32”, and otherwise. Selects data 1 (dat1 [127: 0]) and supplies it to the selector 223b as the selection result dat_sel [127: 0].

  When the input image data Rin [9: 4] is “31”, the selector 223 c validates the subtraction instruction signal subintp (for example, sets it to “1”), and outputs it to the calculation unit 23. At the same time, the selector 223c inverts the bit order of Rin [3: 0], which is the lower 4 bits of the input image data, and sets it to Rin_sel [3: 0]. That is, Rin_sel [3: 0] = Rin [0: 3].

  On the other hand, when the input image data Rin [9: 4] is not “31”, the selector 223c invalidates the subtraction instruction signal subintp (for example, sets it to “0”) and outputs it to the calculation unit 23. At the same time, the selector 223c sets the bit order of Rin [3: 0], which is the lower 4 bits of the input image data, to Rin_sel [3: 0] as it is (without inversion). That is, Rin_sel [3: 0] = Rin [3: 0].

  The selector 223b outputs the selection result dat_sel [127: 0] from the selector 223a as the selection result Rintp [7: 0] based on the input image data Rin [9: 0] and Rin_sel [3: 0]. Specifically, when the input gradation value of the input image data is outside the region 82, the selection result dat_sel [127: 0] = dat1 [127: 0] from the selector 223a is obtained, and therefore dat1 [127: 0 ], The interpolation value corresponding to the input gradation value is read out and output as Rintp [7: 0].

  When the input gradation value of the input image data belongs to the region 82b, the selection result dat_sel [127: 0] = dat2 [127: 0] from the selector 223a and Rin_sel [3: 0] from the selector 223c. Since Rin [3: 0], an interpolation value corresponding to the input gradation value is searched from dat2 [127: 0] in the forward order, read out, and output as Rintp [7: 0]. Thereby, an interpolation value according to the interpolation characteristic 81b shown in FIG. 10B is obtained.

  On the other hand, when the input gradation value of the input image data belongs to the area 82a (that is, when the upper 6 bits Rin [9: 4] of the input gradation value is “31”), the selection result dat_sel [ Since 127: 0] = dat2 [127: 0] and Rin_sel [3: 0] = Rin [0: 3] from the selector 223c, the input grayscale values in reverse order from dat2 [127: 0] The interpolated value corresponding to is retrieved, read out, and output as Rintp [7: 0]. Thereby, an interpolation value according to the interpolation characteristic 81a shown in FIG.

  In the second embodiment, the calculation unit 23 includes a subtracter 234 as shown in FIG. The subtractor 234 receives the interpolation value Rintp [7: 0] and the subtraction instruction signal subintp output from the selector 223b. When the subtraction instruction signal subintp is invalid, the interpolation value Rintp [7: 0] is supplied as it is to the multiplier 232 as the interpolation value Rintp2 [7: 0]. On the other hand, when the subtraction instruction signal subintp is valid, the interpolation value Rintp [7: 0] is subtracted from “1” to generate the interpolation value Rintp2 [7: 0], which is supplied to the multiplier 232.

  As described above, in FIG. 9B, when the input gradation value of the input image data is outside the region 82, interpolation is performed using the interpolation reference data 1 for linear interpolation as the interpolation characteristic. When the input gradation value belongs to the region 82b, interpolation is performed using the interpolation reference data 2 for nonlinear interpolation (see FIG. 10B, corresponding to the characteristic 81b) as an interpolation characteristic as it is. On the other hand, when the input tone value belongs to the region 82a, interpolation is performed using the interpolation reference data for nonlinear interpolation (see FIG. 10A, corresponding to the characteristic 81a) as the interpolation characteristic. Accordingly, the interpolation calculation can be performed on the entire regions 82a and 82b by using one interpolation reference data 2 corresponding to the region 82b, and the storage capacity of the interpolation reference data can be reduced.

(Third embodiment)
Next, a third embodiment of the gradation correction unit will be described with reference to FIGS. The third embodiment is based on the first embodiment, but in the same way as the second embodiment, the interpolation reference data in the interpolation reference data section 22 can be changed from the outside, and the input image data This is characterized in that the correspondence between the input gradation value and the interpolation reference data can be set and changed from the outside.

  FIG. 11 shows the configuration of the gradation correction unit 20c according to the third embodiment. The configurations of the gradation reference data unit 21 and the calculation unit 23 are the same as those in the first embodiment. The interpolation reference data unit 22 includes a register 225, a selector 223, and a region setting register 224. The register 225 is the same as that of the second embodiment, and holds the interpolation reference data INTPRset [7: 0] inputted from the outside in the register 225 at the same timing of INTPRwr inputted from the outside.

