JP4200369B2 - Image converter - Google Patents

Description

  The present invention relates to an image conversion process for converting input image data of three-element color information into output image data of four-element color information.

  When displaying an image from a host processing device such as a personal computer on an output device such as a recording device or a display device, a rendering process is performed in which a drawing object is drawn according to a drawing command output from the host processing device and a bitmap image is generated. Done. In this expansion process, the expansion process is generally performed in accordance with the resolution of the output device. However, in characters and line drawings, irregularities called jaggies are visually recognized at oblique edges at normal resolution, and visual image quality may be degraded. In order to reduce such jaggy, several methods have been proposed.

  One of them is a pulse width modulation method. The pulse width modulation method is one of high resolution methods when an electrophotographic method is used as an output device. Specifically, for halftone video data, the type and threshold value of the triangular wave to be referenced is switched and output, thereby controlling the image formation position and area in one pixel and improving the resolution in a pseudo manner. Technology. For example, it is described also in patent document 1.

  FIG. 6 is an enlarged view of an example of an image drawn by the pulse width modulation method. For example, assuming that the output resolution is doubled for both the main and sub scanning lines, one pixel is divided into four, and at which position drawing is selected according to the density, the direction of the edge, and the like, and image formation is performed. As a specific example, as shown in FIG. 6 (A), when there are diagonal lines of characters or line drawings, stair-like irregularities are easily visually recognized. Here, by using the pulse modulation method, as shown in FIG. 6B, by forming an image of one pixel or less (in this case, 1/4), the unevenness of the stepped portion is reduced visually. A smooth diagonal line is formed.

  FIG. 7 is an explanatory diagram of correspondence to white spots when the pulse width modulation method is applied. When the pulse width modulation method as described above is used, when a black character or line drawing is drawn on the background color, pixels that form an image with high resolution by being divided into four as described above are processed as black pixels. Therefore, there is a problem that a part other than the area where the image is formed out of one pixel area is white. For example, in the example shown in FIG. 7A, the lower right quarter of the center pixel is imaged in black, but the remaining 3/4 area is blanked. Even though the image quality of the character itself is improved due to the white spots, the image quality may deteriorate when viewed on the entire page.

  In order to solve such a problem, it is considered to leave the background color together with shape information indicating which position of 1/4 of one pixel is drawn. Then, for black, an image is formed in a pseudo high resolution with the pattern indicated by the shape information, and for other colors, an image is formed using the background color. As a result, as shown in FIG. 7B, it is possible to prevent white spots even for pixels formed with a pseudo high-resolution image, and a high-quality image can be formed in the printing apparatus. Such white-out prevention processing is described, for example, in Patent Document 2.

  Such high-resolution processing by the pulse modulation method is processing for switching and outputting the type of triangular wave, threshold value, and the like that are referred to during drive control of a light source such as a laser diode or a light-emitting diode as described above. . That is, since the process depends on the image forming engine, it is often performed immediately before sending image data to the image forming engine. In general, when image data is sent to an image forming engine, color information of four elements such as yellow (Y), magenta (M), cyan (C), and black (K) used for image formation is sent in frame sequential order. . For this reason, the above-described high resolution processing is generally performed on image data composed of four-element color sequential color information.

However, in order to perform the high resolution processing by the pulse modulation method as described above, it is not necessary to refer to only the K element, and the other three elements and tag information are also referred to. For example, whether or not a certain pixel is a black part refers to whether or not the other three elements other than black are zero. If you try to perform such pixel-by-pixel processing using frame-sequential image data as the processing target, each element will refer to a different plane (plane), which increases the number of accesses to the memory. There was a problem that it took a long time.
Japanese Patent No. 3266576 JP 2000-242461 A

  The present invention has been made in view of the above-described circumstances, and can perform high-speed image conversion while rapidly converting input image data of three-element color information into output image data of four-element color information. It is an object of the present invention to provide an image conversion apparatus that generates output image data.

  The present invention has color information of three elements dot-sequentially at the first resolution and has shape information representing the shape of the image portion at a second resolution higher than the first resolution for at least a predetermined image portion. In an image conversion apparatus for converting input image data into field-sequential output image data having four-element color information, the three-element color information of the input image data is converted into four-element color information to generate four-element dot-sequential information. Color space conversion means for generating intermediate image data having the color information and the shape information, and one color element of the color information after being converted by the color space conversion means in accordance with the shape information in the intermediate image data Is converted into a value corresponding to the shape information and the density value of the three color elements excluding the one color element out of the four elements is compared with a predetermined threshold value according to the 3 And a point plane conversion unit for converting the dot sequential intermediate image data corrected by the correction unit into frame sequential output image data. .

