JP5675856B2 - Image processing apparatus and method, and storage medium storing program - Google Patents

Description

  The present invention provides an image processing apparatus and method capable of obtaining a good correction result that prevents a feeling of desaturation by performing appropriate saturation control when desired gradation correction is performed on an image. About.

  Conventionally, a technique has been proposed in which a main subject such as a person is detected from an image and gradation correction is performed so that the brightness of the subject is appropriate. In this case, a problem is prevention of a feeling of lower saturation after gradation correction. This will be described with reference to FIGS. 22 and 23. FIG.

  FIG. 22 is an example of pixel values included in a dark area in an image, where the horizontal axis indicates each RGB component and the vertical axis indicates the component value. Since it is a pixel in a dark region, the RGB component value is a small value as a whole.

  FIG. 23 shows an RGB component value converted into a luminance color difference space (for example, YCbCr space) with respect to such a pixel value, only the luminance component is increased by a predetermined amount in the space, and converted into the RGB space again. .

  As shown in FIG. 23, when only the luminance value is increased by a predetermined amount in the luminance component, the predetermined amount 2301 is equally added to all the components in the RGB space.

  As is well known, the state in which the saturation of the pixel value is high is a state in which the relative difference between the RGB components is large. If the relative ratio between RGB components is 1.0, that is, the same value, the pixel is a gray pixel with zero saturation. For example, in FIG. 22, the relative ratio of the G component to the R component is about 2.0 times, but the ratio after the increase in luminance is about 1.3 times, and it can be confirmed that the saturation is lowered.

  As described above, if only the luminance of the pixel value included in the dark area in the image is increased in the luminance / color difference space, the image looks poor in appearance such that the saturation is decreased.

  In order to solve the above problem, Patent Document 1 (Japanese Patent Laid-Open No. 2001-45311) proposes a method of performing saturation control so that a relative distance to a color space boundary is maintained in a luminance color difference space. As a similar example, Patent Document 2 (Japanese Patent Laid-Open No. 2002-271644) is also known. Patent Document 2 discloses various color space boundary examples, and discloses a method of relatively maintaining saturation before and after the luminance increase process by performing almost the same processing as Patent Document 1. .

JP2001-45311 JP2002-271644

  However, the above conventional example has the following problems.

  First, if the method of maintaining the relative distance to the color space boundary as described in the conventional example is uniformly applied to all hues and saturations, the correction result may be overcorrected. For example, the reproducibility of the skin color area is a very important factor in the output of a digital camera or printer. It has been experimentally confirmed that when the above method is applied to this skin color region, the saturation is enhanced too much and the human skin and face become too red, and an undesirable result is obtained.

  Further, problems when applying Patent Document 1 will be described with reference to FIG. In the figure, the vertical axis indicates the luminance, the horizontal axis indicates the saturation in a certain hue, and 2405 and 2404 indicate the saturation value and the luminance value of the target pixel value before and after the luminance correction, respectively. Reference numeral 2401 denotes a boundary of an arbitrary color space (for example, sRGB space). That is, it is assumed that the sRGB color space can exist only in the closed region surrounded by the boundary 2401 and the Y axis. In this case, as shown in the figure, if the relative distances 2403 and 2402 with the color space boundary 2401 before and after the luminance increase of the target pixel is the same, the saturation is not emphasized even if the luminance is increased, and the It will be a poor-looking image with a feeling of deterioration.

  Furthermore, as shown in FIG. 25, a case is considered where the luminance of the target pixel 2503 before correction is higher than the luminance value of the maximum saturation point 2502 in the hue of the target pixel. In this case, if the relative relationship with the color space boundary 2501 is to be maintained, the saturation of the pixel 2504 after the correction is lower than that before the correction as shown in FIG. In this case, the image looks poor with a low saturation.

  The present invention has been made in view of the above problems, and by performing appropriate saturation control, it is possible to prevent over-correction and a feeling of lowering of saturation even after performing gradation correction, and provide a good image that looks better than before. An image processing apparatus and method capable of providing

In order to achieve the above object, according to the present invention, the first correction means for correcting the luminance value of the target lattice point of the lookup table, and the hue and saturation value of the target lattice point are used. Calculating means for calculating a luminance value and a saturation value of a boundary; a determining means for determining a saturation correction line in the color space using the luminance value and the saturation value calculated by the calculating means; Second correction means for correcting the saturation value of the target grid point using the luminance value of the target grid point after correction by the correction means and the saturation correction line determined by the determination means; Generating means for generating a lookup table for correcting the luminance and saturation using the luminance value corrected by the first correction means and the saturation value corrected by the second correction means; , Brightness corrected by the first correcting means Correction amount of the saturation value by the second correction means the larger value to provide an image processing apparatus configured to decrease so.