The area setting register 224 is a register that sets area specifying data SELRset indicating an area of an input gradation value to be subjected to interpolation calculation using the interpolation reference data 2. FIG. 12A shows an example of the gradation characteristic 91 in the third embodiment, and FIG. 12B shows an example of the region designation data SELRset . In this example, a region to which the interpolation reference data 2 corresponding to the region 92 to be subjected to nonlinear interpolation is indicated by region designation data SELRset .

Specifically, the region setting register 224 holds region designation data SELRset [7: 0] indicating a region to which the interpolation reference data 2 is to be applied at the timing of SELRwr input from the outside. The area designation data SELRset is sent from the setting register 224 to the selector 223.

The selector 223 extracts an interpolation value corresponding to the input gradation value from the interpolation reference data 2 when the input gradation value of the input image data Rin [9: 0] belongs to the region of the region designation data SELRset , and Rintp [ 7: 0] is output to the calculation unit 23. Otherwise, the interpolation value corresponding to the input gradation value is extracted from the interpolation reference data 1 and output to the calculation unit 23 as Rintp [7: 0]. Interpolation calculation in the calculation unit 23 is the same as that in the first embodiment, and a description thereof will be omitted.

  As described above, in the third embodiment, since the region where nonlinear interpolation should be performed can be set from the outside, the degree of freedom in setting and changing the gradation characteristics is increased. Therefore, gradation correction can be performed using more appropriate gradation characteristics.

  In the above example, the region designation data designates a region to which the interpolation reference data 2 for nonlinear interpolation is to be applied. However, the present invention is not limited to this, and a region to which the interpolation reference data 1 for linear interpolation is to be applied. May be specified. Further, the number of such areas is not limited to one, and it is configured such that areas to which the interpolation reference data are prepared are designated by area designation data for each of a plurality of interpolation reference data prepared in advance, regardless of whether they are linear or non-linear. It doesn't matter.

[Color reduction processing section]
Next, the color reduction processing unit will be described in detail. As shown in FIG. 13, the color reduction processing unit 30 outputs 8-bit image data D3 of each RGB color output from the gradation correction unit 20, that is, Rout, Gout, and Bout to 6-bit image data of each color by bit slice and dither processing. And output as image data D10. FIG. 13 shows a configuration example of the color reduction processing unit 30. FIG. 13 shows only the portion corresponding to the R data among the three RGB colors, but the G data and the B data have the same configuration.

  In FIG. 13, the color reduction processing unit 30 includes 2-bit counters 31 and 32, a dither matrix circuit 33, an adder 34, a switcher 35, and a register value control unit 36. In the dither matrix circuit 33, a known 4 × 4 dither matrix is used.

  The counter 31 outputs a 2-bit X address Xad to the dither matrix circuit 33 by counting the clock signal CLK synchronized with the image data D3. The counter 31 is reset by the horizontal synchronization signal Hsync. The counter 32 counts the horizontal synchronization signal Hsync, thereby outputting a 2-bit Y address Yad to the dither matrix circuit 33. The counter 32 is reset by the vertical synchronization signal Ysync.

  The dither matrix circuit 33 supplies a value defined in the dither matrix to the adder 34 as R (D_out) based on the input X address Xad and Y address Yad. The adder 34 adds the R data R (lut_out) output from the gradation correction unit 20 and the upper 2 bits of the value R (D_out) output from the dither matrix circuit 33, and the upper 6 bits of the result. Is output to the input terminal b of the switcher 35 as R (ADD_out). In this way, the 8-bit image data D3 for each color of RGB supplied from the gradation correction unit 20 is reduced to 6-bit image data for each color. Since dither processing is applied, 6-bit image data for each color has color characteristics equivalent to 8 bits for each color.

  The output of the switcher 35 is switched according to the register value output from the register value control unit 36 based on the register control signal Sc. When the input terminal a of the switcher 35 is selected, 8-bit image data for each color of RGB that is not subjected to color reduction processing is output as image data D10. When the input terminal b of the switcher 35 is selected, 6-bit image data of each RGB color obtained by the color reduction process is output as the image data D10.

[Modification]
As described above, the example of the image display device to which the color conversion circuit of the present invention is applied has been described mainly using the display device using liquid crystal as an example, but the present invention is not limited to this, and a plasma display (PDP), It can also be applied to organic EL display devices, field emission displays (FED), etc.