At this time, the correction process by the correction means is performed when the density value of the three color elements excluding the one color element among the four elements is equal to or greater than a predetermined threshold. If the density value of the three colors is smaller than a predetermined threshold value, the density information of the three color elements obtained from the color space conversion means is obtained. It can be configured to perform the correction as follows.

  Further, the correction process by the correction unit further includes a second value between the density value of the element of the one color in the color information converted by the color space conversion unit and the density value associated with the shape information. According to the comparison result, the density value of the element of one color can be corrected. In this case, for the second comparison result, when the density value of the element of one color of the color information after being converted by the color space conversion unit is smaller than the density value associated with the shape information, When the density value of the one color element is corrected to the density value associated with the shape information, and the density value is equal to or higher than the density value associated with the shape information, the density value of the one color element is The color space obtained by the color space conversion means can be corrected.

  According to the present invention, the correction processing for increasing the resolution is performed together with the color space conversion while maintaining the dot sequential image data, so that the correction processing can be performed at a high speed, and consequently the processing time of the entire image conversion processing. There is an effect that can be shortened.

  Further, as described with reference to FIG. 7 described above, white background can be prevented by leaving a background color during high resolution processing. However, when the background color is dark, characters and fine lines may appear thick. In order to prevent this, for example, three colors other than black K are limited by threshold values. As a result, image formation can be performed with high image quality. In addition, when a black character or line drawing is drawn on a gray background, white spots may occur at the edge of the character or line drawing. This white spot becomes noticeable when the background gray is dark. In order to prevent this, the value before and after the correction process can be selected for black K by comparing with the background gray. For example, when the background gray is dark and the drawing area of the edge portion is small, drawing with the background color has an effect of preventing apparent white spots.

  FIG. 1 is a block diagram illustrating an embodiment of the present invention, and FIG. 2 is an explanatory diagram of an example of a format of image data. In the figure, 1 is a color space conversion unit, 2 is a gray correction unit, and 3 is a point plane conversion unit. The input image data is dot-sequential image data having color information of three elements, for example, red (R), green (G), and blue (B). FIG. 2A shows a data format for one pixel of the input image data. Since the data is dot sequential, three element values are stored in one pixel of data. In the color information of elements 1 to 3 in the figure, for example, values of R, G, and B are stored, respectively.

  In addition, object information, shape information, and the like are included. The object information includes information such as what attribute the pixel is, for example, whether it is a photographic part, a graphic part, or a character part. This attribute includes an attribute in the case of having shape information. When an attribute including shape information is set as the object information, the shape information of the pixel is stored in the next shape information field. For example, in the case of using the above-described pulse modulation method, it is necessary to indicate a position where an image is formed in one pixel when an image is formed with a higher resolution (second resolution) than a normal resolution (first resolution). There is. Information for this is shape information.

  Note that the data format shown in FIG. 2A is an example, and can be modified, for example, by adding other information or by changing the arrangement order. The width of each field is also arbitrary.

  The color space conversion unit 1 receives input image data having point-sequential three-element color information as described above, and converts the three-element color information into four-element color information for each pixel of the input image data. Intermediate image data is generated. The color information of the four elements to be converted can be color information corresponding to an image forming apparatus that forms an image using the output image data, for example, color information of four elements of C, M, Y, and K. be able to. The conversion algorithm used in the color space conversion unit 1 is arbitrary. For example, the object information in the input image data can be referred to at the time of conversion, and the conversion process can be performed using parameters corresponding to the attributes of each pixel.

  The format of the intermediate image data generated by the color space conversion unit 1 can be substantially the same as the above-described input image data. An example is shown in FIG. For example, when the color space conversion unit 1 converts into four elements of YMCK, the values of Y, M, C, and K are stored in the color information of the elements 1 'to 4' in the drawing. At this time, the K value stores the density value after being converted by the color space conversion unit 1.

  The gray correction unit 2 receives the dot-sequential intermediate image data generated by the color space conversion unit 1 and, in the case where the pixel has an attribute having shape information, correction for increasing the resolution especially for black according to the shape information Process. By this correction processing, the value of the black element is changed as necessary.