In order to achieve the above object, according to the present invention, the first correction step of correcting the luminance value of the target grid point of the lookup table, and the hue and saturation value of the target grid point are used to determine the color. A calculation step of calculating a luminance value and a saturation value of a boundary of the space; a determination step of determining a saturation correction line in the color space using the luminance value and the saturation value calculated by the calculation step; A second correction step of correcting the saturation value of the target lattice point using the luminance value of the target lattice point after the correction in the first correction step and the saturation correction line determined by the determination step; A generation step of generating a lookup table for correcting the luminance and the saturation using the luminance value after the correction by the first correction step and the saturation value after the correction by the second correction step. And after the correction by the first correction step Correction amount of the saturation value by the second correction process larger the luminance value is to provide an image processing method configured to decrease so.

  According to the present invention, it is possible to prevent a decrease in saturation after gradation correction by newly setting an equal saturation line after gradation correction of an image and setting a saturation correction amount based on the equal saturation line. Thus, it is possible to determine an appropriate saturation correction amount according to the luminance value after gradation correction.

Flowchart of correction parameter calculation unit in the first embodiment Hardware configuration example capable of executing the image processing method of the present invention Block diagram of processing in the first embodiment Flow chart of image analysis unit Diagram for explaining luminance histogram Flow chart of exposure appropriateness determination processing Flowchart of correction parameter calculation unit Illustration of gradation correction curve for brightness correction Diagram for explaining the saturation and hue calculation method Diagram for explaining color reproduction space information Explanatory drawing of the calculation method of the color space boundary position corresponding to the target grid point Illustration of iso-saturation line setting method after brightness correction Diagram for explaining variation of iso-saturation line Flowchart of the image analysis unit in the second embodiment Flowchart of correction parameter calculation unit in the second embodiment Illustration of how to set iso-saturation lines depending on the shooting scene A diagram for explaining overcorrection of saturation Flowchart of correction parameter calculation unit in the third embodiment Block diagram of processing in the fourth embodiment Flowchart of correction parameter calculation unit in the fourth embodiment Illustration for explaining the difference in color space boundary due to the difference in color space Diagram for explaining conventional saturation correction Diagram for explaining conventional saturation correction Diagram for explaining conventional saturation correction Diagram for explaining conventional saturation correction

(First embodiment)
A preferred first embodiment of the present invention will be described below. The following description will be made assuming an image processing unit inside the printer body. However, this is merely an example of the embodiment, and the present invention is limited to the following embodiment. is not.

(Hardware description)
FIG. 2 shows an example of a hardware configuration 200 of a printer that can execute the image processing method of the present invention. 201 is a CPU, 203 is a ROM storing various control program tables, 202 is a RAM, and 204 is a secondary storage device (for example, a hard disk). Reference numeral 205 denotes a user interface including an operation unit and a display device. Reference numeral 206 denotes an external interface (for example, USB (registered trademark), memory card reader), which reads image data from the memory card 209 or is connected to the personal computer 210 via the interface. Reference numeral 207 denotes a document reading device (for example, a scanner), and 208 denotes a print engine. The above resources are connected to the system bus 211.
The CPU 201 loads a program (including a processing program described below) stored in the ROM 203 or the secondary storage device 204 into the RAM 202, which is a work memory, and executes the program. Then, the CPU 201 realizes the function of the program by controlling each of the above components via the system bus 109 according to the program.

(Process block diagram)
FIG. 3 is a block diagram of processing in the present embodiment, which is processing executed by the CPU 201. For example, when an instruction (printing instruction) such as a printing process in a printer is input from the user via the UI 205, first, in 301, a processing target is received from a memory card connected to the IF 206 or the image reading device 207 (for example, a scanner). Read the image data.

  Next, the image analysis unit 302 performs image analysis processing. The image analysis unit in this embodiment mainly determines the appropriateness of exposure of a photographic image. Various methods are conceivable for the determination process. For example, it is conceivable to extract a person's face area from the image using a known face detection technique and determine the appropriateness level based on the brightness of the face area. .

  Next, the correction parameter calculation unit 303 calculates various parameters for correcting the image based on the appropriateness determination of the exposure. Details of the parameters to be calculated will be described later.

  Next, the image correction unit 304 corrects the image using the correction parameters, and the color conversion unit 305 converts the corrected image into an ink color component (for example, CMYK). Thereafter, the pseudo gradation processing unit 306 performs pseudo gradation processing (for example, error diffusion and dither) for each ink color component, and the print engine unit 307 performs print processing on the print medium.

  The above is the outline description of the processing using the block diagram.

(Explanation of flowchart)
Hereinafter, detailed description of each processing block described above will be given with reference to a flowchart of processing in the present embodiment.

