JPH0620238B2 - Image signal processing method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image signal processing method for binarizing an image signal obtained by scanning a film projection image with an image sensor and then binarizing the image signal. It is a thing.

(Technical background of the invention) A film projection image is read by an image sensor such as a CCD line sensor or a CCD area sensor, and after this image signal is defocused, it is binarized at a constant binarization level or floating according to the image signal. A method of comparing with the level has already been proposed (see JP-A-62-130068).

The blur correction here emphasizes the edges of the image by emphasizing the high frequency components of the image, and a high-frequency emphasis filter such as an unsharp mask (Unsharp Mask, which uses a first derivative or Laplacian (second derivative)). (Hereinafter referred to as US mask) is used. However, since such a high-frequency emphasis filter essentially uses differentiation, it is vulnerable to noise, and especially noise in the background region of the image signal is also emphasized, which significantly improves the image quality after binarization processing. There was a problem of lowering it. This problem becomes particularly important when reading a projected image of a film on which dust is likely to be dusted or scratched.

Therefore, it has been considered by the applicant of the present application that the image signal is sliced at a predetermined background noise cut level close to the background level to remove noise included in the background. However, in this case, since the background density level differs depending on the image, how to determine the background noise cut level becomes a problem. That is, if this setting is wrong, background noise cannot be sufficiently removed, or the fluctuation range (dynamic range) of the effective signal level of the image signal is narrowed, resulting in deterioration of image quality.

Therefore, it is conceivable to determine this background noise cut level by using a histogram. In this case, if the image of the platen is included in a region other than the original, such as the periphery of the original, in the film projection image, the image of the original The region will create a different mountain in the histogram than the background mountain.
For this reason, it is possible that the background mountain cannot be correctly identified and the background noise cut level is set to an incorrect level.

(Object of the Invention) The present invention has been made in view of such circumstances, and while reading the projected image of the film to obtain the image signal, the image signal is sliced by the background noise cut level close to the background level and backed. When noise in the ground is removed, blurring is corrected by high-frequency emphasis, and then binarization processing is performed, the background noise cut level can always be set appropriately without being affected by the image signal for areas other than the original. An object of the present invention is to provide an image signal processing method capable of improving the image quality after binarizing a projected image.

According to the present invention, this object is to remove the background noise by slicing the image signal obtained by scanning the projected image of the film by the background noise cut level close to the background level, In an image signal processing method for sharpening an image signal by performing blur correction and further binarizing the image signal with a predetermined binarization level, a projection that does not include the original in a histogram obtained from the image signal for a projection image including the original. The background peak is detected in the histogram excluding the output range of the peak of the histogram obtained from the image signal for the image, and the background noise cut level is found near the skirt on the signal level side including the image in the left and right skirts of this mountain. Is achieved by an image signal processing method.

(Principle) FIG. 8 is a view showing the principle of the present invention. FIG. 8A shows a projected image of a film on which an image of an original 1 placed on an original table 2 wider than this is projected, and this projected image is a line. First, the sensor scans an area not including the original 1, that is, a portion o of the projected image of the original 2 on the outer periphery of the original 1. If the projected image on the document table 2 is dark and the amount of light incident on the line sensor from this is small, the histogram at this time becomes one peak p as shown in FIG. 8B. Next, when the projected image portion q including the original 1 is scanned, the histogram r at this time includes the peak p ′ for the projected image of the original table 2 and the peak s for the background as shown in FIG. 8C. According to the present invention, the output level in the distribution range t of the mountain p in FIG. 8B is excluded from the histogram r in FIG. 8C, and the background noise cut level is obtained using the histogram only in the range u.

(Embodiment) FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a conceptual diagram of a part of a concrete example thereof, and FIGS. 3A to 3C are diagrams showing signal waveforms during processing, and FIG. Shows a 3 × 3 matrix, and FIG. 5 shows a histogram for a negative film.

In FIG. 1, reference numeral 10 is a light source. Light from the light source 10 is guided to an image sensor 20 via a condenser lens 12, a film 14, a projection lens 16 and a mirror 18, and a projected image of the film 14 is directed to the image sensor 20. Form an image. The image sensor 20 is a CCD line sensor or C
It is formed of a CD area sensor or the like and is driven by a pulse supplied from a pulse circuit (not shown) to scan an image and output a time-series image signal a.

This image signal a is the third A when the film 14 is negative.
The output waveform is as shown. 3A to 3C, 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 the image of the original is temporarily stored in the semiconductor memory 22 such as a line memory, and the histogram p (8th
(Fig. B) is obtained by the histogram means 24 constituted by a CPU (not shown), and the mountain area t is obtained. The area to be scanned next is the area q including the original (FIG. 8A)
Instead, the histogram means 26 of the CPU obtains the histogram r of this area. The CPU removes this range t from this histogram r and uses only the histogram in the remaining range u to obtain the background noise cut level b from this histogram by the background noise cut level detection means 30 of this CPU itself.