  FIG. 3 is an explanatory diagram showing an example of a pattern of image forming positions in one pixel formed by the resolution enhancement process. Here, assuming that the normal resolution is doubled for both the main / sub scanning lines, one pixel is divided into four, and any one of the positions is selected according to the density to form an image. In this case, the pattern of image forming positions is the 16 patterns shown in the figure. A numerical value is shown at the bottom of each pattern, and this numerical value is a code for specifying the pattern. In addition, the schematic density of each pattern when the number of gradations is 256 gradations is shown. For example, in a pattern in which only 1/4 is formed, since the density is about 1/4, a value of 62 to 65 is assigned. The same applies to other patterns. The gray correction unit 2 sets these values as the values of the black elements in the data format shown in FIG.

  The gray correction unit 2 also performs correction processing for the elements of the other three colors (YMC) as the background, along with the correction processing for increasing the resolution of the black elements. For example, when the values of the three color elements are equal to or greater than a predetermined threshold value, the processing can be performed so as to limit the values of the three color elements to the threshold value. As a result, it is possible to eliminate the problem that the lines appear to be thicker by forming a high-density background at the edges of black characters and black thin lines.

  Furthermore, the black value obtained by the high-resolution processing and the color space conversion unit 1 were used to prevent white spots occurring at the edge when black characters or black line drawings were drawn on a gray background. For example, the larger value can be selected by comparing the black values. For example, when the black value output from the color space conversion unit 1 is larger than the processing result for higher resolution, the black value output from the color space conversion unit 1 is selected. In this case, shape information is not used. In addition, when the processing result of high resolution is larger than the black value output from the color space conversion unit 1, the processing result of high resolution may be selected.

  Note that the data format of the intermediate image data passed from the gray correction unit 2 to the next point plane conversion unit 3 is substantially the same as the format shown in FIG. 2B. Since the shape information is stored in the color information field as the element value, there is no need to provide a shape information field.

  The point plane conversion unit 3 converts the dot sequential intermediate image data after the correction processing by the gray correction unit 2 into frame sequential output image data. The data format of the output image data is shown in FIG. In this example, the data format is such that object information is added to each element of each pixel. In frame sequential image data, pixel data is arranged for each color element as indicated by broken lines in the figure. Also, in the output image data, when shape information is required, the shape information is stored in the black plane, so a field for shape information is not provided.

  Here, object information is added for each color element. However, the present invention is not limited to this, and a plane including only object information may be generated and configured with five planes together with four planes generated for each four-color element. it can.

  The output image data output from the point plane conversion unit 3 is generally input to an image forming engine or the like. The image forming engine forms an image using the output image data, but the image forming method is arbitrary. For example, a high-resolution image part needs to have a function to form an image at a higher resolution than the normal resolution, but it is not limited to the conventional pulse modulation method using a reference wave such as a triangular wave. Absent. For example, there is a case where the resolution of the image forming engine itself can be output at a higher resolution than the normal resolution in the image conversion apparatus of the present invention. In such a case, a high resolution utilizing the capability of the image forming engine. Image formation is possible.

  Next, an example of the operation in the embodiment of the present invention will be described. When dot-sequential input image data having three-element color information is input to the color space conversion unit 1, the three-element color information is converted to four-element color information, and the dot-sequential input image data having four-element color information is converted. Intermediate image data is generated. Here, as an example, it is assumed that the colors of the four elements after conversion are YMCK.

  When the gray correction unit 2 receives the intermediate image data generated by the color space conversion unit 1, the gray correction unit 2 performs the following processing for each pixel. FIG. 4 is a flowchart illustrating an example of processing for one pixel in the gray correction unit 2. First, in S11, the object information in the intermediate image data is referred to and it is determined whether or not the pixel has an attribute having shape information. If the pixel has an attribute that does not have shape information, in S12, the correction process in the gray correction unit 2 is not performed, and the process for the pixel is finished. In this case, the object information of the intermediate image data generated by the color space conversion unit 1 and dot sequential image data based on the color information of four colors are passed to the point plane conversion unit 3.

  If the pixel has an attribute having shape information, whether or not YMC other than black is 0 in S13, that is, color information YMCK = [0, 0, 0, Kx] (the value of Kx is arbitrary) Determine whether or not. A pixel whose YMC is all 0 is a pixel having no background color. The value of the black element passed from the color space conversion unit 1 is also referred to as Kx in the following description.

  If the values of YMC are all 0, the value of the black element is converted in S14. Let Ks be the value of the black element after conversion. This conversion process is optional, but here it is assumed to be a conversion process for increasing the resolution, and is shown in FIG. 3 based on the shape information in the intermediate image data received from the color space conversion unit 1. It is determined which of the patterns corresponds, and a value corresponding to the pattern is set as Ks. In this case, since YMC as the background color is 0, conversion processing is not performed on YMC.