(Image reading part)
The image reading unit 301 reads processing target image data. Reading can be realized by reading image data included in a memory card connected via the IF 206, or reading data obtained by optically scanning a document by the reading device 207. The read image data is flag information for identifying whether the color space is, for example, sRGB space, adobe RGB (registered trademark of Adobe Corporation) space, or other color space. At the same time, it is sent to the next processing.

(Image Analysis Department)
FIG. 4 shows a flowchart executed by the image analysis unit 302. First, in S401, the input image is resized. For the subsequent histogram calculation and face detection processing, for example, when the input image is 4000 × 3000 pixels, if all pixels are processed, a huge amount of processing is required. In the face detection process and the histogram calculation process, the overall processing amount can be reduced by reducing the resolution to a level that captures the characteristics of the image and processing the reduced image. In the present invention, the reduction algorithm and the reduction resolution are not particularly defined, but for example, it is effective to reduce it to the VGA size (equivalent to 640 × 480 pixels).

  Next, in S402, color conversion processing is performed on the reduced image. This is because parameter design in the exposure appropriateness determination unit or the like is easier when processing is performed in a common color space in subsequent analysis processing. In the present embodiment, the common color space is set as the sRGB space, and when other color spaces are input, conversion into the sRGB space is performed in S402 using a known conversion unit.

  Next, in S403, histogram calculation of the entire image is performed. Any method may be used for calculating the histogram. Here, the sRGB value of each pixel is converted into a luminance color difference space (for example, YCbCr space), and a histogram is calculated for each component. When the input image is taken in a state of being underexposed as a whole, the luminance component histogram (with 8-bit accuracy) has a distribution biased toward the dark part as shown in FIG.

  Next, in S404, a desired subject is detected from the image. Although the subject is not particularly limited in the present invention, here, main subject detection will be described by taking a human face as an example. Various methods have been proposed so far for detecting a human face, and any of them may be used in the present embodiment.

  Next, in S405, the appropriateness of exposure is determined using the above-described histogram information and face detection information.

  FIG. 6 shows an example of simple appropriateness determination in the present embodiment. In S601, it is determined whether or not a face has been detected. If it has been detected, the average luminance value in the face area is compared with a threshold value for determining the appropriate degree of exposure in S602. If the average luminance value in the face area exceeds the threshold value as a result of the comparison, it is determined that the exposure is appropriate, and if it is below the threshold value, it is determined that the exposure is insufficient. If the face cannot be detected, in S603, the average luminance value of the entire image calculated from the histogram is compared with a predetermined threshold value.If the face value is above the threshold value, it is determined that the exposure is appropriate. Judged as insufficient.

  The above is the description of the image analysis unit 302 in the present embodiment.

(Correction parameter calculation unit)
Hereinafter, processing executed by the correction parameter calculation unit 303 will be described with reference to the flowchart of FIG.

  First, in S701 of the figure, a tone curve Tc [256] (tone correction curve) for luminance correction is generated according to the result of the appropriateness of exposure determination. FIG. 8 shows an example of tone curve generation when it is determined that exposure is insufficient in this embodiment. In FIG. 8, the horizontal axis indicates the luminance before correction, and the vertical axis indicates the luminance after correction. ObjY and TarY mean that when a face can be detected, ObjY is the average luminance in the face area, and TarY is the target luminance value of the face area that is preset. On the other hand, when a face cannot be detected, ObjY means the average luminance of the entire image, and TarY means a preset target value of the average luminance of the entire image. If it is determined that there is no underexposure, the tone curve may be a straight line such that the luminance value before correction and the luminance value after correction match. The tone curve may be generated with any luminance range and bit accuracy. However, for the sake of simplicity in this embodiment, the output luminance value for each input luminance value with 8-bit accuracy (0 to 255) is arranged in 256 arrays. Assume a one-dimensional lookup table stored in.

(Generate 3DLUT)
Next, in S702 of FIG. 7, a correction three-dimensional lookup table (hereinafter referred to as 3DLUT) is generated using the tone curve (tone correction curve).

  Here, consider a case where the resolution of the input image data is a high-resolution image such as 3000 × 4000 pixels as described above. In this case, if the luminance correction is performed for each pixel using the tone curve, and then a saturation enhancement amount (saturation correction amount) to be described later is calculated to perform the saturation correction, a huge amount of calculation is required. It becomes. If a hardware equipped with a high-speed CPU and a high-speed access RAM like a personal computer in recent years, a huge amount of arithmetic processing as described later can be realized. However, in a device embedded device (for example, a digital camera or a printer), it is impossible to process at a realistic speed.

  Therefore, in this embodiment, a correction method using 3DLUT will be described. In the 3DLUT method, the above correction calculation is performed only on pixel values at predetermined grid points, and the corrected pixel value is calculated using a known interpolation calculation for pixel values between grid points. For example, the known tetrahedral interpolation calculation has the merit that the value of one component per pixel can be calculated by performing several comparison operations and four multiplications, thereby enabling high-speed processing.

