JPH01160176A - Picture signal processing method - Google Patents

Abstract

PURPOSE:To attain binarization processing with high picture quality by setting background noise cut level in the vicinity of the skirt of a peak while obtaining the peak from a hitogram after subtraction with respect to background. CONSTITUTION:A histogram (p) of a picture signal (a) of an area (o) not including a picture of an original 1 is obtained by a hitogram means 24 and the area is changed ito an area (r) including the original 1 and a histogram means 26 obtains the histogram (s) of the area. Then the histogram (p) is subtracted from the historam (s) by a subtraction means 28 and the histogram after subtraction is used to detect a peak with respect to the background. Moreover, the background noise cut level (b) is set in the vicinity of the skirt of the signal level including the picture in the left/right skirts of the peak. Thus, the background noise cut level (b) is always set properly without being affected by the picture signal with respect to the area except the original in the application of blur correction after the noise is eliminated and the picture quality after binarization processing of the picture is improved.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to an image signal processing method for binarizing an image signal obtained by scanning an image with an image sensor after performing blur correction processing. It is.

(Technical Background of the Invention) An image is read by an image sensor such as a CCD line sensor or a CCD area sensor, and after the image signal is corrected for blur, it is converted to a fixed binarization level or a binarization level that floats depending on the image signal. A comparison method has already been proposed (see Japanese Patent Laid-Open No. 130068/1983).

Here, blur correction is a method of emphasizing the edges of an image by emphasizing the high frequency components of the image. ,
(hereinafter referred to as US mask) is used. However, since such high-frequency enhancement filters essentially use differentiation, they are susceptible to noise, and in particular, noise in the background area of the image signal is also emphasized at the same time, which significantly deteriorates the image quality after binarization processing. There was a problem of lowering the

Therefore, the applicant of the present application has considered slicing the image signal at a predetermined background noise cut level close to the background level to remove the noise contained in the background. However, in this case, since the density level of the background differs depending on the image, the problem is how to determine the background noise cut level. In other words, if this setting is incorrect, problems may arise in that background noise cannot be removed sufficiently or the range of variation (dynamic range) of the effective signal level of the image signal is narrowed, resulting in a reduction in image quality.

Therefore, it may be possible to determine this background noise cut level using a histogram, but in this case, if the image contains an area other than the original, such as the image of the original platen around the original, the background noise cut level may be determined using a histogram. The area will create a background mountain and another mountain in the histogram. For this reason, it is possible that the mountains in the background cannot be correctly determined, and the background noise cut level may be set to an incorrect level.

(Objective of the Invention) The present invention was made in view of the above circumstances, and it slices the image signal at a background noise cut level close to the background level to remove background noise, and then emphasizes high frequencies to remove blur. When making corrections and then binarizing the image, the background noise cut level can always be set appropriately without being affected by the image signal for areas other than the original, improving the image quality after the image is binarized. The purpose of the present invention is to provide an image signal processing method that can perform the following steps.

(Structure of the Invention) According to the present invention, this purpose is to slice an image signal obtained by scanning an image including a document using a background noise cut level close to the background level, and then remove background noise. In an image signal processing method that sharpens an image signal by performing blur correction and further binarizes it at a predetermined binarization level, an image for an area that does not include the original is determined from a histogram obtained from an image signal for an area that includes an original. The histogram obtained from the signal is subtracted, the subtracted histogram is used to detect a mountain against the background, and the background noise cut level is applied near the base on the signal level side that includes the image among the left and right bases of this mountain. This is achieved by an image signal processing method characterized by setting.

(Principle) FIG. 8 is a diagram showing the principle of the present invention. The original l shown in FIG. A portion O of the document table 2 is scanned. If the document table 2 is dark and the amount of light incident on the line sensor from there is small, the histogram p at this time will have one peak q located on the low output level side, as shown in FIG. 8B. Next, when scanning the partial molecule containing original 1, if original 1 is negative at this time, the histogram S is
As shown in FIG. 8C, it includes a mountain q' for the document table 2 and a mountain t for the background. The present invention subtracts the histogram p shown in FIG. 8B from the histogram S shown in FIG. 8C, and uses this subtracted histogram to determine the lower limit noise cut level.

(Example) Fig. 1 is a block diagram of an embodiment of the present invention, Fig. 2 is a conceptual diagram of a part of the specific example, Figs. 3A to 30 are diagrams showing signal waveforms in the processing process, and Fig. 4 5 is a diagram showing a 3×3 matrix, and FIG. 5 is a diagram showing a histogram for a negative film.