1 is a block diagram of an image display device to which a gradation correction circuit of the present invention is applied. FIG. 2 is a block diagram showing an internal configuration of the image processing circuit shown in FIG. 1. It is a block diagram which shows the structure of a color conversion circuit. It is a block diagram which shows the structure of the gradation correction | amendment part which concerns on 1st Example. 2 shows an example of gradation characteristics and interpolation reference data in the first embodiment. A numerical example of interpolation reference data is shown. It is a block diagram which shows the structure of the gradation correction part which concerns on 2nd Example. It is a block diagram which shows the structure of the interpolation reference | standard data part in FIG. The example of the gradation characteristic in 2nd Example is shown. An example of gradation characteristics of a region where nonlinear interpolation is performed will be shown. It is a block diagram which shows the structure of the gradation correction part which concerns on 3rd Example. The example of the gradation characteristic in 3rd Example is shown. FIG. 2 is a block diagram of a color reduction processing unit shown in FIG. 1.

Explanation of symbols

  10 color conversion operation unit, 20 gradation correction unit, 21 gradation reference data unit, 22 interpolation reference data unit, 23 calculation unit, 30 color reduction processing unit, 100 image display device, 101 image processing circuit, 102 image display unit, 211 SRAM, 212 selector, 220 ROM, 221, 222 interpolation reference data, 223 selector, 224 area setting register, 231 subtractor, 232 multiplier, 233 adder

Claims (6)

A gradation reference data holding unit that holds a gradation reference data group for the number of gradations smaller than the number of gradations of the input image data;
A gradation reference data selection unit that selects two adjacent gradation reference data from the gradation reference data group based on an input gradation value of the input image data;
An interpolation reference data holding unit for holding a plurality of interpolation reference data groups according to the input gradation value;
An area designation data holding unit for holding area designation data indicating an area of an input gradation value to be subjected to interpolation calculation using a predetermined interpolation reference data group among the plurality of interpolation reference data groups;
An interpolation reference data selection unit that selects interpolation reference data corresponding to the input gradation value from the predetermined interpolation reference data group when the input gradation value of the input image data belongs to the region indicated by the region designation data When,
An arithmetic unit that interpolates between the two adjacent gradation reference data using the interpolation reference data in the selected interpolation reference data group, and calculates and outputs an output gradation value corresponding to the input image data; A gradation correction circuit comprising:   The gradation correction circuit according to claim 1, wherein the plurality of interpolation reference data groups include an interpolation reference data group for linear interpolation and an interpolation reference data group for nonlinear interpolation.   The gradation correction circuit according to claim 1, wherein the interpolation reference data holding unit holds the interpolation reference data input from the outside in a rewritable manner. The interpolation reference data selection unit is
Creating means for creating an interpolation reference data group corresponding to an input gradation value in another predetermined range by performing a predetermined calculation on the interpolation reference data group corresponding to an input gradation value in a predetermined range;
The gradation correction according to claim 1, further comprising means for selecting the created interpolation reference data group when an input gradation value of the input image data is included in the other predetermined range. circuit. The interpolation reference data selection unit outputs one interpolation reference data corresponding to the input gradation value of the input image data from the selected interpolation reference data group to the calculation unit,
The computing unit is
Means for calculating a difference between the two adjacent gradation reference data;
Means for multiplying the difference by the one interpolation reference data supplied from the interpolation reference data selection unit and outputting a multiplication result;
The gradation correction circuit according to claim 1, further comprising means for adding the multiplication result to one of the two gradation reference data to calculate the output gradation value. An image executed by an image processing apparatus including a gradation reference data group for the number of gradations smaller than the number of gradations of the input image data and a holding unit that holds a plurality of interpolation reference data groups corresponding to the input gradation value A processing method,
A gradation reference data selection step of selecting two adjacent gradation reference data from the gradation reference data group based on the input gradation value of the input image data;
Based on area designation data indicating an area of an input gradation value to be subjected to an interpolation operation using a predetermined interpolation reference data group among the plurality of interpolation reference data groups, the input gradation value of the input image data is When belonging to the region indicated by the region designation data, an interpolation reference data selection step of selecting interpolation reference data corresponding to the input gradation value from the predetermined interpolation reference data group;
A calculation step of interpolating between the two adjacent gradation reference data using the interpolation reference data in the selected interpolation reference data group, and calculating and outputting an output gradation value corresponding to the input image data; An image processing method comprising:

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