  FIG. 1 shows a flowchart of 3DLUT generation processing in the present embodiment. In this embodiment, as an example, a 3DLUT having 729 grid points in the sRGB space is generated. A 3DLUT with 729 grid points can be obtained by dividing 8-bit RGB component values (0 to 255) evenly at 32 intervals.

  First, the first grid point RGB value (R, G, B) is converted into a luminance color difference component (here, YCbCr component) in S101. The conversion may be performed by a known formula.

Next, in S102, the tone curve Tc shown in FIG. 8 is applied to the obtained luminance component Y by the following equation to obtain a corrected luminance value Y ′.
Y '= Tc [Y]

  Next, in S103, the hue and saturation of the target lattice point are calculated using the color difference component CbCr value calculated in S101. FIG. 9 is a diagram for explaining a method of calculating the hue and saturation of the target lattice point 901. In FIG. In the figure, the horizontal axis indicates the Cb value, and the vertical axis indicates the Cr value. The hue can be expressed by an angle H in the CbCr plane as shown in FIG. Further, the saturation can be expressed by the distance S from the origin on the CbCr plane as shown in FIG.

  Next, in S104, using the YCbCr value of the target grid point, the Y ′ value after luminance correction, and the color reproduction space information 109 stored in advance, the color space boundary position corresponding to the target grid point Is calculated.

  Before describing this step, the color reproduction space information 109 will be described. FIG. 10 shows the boundary of the sRGB color space in the luminance color difference space. In FIG. 10, the horizontal axis indicates the saturation value (distance from the origin) in a certain hue, and the vertical axis indicates the luminance component (0 to 255). In the figure, the inside of the rectangular area surrounded by a straight line indicates the existence range of the sRGB color space, and the boundary can be approximated by a straight line as shown in the figure. Therefore, in this embodiment, as the boundary information of the sRGB color space, the saturation value Smax [hm] and the luminance value Ymax [hm] of the maximum saturation point 1001 in an arbitrary hue hm as shown in the figure are used. It is calculated in advance and stored in the ROM 203. The hue interval when holding Smax and Ymax is not particularly limited in the present embodiment.For example, if the hue interval is held from 0 degree to 360 degrees, the hue interval is held every 1 degree. The accuracy is high and effective. For the sake of simplicity, this figure shows a two-dimensional plane only in a certain hue direction, but in reality, a similar color space boundary is formed for each hue in the YCbCr three-dimensional space.

  In S104, a color space boundary position corresponding to the target lattice point is calculated based on the color reproduction space information. Hereinafter, the color space boundary position calculation process will be described with reference to FIG. First, assuming that the hue of the target lattice point 1101 is h, the values Smax [h] and Ymax [h] of the color reproduction space information are referred to, and the color space boundary lines Lu and Ll on the luminance / saturation plane are referred to. Form. Lu and Ll are straight lines connected from the point (Smax [h], Ymax [h]) to (255,0) and (0,0), respectively. Further, a straight line 1102 having the same saturation as the target grid point 1101 is formed, and points 1103 and 1104 where the straight line intersects with Lu and Ll are the highest points (Su, Yu) of the color space boundary corresponding to the target grid point. , The lowest point (Sl, Yl).

  Next, in S105, the tone correction tone curve is applied to the calculated most significant point (boundary pixel) and least significant point (boundary pixel), and the most significant point and the least significant point after gradation correction are set. (Update the color space boundary position) and use them to set the isochromatic line after gradation correction. Here, the iso-saturation line is a line that can be considered to have the same saturation even if the luminance values are different.

The iso-saturation line setting process in the present embodiment will be described with reference to FIG. In the figure, first, the corrected luminance values Yu ′ and Yl ′ are calculated by applying the tone curve (gradation correction curve) to the luminance components of the highest point and the lowest point.
Yu '= Tc [Yu]
Yl '= Tc [Yl]

  Next, for the lowest point, since the lowest point originally existed on the color space boundary line Ll, the lowest point after luminance correction is also located on the straight line Ll (Sl ′, Yl '). Next, for the highest point, the highest point originally existed on the color space boundary line Lu, but if the highest point after luminance correction is also set to be located on the straight line Lu, the saturation will be higher than before the correction. The image may also be lowered, and there is a risk of forming an unsightly image. Therefore, in this embodiment, the highest point is set to (Su, Yu ′) in the figure so as not to cause a decrease in saturation. In this way, the color space boundary position update process is performed. The straight line Lz connecting the above points (Su, Yu ') and (Sl', Yl ') is used as an isochromatic line after luminance correction (after updating the color space boundary position), and after the gradation correction of the target lattice point The saturation is controlled so as to be located on the straight line. In other words, the isochromatic line of the pixel of interest before correction was a straight line connecting the points (Su, Yu) and (Sl, Yl), but it can be considered that it changes to a straight line Lz by gradation correction. .