In FIG. 1, reference numeral 10 indicates a light source, and the light from this light source 10 is guided to the image sensor 20 via the condenser lens 12, film 14, projection lens 16, and mirror 18, and the projected image of the film 14 is sent to the image sensor 20. Form an image. The image sensor 20 is a CCD line sensor or CC
D formed by area sensor etc., pulse circuit (not shown)
It scans the image by being driven by pulses supplied from the oscillator and outputs a time-sequential image signal a.

If the film 14 is negative, this image signal a is
The output waveform will be as shown in the figure. In FIG. 3A-0, the horizontal axis represents time or pixel order, and the vertical axis represents voltage.

The image signal a for one scanning line for an area (O in FIG. 8A) that does not include an image of the original is temporarily stored in the semiconductor memory 22 such as a line memory, and the histogram p (the 8th
(Fig. B) is obtained by a histogram means (I) 24 constituted by a CPU (not shown). Next, the area to be scanned is changed to area r (FIG. 8A) containing the document, and a histogram S of this area is obtained by the histogram means (II) 26 of the CPU. The CPU subtracts the histogram p from the histogram S using the built-in subtraction means 28. At this time, if necessary, the level of the histogram p is changed by the attenuator 30 to αp. Find cut level b.

The histogram has the output level ■ of the image signal a on the horizontal axis and the frequency H on the vertical axis, and the histogram (S-αp) after subtraction for the negative film 14 has a peak B corresponding to the background on the low output level side. It will have the following. C
The PU determines the background noise cut level b near the base of the right side (high output level side) of the mountain B of this histogram.

For example, if the total number of pixels of this histogram b is N, then N/1
The output level with a high frequency of 00 can be determined as the background noise cut level b. The background noise cut level b can be set by various setting methods using the slope of the histogram, the distance D from the intersection C where the peak B of the histogram intersects with the height of a certain frequency (FIG. 5), and the like.

The image signal a and the background noise cut level b are input to the background noise cut circuit 34, where the noise included in the background area is removed. That is, this circuit 34
For example, as shown in FIG.
The comparator 36 compares the image signal a and the background noise cut level b. Further, the switch 38 selects the background noise cut level b when the comparator 36 judges that a<b, and selects the image signal a when a≧b.

As a result, the output C of this circuit 34 is sliced at the background noise cut level b, and the noise in the background area is cut, resulting in a signal as shown in FIG. 3B. In this way, the background noise cut level b is set to a level close to and slightly higher than the background level d (FIG. 3A).

Reference numeral 40 denotes a blur correction circuit, which emphasizes the high frequency components of the signal C using, for example, a US mask 42 as a high frequency emphasis filter, thereby emphasizing the edges of the image.

This US mask 42 functions to emphasize the central pixel, for example, when the image space is an odd matrix in which the central pixel appears. For this US mask 42, for example, when each pixel data of a 3×3 matrix is ai as shown in FIG. -(b+d+h+f) and use this E as a new image signal. In this case U
The S mask 42 is set as shown in FIG. 2, and each element of this matrix is integrated into the image data of four surrounding pixels around the center pixel, and the sum E of the integrated values is determined. The image signal e emphasized in this way is compared with a binarized level f in a comparator 44, and a binarized signal g is obtained.

In this way, the histogram of the area that does not contain the original is subtracted from the histogram of the area that contains the original.

Since the background noise level is determined from the histogram after this subtraction, it is possible to correctly identify the background peaks in the histogram without being affected by the document table, etc., and then correctly determine the background noise cut level b from the background peaks. can do.

FIG. 6 is a block diagram of another embodiment, and FIGS. 7A to 7D are output waveform diagrams of each part.

In this embodiment, the binarization level is varied using the image signal a. That is, the image sensor 20
The image signal a is stored in memory and smoothed in a smoothing circuit 50 to form a smoothed signal h (FIG. 7A), which is compressed in a compression circuit 52. This compressed signal i (
The voltage level of FIG. 7B) is further adjusted by the level shift circuit 54.
The level will be raised by k. This level-shifted signal j (FIG. 7C) is compared with the signal d, which has already undergone blur correction, in a comparator 44 to obtain a binarized level.

