JP5273869B2 - Image processing apparatus and image processing method - Google Patents

Description

  The present invention relates to an image processing apparatus and an image processing method, and more particularly to a technique for determining whether a target pixel is located on an edge.

An apparatus such as a printer that binarizes and prints an original image needs to perform a screening process on the original image before printing. Since the screening image after the screening process is discrete compared to the original image composed of the continuous tone image, it is quite difficult to process the screening image directly. Therefore, conventionally, before processing the screening image, the edge strength (and edge direction) of the screening image is first confirmed before processing.
For example, Patent Document 1 discloses an example in which an edge is detected in a screening image and a smoothing process is performed on the detected edge.

Chinese Patent Application No. 1310430

  In general, a matrix filter is used in image processing. However, in the conventional technique, it is easy and accurate to determine whether or not the pixel of interest located at the center of the filter is located on the edge. However, the problem remained.

  The present invention has been made to solve the above problem, and provides an image processing apparatus and an image processing method capable of easily and accurately determining whether or not a target pixel is located on an edge. For the purpose.

The image processing apparatus according to the present invention performs an operation to apply an N row × N column (N is an odd number of 3 or more) edge detection filter to the original image data, thereby obtaining an edge position in the original image data and An edge detection means for detecting an edge direction at the edge position; an average value of distances between an edge reference line passing through a pixel of interest located at the center of the edge detection filter and perpendicular to the edge direction; and each pixel at the edge position A distance calculating means for calculating each edge direction, and a determining means for determining whether or not the target pixel is located on an edge based on an average value of the distances calculated by the distance calculating means. wherein the determining means, by comparing the minimum value with respect to all the edge direction in the average value of the distance calculated for each of the edge direction with a predetermined threshold value, Carry out the serial judgment.

The image processing apparatus according to the present invention can easily and accurately determine whether or not a target pixel is located on an edge.

FIG. 1 is a block diagram showing an image processing apparatus 100 according to the present invention. FIG. 2 is a schematic diagram showing detection of an edge position and an edge direction by the edge detection unit 110 of the image processing apparatus 100 according to the present invention. FIG. 3 is a schematic diagram showing edge directions detected by the image processing apparatus 100 according to the present invention. FIG. 4 is a schematic diagram showing edge positions and edge directions detected by the edge detection unit 110 of the image processing apparatus 100 according to the present invention. FIG. 5 is a schematic diagram showing the edge direction of effective pixels detected by the image processing apparatus 100 according to the present invention. FIG. 6 is a schematic diagram showing the edge direction of effective pixels detected by the image processing apparatus 100 according to the present invention. FIG. 7 is a schematic diagram showing the distance between the edge reference line and each effective pixel calculated by the distance calculation unit 120 of the image processing apparatus 100 according to the present invention.

  FIG. 1 is a block diagram showing an image processing apparatus 100 according to the present invention.

  As shown in FIG. 1, the image processing apparatus 100 includes at least an edge detection unit 110, a distance calculation unit 120, a determination unit 130, and a smoothing unit 140. The document image input to the image processing apparatus 100 is input to the edge detection unit 110.

The edge detection unit 110 performs an operation of applying an N row × N column (N is an odd number greater than or equal to 3) edge detection filter using a known method, whereby the edge position and the edge of the input document image are detected. Detect direction.
FIG. 2 is a schematic diagram showing the detection of the edge position and the edge direction by the edge detection means 110. As shown in FIG. 2A, in an image in which a white (high luminance) region and a black (low luminance) region are adjacent to each other, as shown in FIG. The position where the luminance changes rapidly, that is, the edge position is detected, and the direction from the high luminance region to the low luminance region across this boundary line is detected as the edge direction. For example, as shown in FIG. 3, the edge direction is represented by each value of 0 to 15 corresponding to a direction obtained by dividing 360 ° into 16 parts. The procedure for detecting whether or not the pixel of interest located at the center of the filter is on the edge will be described below, taking the case of using a 5 × 5 filter matrix as an example.
FIG. 4 is a diagram showing edge positions and edge directions detected by the edge detection unit 110 with respect to the image of FIG. As shown in FIG. 4A, the edge position in FIG. 2A is set to 1 for the pixel corresponding to the boundary line in FIG. 2B (effective pixel) and 0 for the other pixels. It is detected as a matrix of rows × 5 columns. Further, as shown in FIG. 4B, the edge direction in FIG. 2A is detected in the direction from 5 to 8, that is, the direction from the top of the paper (right) to the bottom of the left.

  Next, as shown in FIG. 5, the edge detection unit 110 applies the effective pixel whose edge position is detected as 1 in FIG. 4A among the edge directions detected as shown in FIG. Only the corresponding edge direction is adopted as the effective edge direction.