Next, in S106, the saturation enhancement amount (saturation correction amount) after luminance correction of the target lattice point is calculated. The calculation process will also be described with reference to FIG. In the present embodiment, as described above, the saturation of the target lattice point is controlled on the straight line Lz. That is, the target lattice point (S, Y) before luminance correction is set to (S ′, Y ′) on the straight line Lz after luminance correction. Since the saturation enhancement coefficient e = S ′ / S is obtained from S, and S ′, the gradation-corrected values (Y ′, Cb ′, Cr ') is obtained as follows.
Y '= Tc [Y]
Cb '= Cb × e
Cr '= Cr × e

  The corrected lattice point pixel values (Y ′, Cb ′, Cr ′) obtained as a result are converted into (R ′, G ′, B ′) by a known conversion formula in S107 and stored in 3DLUT.

  By performing the above processing at all grid points (S110), it becomes possible to generate a corrected 3DLUT.

  After the 3DLUT generation, the image correction unit 304 applies the 3DLUT to each pixel value of the input image data by a known interpolation method (for example, tetrahedral interpolation, triangular prism interpolation, cube interpolation), and the corrected image data is Generate. The corrected image data is converted into an ink color component (for example, CMYK) by the color conversion unit 305, and after being subjected to pseudo gradation processing 306 (for example, error diffusion, dither), is formed as an image on the print medium by the print engine unit 307. The

  As described above, according to the image processing described in the present embodiment, the saturation is enhanced over the entire intermediate length by controlling the saturation after luminance correction as shown by the straight line Lz in FIG. Therefore, it is possible to prevent a feeling of desaturation and to generate image data with good appearance.

  The essence of the present embodiment is to reset the isochromatic line after the luminance correction. An iso-saturation line is a line that can be considered to have the same saturation even if the luminance is different. Before luminance correction (before color space boundary position update), the isochromatic line of the target grid point is a line parallel to the Y axis that connects the point (Su, Yu) and the point (Sl, Yl) in FIG. All the points that exist in this straight line have the same saturation. On the other hand, the isochromatic line after gradation correction in this embodiment is reset as a straight line connecting the lowest point (Sl ', Yl') and the highest point (Su, Yu ') after gradation correction. The saturation after correction of the target lattice point is controlled on a new isochromatic line. Further, the new isochromatic line is set not to be parallel to the Y axis but closer to the Y axis as the luminance increases. As a result, saturation control can be sufficiently enhanced for grid points with low brightness, and saturation control is not performed for grid points with high brightness, which does not decrease saturation and is less prone to overcorrection. There is a special effect that can be done.

  Therefore, the object of the present invention can be achieved as long as the new isochromatic line as described above is a line that is closer to the Y-axis as the luminance is higher and away from the Y-axis as the luminance is lower. For example, as shown in FIG. 13, the isochromatic line of the present invention is not limited to a straight line as indicated by Lz, and even if it is a curved line such as Lz1 or Lz2, it is included in the category of the present invention. It will be.

  In addition, the iso-saturation line resetting in the present invention is determined by the inclination of the color space boundary straight lines Ll and Lu. This inclination changes variously according to the hue of the target lattice point. In other words, the present invention also includes the advantage that an optimal isochromatic line can be set for each hue according to the color space boundary.

  In this embodiment, the correction method using 3DLUT has been described for speeding up, but the present invention is not limited to this. Although the overall processing amount increases, the saturation enhancement processing described in the present embodiment may be performed on all pixel values in the input image without generating 3DLUT.

  The above is the description of the first embodiment.

(Second embodiment)
In the present embodiment, a method for changing the equal saturation line described in the first embodiment in accordance with the shooting scene of the input image will be described.

FIG. 14 is a flowchart executed by the image analysis unit 302 in the present embodiment. The difference from the first embodiment is that the shooting scene is determined in S1406. S1401 to S1405 in FIG. 14 are the same as S401 to S405 in FIG. 4 of the first embodiment.
In this embodiment, any algorithm may be used for this shooting scene determination. For example, the shooting scene determination may be performed based on additional information added to the input image. A scene may be estimated. In this embodiment, for the sake of simplicity, it is assumed in S1406 whether the scene is (landscape scene, person scene, night scene, or standard scene). The determined scene type information is input to the correction parameter calculation unit 303, which generates a 3DLUT in which appropriate saturation control is performed for each shooting scene.