  If it is determined in S13 that any of YMC is not 0 and a background color exists (color information YMCK = [Yx, Mx, Cx, Kx] (values of Yx, Mx, Cx, Kx are arbitrary)) In S15, the value of the black element is converted in the same manner as in S14 to obtain Ks. In S16, referring to Ks obtained in S15, it is determined whether or not the pattern forms all the pixels. For example, in the case of the pattern shown in FIG. 3, it is determined whether or not the pattern has the value 255 at the lower right. FIG. 5 is an explanatory diagram of a specific example of correspondence between a shape pattern and an image. The converted black value Ks obtained in S15 is a pattern that forms one pixel, for example, as indicated by a point P1 in FIG. 5, which is inside a black character or a black line segment. Conceivable.

  If the converted black value Ks obtained in S15 indicates a pattern that forms all one pixel, all the pixels are filled with black, so that the background color is visible even if there is a background color. There is almost no. Therefore, in S17, the YMC value is set to a predetermined fixed value [Yy, My, Cy], and the corresponding field of the intermediate image data is changed. The fixed value depends on the image forming engine that receives the output image data, but there may be a limit imposed on the total of the color information of four elements, for example. The fixed value may be determined in consideration of such various conditions. Of course, it may be 0 as a fixed value.

  If it is determined in S16 that the pattern indicated by Ks is not a pattern that forms all one pixel, this is the case of an edge portion of a black character or a black line segment as indicated by a point P2 in FIG. In the edge portion, as described in the background art, since white spots occur only in the black pattern, the image data is generated so that the background color is also formed. However, at this time, there is a problem that if the background color density is high, the black line looks thick. Therefore, in S18, it is determined whether or not the values of Y, M, and C indicating the background color are dark. As a specific example, it is determined whether or not the values [Yx, Mx, Cx] of Y, M, and C output from the color space conversion unit 1 are greater than or equal to a threshold [Yz, Mz, Cz], and the background color is The processing is divided into a case where it is dark and a case where it is light. Note that the threshold value used at this time can be set to an optimum value for high-quality printing, for example, based on the solution to the problem described in Japanese Patent Laid-Open No. 2000-232590 and its method. In addition, the determination with respect to this threshold value is that all of Y, M, and C are equal to or greater than the threshold value, and if it is close to black, the background color is dark, and if any of Y, M, and C is equal to or greater than the threshold value Determination conditions relating to the intensity of the background color, such as determining that the background color is dark, can be arbitrarily determined.

  If it is determined in S18 that the background color is dark, processing for suppressing the background color is performed in S19. As a specific example, here, the values of Y, M, and C are set to the threshold values [Yz, Mz, Cz] used in the determination of S18, and the respective fields of the intermediate image data are rewritten. As a result, the background color is suppressed by the threshold value, and it is possible to solve the problem that black characters and black line drawings appear thick due to the dark background.

  If it is determined in S18 that the background color is light, the occurrence of problems is relatively small even if the background color is used as it is, so that the values of Y, M, and C obtained from the color space conversion unit 1 in S20 [ Yx, Mx, Cx] are used as they are.

  When a value obtained by correcting the black element converted value Ks and YMC as necessary is obtained in this way, the black converted value Ks is finally used as the black element value. Determine whether or not. As described in Patent Document 1 described above, when a black character or a black line drawing is formed on a gray background, white spots may occur in the edge portion when high resolution processing is performed. In particular, such a problem is likely to occur when the background is dark gray and the area formed in one pixel is small due to the high resolution processing.

  In order to prevent such a problem, in S21, the value Ks obtained after the conversion processing for increasing the black resolution obtained in S14 or S15 and the value Kx of the black element obtained from the color space conversion unit 1 are used. Compare. The value Ks is, for example, a value indicating one of the patterns shown in FIG. 3, and the value Kx is a value indicating the density of black, and is a value having different properties. However, since the value Ks is associated according to the density corresponding to each pattern as described above, the value Ks can be used as it is as the density.

  If the value Ks is larger than the value Kx, the converted value Ks for high resolution is selected as the black element value in S22 and set in the black field of the intermediate image data. To do. If the value Kx is equal to or greater than the value Ks, the black element value Kx obtained from the color space conversion unit 1 is used as it is. In this case, since the value Kx does not represent shape information, the attribute in the object information is changed to an attribute that does not use shape information. As a result, white spots can be prevented by filling a portion where white spots occur in a dark gray background with the background color.