  Next, as illustrated in FIG. 6, the edge detection unit 110 separates the effective edge direction in FIG. 5 as a separate matrix for each value. In FIG. 5, since only 6 and 8 are employed as the effective edge directions, they are separated as matrices corresponding to 6 (FIG. 6 (a)) and 8 (FIG. 6 (b)).

  Next, as shown in FIG. 7, the distance calculating unit 120 (absolute value of the distance between each effective pixel and an edge reference line (dotted line) passing through the target pixel located in the center of the filter and perpendicular to each edge direction. Of) average value. This distance is given a positive or negative sign based on the positional relationship between the effective pixel and the edge reference line. Specifically, when the effective pixel is located (left) down, that is, on the low luminance side with respect to the edge reference line, a positive sign is located (right) above, that is, on the high luminance side. In some cases, a negative sign is given. The distance (absolute value) may be determined in any way as long as the number of pixels is a unit.

  That is, in the matrix corresponding to the effective edge direction of 6 (FIG. 7A), of the 5 effective pixels, 4 effective pixels are on the edge reference line, that is, the distance is 0, and 1 effective pixel. (Right) upward 0.7 (in this case, the distance has a negative sign and is −0.7). Therefore, the average value of the distance is | 0 + 0 + 0 + 0 + (− 0.7) | /5=0.14.

  Further, in the matrix in which the effective edge direction corresponds to 8 (FIG. 7B), one effective pixel is separated by 1 in the upward direction (in this case, the distance has a negative sign, and −1 and Become). Therefore, the average value of the distance is | −1 | = 1.

  Next, the determination unit 130 determines whether or not the target pixel is on the edge by comparing the minimum value among the average values of the distances calculated for all edge directions with a predetermined threshold value, and The edge direction when the target pixel is on the edge is determined. For example, when the threshold value is 0.5, in the example of FIG. 7, the minimum value among the average values of the distance is 0.14 corresponding to the effective edge direction of 6 (FIG. 7A). Less than the threshold of 0.5. Therefore, the determination unit 130 determines that the target pixel is on the edge and determines that the edge direction is 6.

  Next, the smoothing unit 140 performs a smoothing process using a known method based on the target pixel determined to be on the edge.

  Similarly, the image processing apparatus 100 repeats the above procedure while moving the filter (that is, moving the target pixel).

  As described above, the image processing apparatus 100 according to the present invention determines whether or not the target pixel is located on the edge based on the edge reference line and the average value of the distance between each pixel at the edge position. . Therefore, the determination can be performed easily and accurately.

  Also, by adding a positive or negative sign to the distance according to the positional relationship between the pixel and the edge reference line, the pixel of interest is located not only on the edge but also on the inner side of the edge It is also possible to make a determination for the case of being located outside the edge.

DESCRIPTION OF SYMBOLS 100 Image processing apparatus 110 Edge detection means 120 Distance calculation means 130 Determination means 140 Smoothing means

Claims (4)

An edge position in the original image data and an edge direction in the edge position are detected by performing an operation for applying an N row × N column (N is an odd number of 3 or more) edge detection filter to the original image data. Edge detection means;
Distance calculating means for calculating an average value of the distance between the edge reference line passing through the target pixel located at the center of the edge detection filter and perpendicular to the edge direction and each pixel at the edge position for each edge direction;
Determination means for determining whether or not the target pixel is located on an edge based on an average value of the distance calculated by the distance calculation means ;
The image processing apparatus performs the determination by comparing a minimum value with respect to all the edge directions among a mean value of the distances calculated for each edge direction with a predetermined threshold . The image processing apparatus according to claim 1,
The distance calculation unit is an image processing apparatus that gives a positive or negative sign to the distance according to a positional relationship between the pixel and the edge reference line. The image processing apparatus according to claim 1, wherein:
Smoothing means for performing a smoothing process based on the target pixel when the target pixel is determined to be on an edge by the determination unit
An image processing apparatus further comprising: N row x N column (N is an odd number greater than 3) edge detection filter is applied to the original image data.
An edge detection step of detecting an edge position in the original image data and an edge direction at the edge position by performing an operation
A distance calculation step of calculating an average value of the distance between the edge reference line passing through the target pixel located at the center of the edge detection filter and perpendicular to the edge direction and each pixel at the edge position for each edge direction;
A determination step of determining whether or not the pixel of interest is located on an edge based on an average value of the distances calculated in the distance calculation step;
With
The image processing method includes a step of performing the determination by comparing a minimum value with respect to all the edge directions among a mean value of the distances calculated for each of the edge directions with a predetermined threshold.