  FIG. 15 shows a flowchart of processing of the correction parameter calculation unit 303 in the present embodiment. The difference from the first embodiment is that the shooting scene information 1511 and the scene-specific isochromatic line information 1512 are input to S1505. S1501 to S1510 in FIG. 15 are the same as S101 to S110 in FIG. 1 of the first embodiment.

  Here, the scene-specific isochromatic line information will be described with reference to FIG. FIG. 16 is a diagram in which the isochromatic line Lz after gradation correction in FIG. 12 is rotated in the horizontal direction for the sake of simplicity. For the sake of simplicity in this embodiment, as shown in FIG. 16, when the shooting scene is standard, the straight line Lst is set as an isochromatic line. When the shooting scene is a night view, a straight line Lya with a slightly higher saturation enhancement amount is set, and when the shooting scene is a landscape, Lfu with a higher saturation enhancement amount is set. Furthermore, when the shooting scene is a person, it is conceivable to set a straight line Lji in order to suppress saturation enhancement as much as possible.

  In consideration of such an embodiment, the iso-saturation line information in S1505 may hold, for example, the deviation amount of the central portion in FIG. 16 for each scene (iso-saturation line information for each scene). Holding means). In the figure, the landscape deviation is dfu, the night scene deviation is dya, the person deviation is dji, and the standard deviation is dst, but is 0 in this case. In addition, the isochromatic line may hold each linear shape itself as a table.

  In any case, the feature of this embodiment is that the shape of the isochromatic line is changed for each scene, and it is the isochromatic line information for each scene that has enough information to form the line. become.

  Other processing is the same as that described in the first embodiment, and is omitted here.

  As described above, in the present embodiment, it is possible to perform more appropriate saturation control for each scene by changing the isochromatic line shape according to the shooting scene of the input image.

  The above is the description of the second embodiment.

(Third embodiment)
In this embodiment, a control method for preventing overcorrection for each hue of the target lattice point in the above-described embodiment will be described.

  In the first embodiment, Smax [h] and Ymax [h] as shown in FIG. 10 are held in advance as color reproduction space information for each hue in the YCbCr space, and a color space boundary is formed by straight lines. I explained that. At this time, as shown on the right side of the Y axis in FIG. 17, for example, the hue of the blue system has a high saturation Smax [h] and a low luminance Ymax [h]. Is very low. If such a first embodiment is applied as it is, the following problems occur.

  That is, for example, as shown in FIG. 17, the saturation of the lowest point (Sl ′, Yl ′) after correction is excessively emphasized with respect to the lowest point (Sl, Yl) before correction, and as a result, the straight line Lz If the saturation of the target lattice point is controlled above, overcorrection may occur.

  In view of the above problems, this embodiment describes a method for preventing overcorrection by setting an upper limit on the saturation enhancement amount for each hue when setting the second color reproduction space information.

A flowchart of the parameter calculation unit 303 in this embodiment is shown in FIG. The difference from the first embodiment is that a suppression coefficient 1811 for each hue is input to S1805. S1801 to S1810 in FIG. 18 are the same as S101 to S110 in FIG. 1 of the first embodiment.
The suppression coefficient for each hue in the present embodiment is used to suppress the amount of change in saturation at the lowest point after luminance correction. In this embodiment, the suppression coefficient for the lowest point is Rl [360], and the range of possible values is set as follows, for example.
Rl [360] = 1.0-2.5

  Rl [h] is set to a value from 1.0 to 2.5 according to the hue. In particular, the table is set so that a value close to 2.5 is near the blue hue where the slope of the straight line Ll is low, and a value close to 1.0 near the yellow hue where the slope is steep (saturation suppression coefficient holding means). .

When the hue of the target lattice point is h, the saturation enhancement amount Pl at the lowest point before and after the gradation correction can be calculated as follows.
Pl = Sl '/ Sl

When Pl is larger than Rl [h], Sl ′ is suppressed as follows.
Sl '= Sl x Rl [h]

  Using the lowest point (Sl ′, Yl ′) value suppressed as described above, an isochromatic line is set, and the chroma value of the target lattice point is determined.

  Since other processes are the same as those in the first embodiment, they are omitted here.

  As described above, according to the present embodiment, it is possible to prevent overcorrection particularly in a hue region where overcorrection is likely to occur by maintaining the saturation suppression coefficient at the lowest point for each hue. It becomes.

  It should be noted that the specific numerical values described in the above-mentioned examples are all examples for explanation, and any numerical values can be used as long as the object of the present invention can be achieved and are included in the scope of the present invention. Needless to say.

  The above is the description of the third embodiment.

(Fourth embodiment)
In the embodiments so far, the description has been made by limiting the color space to sRGB for simplicity. However, for example, when this method is incorporated in a printer printing system, the color space and output color space of the input image are not necessarily limited to sRGB, so a form applicable to other color spaces is also possible. I need to think about it. In the present embodiment, a method corresponding to the above problem is described by assuming a plurality of input color spaces, holding color reproduction space information for each color space, and switching and using them as necessary.