Note that as the smoothing circuit 50 of this embodiment, for example, a median filter can be used that uses the median of a 3×3 matrix centered on the center pixel as the image data of the center pixel.

According to this embodiment, since the binarization level j itself varies depending on the image signal a, it is suitable for highly accurate binarization processing of images with low contrast.

Although each of the above embodiments uses the US mask 42 in the blur correction circuit, the present invention is not limited to this.

Further, when using a negative film image, the background noise cut level b is set slightly higher than the background d as in the above embodiment, but on the other hand, when using a positive film, the background noise cut level b is set slightly higher than the background. Of course, it should be set as low as possible; in this case, it is set at the base of the left side (low output level side) of the histogram peak.

(Effects of the Invention) As described above, the present invention subtracts the histogram obtained based on the signal of a portion other than the document included in the image signal, such as the document table, from the histogram obtained from the region including the document, and performs this subtraction. The method calculates the peak in the background from the subsequent histogram and sets the background noise cut level near the base of this peak, so even if parts other than the original are included in the image, the background peak can be correctly identified. Additionally, the background noise cut level can always be set to an appropriate level even if the image changes. For this reason, in the binarization process after blur correction, this background noise cut level may be too close to the background and noise cannot be removed sufficiently, or conversely, it may be too far from the background level to reduce the dynamic range of the blurred image signal. It is possible to perform high-quality binarization processing without inconveniences such as sacrificing images.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a conceptual diagram of a specific example, FIGS. 3A to 30 are diagrams showing signal waveforms in the processing process, and FIG. 5 is a diagram showing a histogram, FIG. 6 is a block diagram of another embodiment, FIGS. 7A to 7D are output waveform diagrams of various parts, and FIG. 8 is a diagram explaining the principle of the present invention. 20... Image sensor, 34... Base noise cut circuit, 40... Blur correction circuit, 42... Unsharp mask, 44... Comparator, a
...image signal, b...base noise cut level, d
...Background level, f, j...Binarization level. Patent Applicant Fuji Photo Film Co., Ltd. Agent Patent Attorney Yama 1) Yu Fumi (1 other person) Figure 3A Figure 4 Figure 5 Figure 7A r Figure 7B Procedural Amendment ([1 Plow January 1988 25th Japan Patent Office Commissioner Kunio Ogawa 1. Indication of the case 1988 Patent Application No. 317361 2. Name of the invention Image signal processing method 3 - Person making the amendment Relationship with the case Patent applicant address Minamiashigara City, Kanagawa Prefecture Nakanuma 210 address name (520
) Fuji Photo Film Co., Ltd. Representative: Minoru Ohnishi 4, Agent Address: Yamato Bank Toranomon Building, 1-6-21 Nishi-Shinbashi, Minato-ku, Tokyo (Telephone: 591-7556) Spontaneous 6, Number of inventions increased by amendment 07 , Detailed Description of the Invention column of the specification to be amended and Drawing 8, Contents of the amendment (1) In the third line of page 7 of the specification, "lower limit noise" is amended to "base noise". (2) In the same book, page 7, line 19, "r time series" is corrected to "time series." (3) Correct Figure 2 as attached. (1×→

Claims (6)

[Claims] (1) After removing the background noise by slicing the image signal obtained by scanning the image including the original using a background noise cut level close to the background level, the image signal is sharpened by performing blur correction, Furthermore, in the image signal processing method of binarizing at a predetermined binarization level, the histogram obtained from the image signal for the area not containing the original is subtracted from the histogram obtained from the image signal for the area containing the original, and this subtraction is performed. An image signal processing method characterized in that a mountain against the background is detected using a subsequent histogram, and the background noise cut level is set near the base on the signal level side containing the image among the left and right bases of this mountain. . (2) The image signal processing method according to claim 1, wherein the blur correction is performed using an unsharp mask. (3) The binarization level is determined by smoothing the image signal,
Claim 1 characterized in that the smoothed signal is obtained by compressing the smoothed signal and further level-shifting the smoothed signal.
Image signal processing method described in Section 2. (4) The image signal processing method according to claim 3, wherein the smoothing is performed using a median filter. (5) The image is a projected image obtained from a negative film, and the background noise cut level is set near the foot of a high output level side of a peak of a histogram with respect to the background. 1st
Image signal processing method described in Section 2. (6) The image is a projected image obtained with a positive film, and the background noise cut level is set near the foot of a low output level side of the histogram peak with respect to the background. The image signal processing method according to item 1.