  In this way, the processing in the gray correction unit 2 is finished. The processing of the gray correction unit 2 as described above refers not only to black but also to other color elements (YMC). Since the plane of each color element is accessed in the field sequential image data as in the past, if object information or shape information is included, memory access is required five times or more, which is a very time-consuming process. . However, in the present invention, since the image data is dot-sequential as shown in FIG. 2B, memory access is performed once or twice even if YMCK color information, object information, shape information, etc. of one pixel are included. Just do it. Therefore, processing can be executed at high speed. Even if input image data is directly received, processing can be performed in the order in which dot-sequential image data for one pixel is received without storing each plane as in frame-sequential image data. Processing can be performed along the flow, and high-speed processing is possible. Conversely, even when processing using color elements other than black is performed as described above, processing can be performed without causing a decrease in processing speed.

  Note that the point-sequential intermediate image data passed to the point plane conversion unit 3 does not require the shape information field, and can be deleted at this point.

  The four-element dot-sequential intermediate image data output from the gray correction unit 2 is input to the point-plane conversion unit 3 so that the four-element frame-sequential output image data as shown in FIG. The information is rearranged, and output image data is generated and output by combining the information of each element and the object information.

  The generated output image data is transferred to, for example, an image forming engine, and image formation according to the output image data is performed. At this time, when an attribute having shape information is set as the object information, the value of the black plane is interpreted as the pattern shown in FIG. 3, for example, and high-resolution image formation is performed. . At this time, since the background color is also corrected by the gray correction unit 2, a high-quality image can be obtained without causing white spots in pixels in which high-resolution image formation is performed on black. . In addition, since the density of the background color is suppressed, black characters and black line drawings do not appear to be thick. Furthermore, even for black pixels in dark gray areas and defects that cause white spots at the edges of black line drawings, such pixels are corrected in advance so that image formation is performed with the background color. Can be avoided. In this way, it is possible to obtain a high-quality image as a whole.

  In the above description, RGB and YMCK are shown as specific color elements. However, it goes without saying that the present invention is not limited to this.

It is a block block diagram which shows one Embodiment of this invention. It is explanatory drawing of an example of the format of image data. It is explanatory drawing of an example of the pattern of the image formation position in 1 pixel formed by the process of high resolution. 4 is a flowchart illustrating an example of processing for one pixel in a gray correction unit 2; It is explanatory drawing of the specific example of a response | compatibility with a shape pattern and an image. It is an enlarged view of an example of the image drawn by the pulse width modulation system. It is explanatory drawing of a response | compatibility to a white spot at the time of applying a pulse width modulation system.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Color space conversion part, 2 ... Gray correction part, 3 ... Point-plane conversion part.

Claims (6)

  Input image data having color information of three elements dot-sequentially at the first resolution and shape information representing the shape of the image portion at a second resolution higher than the first resolution for at least a predetermined image portion Converting image data into plane sequential output image data having four-element color information, converting the three-element color information of the input image data into four-element color information and dot-sequential four-element color information And color space conversion means for generating intermediate image data having the shape information, and the shape of one color element of the color information after being converted by the color space conversion means in accordance with the shape information in the intermediate image data The three-color elements are converted into values corresponding to the information, and the three-color elements are compared according to a comparison result between the density values of the three-color elements excluding the one-color elements of the four elements and a predetermined threshold value. Image converter, characterized in that it comprises a correcting means for correcting the density value, the sequential intermediate image data point-plane conversion means for converting the field sequential output image data points after correction by said correcting means.   When the density value of the three color elements excluding the one color element among the four elements is equal to or higher than a predetermined threshold, the correction unit determines the density value of the three color elements. The image conversion apparatus according to claim 1, wherein correction is performed with the threshold value being a density value of the threshold value.   When the density value of the three color elements excluding the one color element among the four elements is smaller than a predetermined threshold, the correction unit determines the density value of the three color elements. The image conversion apparatus according to claim 1, wherein correction is performed using color information of the three color elements obtained from the color space conversion unit.   The correction means further includes a second comparison result between the density value of the element of one color in the color information converted by the color space conversion means and the density value associated with the shape information. 4. The image conversion apparatus according to claim 1, wherein the density value of the one color element is corrected accordingly. 5.   The correction means has a density value of the element of one color of the color information after the color space conversion means converts the second comparison result smaller than a density value associated with the shape information. 5. The image conversion apparatus according to claim 4, wherein the correction is performed so that the density value of the element of one color is a density value associated with the shape information.   The correction unit has a density value of the element of one color of the color information after the color space conversion unit converts the second comparison result equal to or higher than a density value associated with the shape information. 5. The image conversion apparatus according to claim 4, wherein in some cases, correction is performed using the density value of the element of one color as the color information obtained by the color space conversion unit.

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