  FIG. 19 shows a block diagram of processing in this embodiment. The difference from the first embodiment is that a UI setting unit 1901 is added. The UI setting unit represents steps for a user to set and instruct execution of desired print processing via the UI 205 of the printer. Subsequent processing is executed according to the content of instructions from the user.

  For example, when the user instructs execution of photo printing from the memory card, the image reading unit 1902 reads desired image data from the memory card 209 connected via the IF 206. The read image data is input to the next-stage image analysis unit together with attribute information for specifying the color space of the input image.

  When the user instructs execution of copying, the image reading unit 1902 drives a scanner mounted on the printer to read a document. At this time, general printed materials (eg, magazines, catalogs) often have a wide color gamut that exceeds the sRGB color space, so an image is read as Adobe RGB data. The image data obtained as a result is input to the next-stage image analysis unit together with additional information that can identify that it is adobe RGB.

  Since the processing of the image analysis unit is the same as in the previous embodiments, detailed description thereof is omitted here.

  FIG. 20 shows a flowchart of processing of the correction parameter calculation unit 1904 in the present embodiment. The differences from the previous embodiments are that step S2001 for determining the output color space is inserted and that a plurality of color reproduction space information 2011 is held.

  In this embodiment, the output color space, that is, the 3DLUT color space calculated by the correction parameter calculation unit, is two types of sRGB and Adobe RGB for simplicity. FIG. 21 is a schematic diagram showing a color space boundary between the sRGB and adobe RGB spaces on a plane in which the vertical axis represents luminance and the horizontal axis represents saturation for a certain hue. In general, adobe RGB indicated by a thick line can cover a wider color space than sRGB. In this embodiment, in order to correspond to these two color spaces, S_Smax [h] and S_Ymax [h] are used as sRGB color space information, and A_Smax [h] and A_Ymax [h] are used as adobeRGB color space information. Calculate and hold in advance. Since the meaning of the arrangement is the same as that described in the first embodiment, a detailed description thereof is omitted here.

  In the output color space determination process of S2001, reference is made to additional information representing the color space added to the input image. If the input image is sRGB, the color reproduction space information for 2011 sRGB is selected, and the 3DLUT of sRGB is selected. Generate. On the other hand, if the input image is adobe RGB, the color reproduction space information for adobe RGB of 2011 is selected, and 3DLUT of adobe RGB is generated.

  Since the processes other than those described above are the same as those in the previous embodiments, detailed description thereof is omitted here.

  As described above, in this embodiment, the color reproduction space information in the luminance color difference space is held in advance for various color spaces assumed to be input. Furthermore, by selecting and using desired color reproduction space information according to the color space of the image data read by the reading device, adaptive processing can be performed for the color space.

  In the present embodiment, a method of switching a plurality of color space boundary tables held according to the color space of the input image has been described, but the present invention is not limited to this. For example, it goes without saying that switching according to selection of the paper type, print quality, and profile set in the UI setting unit is also included in the scope of the present invention.

  Needless to say, the color space is not limited to sRGB and adobeRGB, and can be applied to other spaces.

  Further, in the present embodiment, the output color space determination method in S2001 has been described as being determined according to the color space of the input image for simplicity, but the present invention is not limited to this. For example, the output color space may be determined according to a preset printer operation mode, paper, and print quality.

  The above is the description of the fourth embodiment.

(Other examples)
In the above embodiment, for the sake of simplicity, the luminance color difference space has been described as being limited to YCbCr, but the present invention is not limited to this. For example, even when applied to a known luminance color difference space (for example, Lab), it goes without saying that it is included in the category of the present invention.

  In this embodiment, the color reproduction space information is retained, and in this embodiment, the maximum saturation value and the luminance value at the maximum saturation value are retained for each hue of the color space, and an arbitrary saturation value is stored. Although the method of calculating the color space boundary position in the above has been described, the present invention is not limited to this.

  The color space boundary shape itself may be retained for each hue, or not at all hue angles, for example, boundary information is retained at intervals of 5 degrees, and the boundary information is obtained by interpolation between them. You can also. It goes without saying that the color reproduction space information is included in the scope of the present invention even if any other similar holding means is adopted.

  In the first embodiment, it is described that the highest point after gradation correction is set so as not to cause a decrease in saturation, but the present invention is not limited to this. For example, even if the highest point (Su, Yu ′) is set on the luminance axis side within a range where the appearance of the image after the correction processing does not deteriorate, the range of the present invention Included in the category.

  The embodiment of the present invention has been described as the incorporation process in the printer 200 for simplicity, but the present invention is not limited to this. For example, the present invention can be easily applied to an image correction unit of another image processing apparatus (for example, a digital camera or a facsimile apparatus). Further, the processing of the above-described embodiment may be executed by a PC (personal computer).

  In addition, the present invention can be applied to an apparatus (for example, a copier or a facsimile machine) composed of a single device even if it is applied to a system composed of a plurality of devices (for example, a host computer, an interface device, a reader, and a printer). You may do it.

  The object of the present invention can also be achieved by executing the following processing. That is, a storage medium that records a program code of software that realizes the functions of the above-described embodiments is supplied to a system or apparatus, and a computer (or CPU, MPU, etc.) of the system or apparatus is stored in the storage medium. This is the process of reading the code. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the program code and the storage medium storing the program code constitute the present invention.

301 ・ ・ ・ Image reading part
302 ・ ・ ・ Image analysis unit
303 ... Correction parameter calculation unit
304 ・ ・ ・ Image correction unit
305 ... Color converter
306 ・ ・ ・ Pseudo gradation processing part
307 ... Print engine

Claims (12)

A first correction means for correcting the luminance value of the target grid point of a look-up table,
A calculation means for calculating a luminance value and a saturation value of a boundary of a color space using the hue and saturation value of the target lattice point;
Determining means for determining a saturation correction line in the color space using the luminance value and the saturation value calculated by the calculating means;
Second correction means for correcting the saturation value of the grid point of interest using the luminance value of the grid point of interest after correction by the first correction unit and the saturation correction line determined by the determination unit. When,
Generation means for generating a look-up table for correcting the luminance and saturation using the luminance value corrected by the first correction means and the saturation value corrected by the second correction means; And
The image processing apparatus according to claim it small Kunar correction amount of the saturation value by the higher luminance value after compensation that by the first correction means is larger the second correction means.
The image processing apparatus according to claim 1, further comprising a third correction unit that corrects luminance and saturation of the image using the lookup table generated by the generation unit.
The image processing apparatus according to claim 2 , wherein the correction amount of the luminance value in the first correction unit is determined based on the brightness of the face included in the image.
4. The image processing apparatus according to claim 1 , wherein a correction amount of the saturation value in the second correction unit is different according to a hue of the target lattice point . 5.
5. The image processing apparatus according to claim 4 , wherein a saturation value correction amount in the second correction unit is larger in a case where the hue of the target lattice point is blue than yellow . 6.
The image processing apparatus according to claim 1, wherein the color space is an sRGB space or an adobeRGB space .
Furthermore, for each hue, the image processing apparatus according to any one of claims 1 to 6, characterized in that it has a holding means for holding a maximum saturation value of saturation point and the luminance value of the color space.
The image processing apparatus according to claim 1, wherein the saturation correction line determined by the determination unit is a line that approaches the luminance axis as the luminance increases in the color space.
First correction means for correcting the luminance value of the pixel of the image;
Calculating means for calculating a luminance value and a saturation value of a boundary of a color space using a hue and a saturation value of a pixel of the image;
Determining means for determining a saturation correction line in the color space using the luminance value and the saturation value calculated by the calculating means;
Second correction means for correcting the saturation value of the pixel of the image using the luminance value of the pixel of the image after correction by the first correction means and the saturation correction line determined by the determination means. And
The image processing apparatus according to claim 1, wherein the amount of correction of the saturation value by the second correction unit decreases as the luminance value after correction by the first correction unit increases.
A first correction step of correcting the luminance value of the target grid point of the lookup table;
A calculation step of calculating a luminance value and a saturation value of a boundary of a color space using the hue and saturation value of the target lattice point;
A determination step of determining a saturation correction line in the color space using the luminance value and the saturation value calculated by the calculation step;
A second correction step of correcting the saturation value of the target lattice point using the luminance value of the target lattice point after the correction by the first correction step and the saturation correction line determined by the determination step. When,
A generation step of generating a lookup table for correcting the luminance and the saturation using the luminance value corrected by the first correction step and the saturation value corrected by the second correction step; And
The image processing method according to claim 1, wherein as the luminance value after the correction in the first correction step is larger, the saturation value correction amount in the second correction step is smaller.
A first correction step of correcting the luminance value of the pixel of the image;
A calculation step of calculating a luminance value and a saturation value of a boundary of a color space using a hue and a saturation value of a pixel of the image;
A determination step of determining a saturation correction line in the color space using the luminance value and the saturation value calculated by the calculation step;
A second correction step of correcting the saturation value of the pixel of the image using the luminance value of the pixel of the image after the correction by the first correction step and the saturation correction line determined by the determination step. And
The image processing method according to claim 1, wherein as the luminance value after the correction in the first correction step is larger, the saturation value correction amount in the second correction step is smaller.
The program for functioning a computer as an image processing apparatus as described in any one of Claims 1 thru | or 9 .

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