In the histogram, the horizontal axis represents the output level V of the image signal a and the vertical axis represents the frequency H. For the negative film 14, the peak B corresponding to the background appears on the low output level side. The CPU obtains the background noise cut level b near the skirt on the right side (high output level side) of the mountain B in this histogram. For example, assuming that the total number of pixels of this histogram b is N, the output level having a high frequency of N / 100 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 inclination of the histogram, the distance D (FIG. 5) from the intersection C at which the mountain B of the histogram intersects the height of the fixed characteristic, and the like. .

The image signal a and the background noise cut level b are input to the background noise cut circuit 32, where noise included in the background region is removed. That is, this circuit 32
Is a comparator 34 and a switch 36 as shown in FIG.
The comparator 34 compares the image signal a with the background noise cut level b. The switch 36 selects the background noise cut level b when the comparator 34 judges that a ≦ b, and selects the image signal a when a> b.
As a result, the output c of this circuit 32 is sliced at the background noise cut level b, and the noise in the background region is cut, as shown in FIG. 3B. In this way, the background noise cut level b is set close to the background level d (FIG. 3A) and slightly higher than this.

A blur correction circuit 38 emphasizes the high-frequency component of the signal c by using a US mask 40 as a high-frequency emphasis filter to emphasize the edge of the image.

This US mask 40 functions to emphasize the center pixel when the image space is an odd matrix in which the center pixel appears, for example. As the US mask 40, for example, when each pixel data of a 3 × 3 matrix is set to a to i as shown in FIG. 4, data e for a central pixel is obtained by using data of four surrounding pixels E = 5e It is converted into − (b + d + h + f) and this E is used as a new image signal. In this case U
The S mask 40 is set as shown in FIG. 2, and each element of this matrix is integrated with the image data of four surrounding pixels centering on the central pixel, and the sum E of the integrated values is obtained. The image signal e thus emphasized is compared with the N-valued level f in the comparator 42 to obtain the binarized signal g.

In this way, the range of the peaks of the histogram that does not include the original is excluded from the range of the histogram of the area that includes the original, and the background noise level is determined from the remaining histograms. The inside background mountain can be correctly identified, and then the background noise cut level b can be correctly determined from the background mountain.

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

In this embodiment, the binarization level is changed by using the image signal a. That is, the image sensor 20
The image signal a is stored in the memory and smoothed by the smoothing circuit 50 into a smoothed signal h (FIG. 7A). The smoothed signal h is compressed by the compression circuit 52. The voltage level of the compressed signal i (FIG. 7B) is further raised by k in the level shift circuit 54. This level-shifted signal j
The comparator 24 (FIG. 7C) is compared with the signal d for which the blur correction has already been completed, and the binarized signal 1 is obtained.

As the smoothing circuit 50 of this embodiment, it is possible to use, for example, a median filter that adopts an intermediate value (median) of a 3 × 3 matrix centered on the center pixel as image data of the center pixel.

According to this embodiment, since the binarization level j itself changes depending on the image signal a, it is suitable for highly accurate binarization processing of an image having a small contrast.

In each of the above embodiments, the US mask 40 is used for the blur correction circuit, but the present invention is not limited to this.

Further, the background noise cut level b is set to be slightly higher than the background d when the film projection image by the negative film is used, as in the above embodiment,
On the contrary, when a positive film is used, it is, of course, set slightly lower than the background, and in this case, it is set on the left side of the histogram peak (low output level side).

(Effects of the Invention) As described above, the present invention obtains an image signal by reading a film projection image, finds an output level range of a histogram based on a signal other than a document included in the image signal, for example, a signal of a document table. The range is removed from the range of the histogram obtained from the area containing the original, the peak for the background of the histogram is found within the remaining range, and the background noise cut level is set near the skirt of this peak. Even if the part of is included in the image, the background mountain can be correctly identified, and even if the image changes, the background noise cut level can be always set to an appropriate level.
Therefore, in the binarization process after blur correction, the background noise cut level is too close to the background to sufficiently remove noise, or conversely, it deviates from the background level and sacrifices the dynamic range of the image signal. It is possible to perform high-quality binarization processing without causing inconveniences such as.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a conceptual diagram of a specific example thereof, FIGS. 3A to 3C are diagrams showing signal waveforms in a processing process, and FIG. 4 is a 3 × 3 matrix. FIG. 5, 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 respective portions, and FIG. 8 is a principle explanatory diagram of the present invention. 20: image sensor, 32: background noise cut circuit, 38: blur correction circuit, 40: unsharp mask, 42: comparator, a: image signal, b: background noise cut level, d ... … Background level, f, j …… Binarization level.

Claims (6)

[Claims] 1. A method for sharpening an image signal by slicing an image signal obtained by scanning a projected image of a film at a background noise cut level close to the background level to remove background noise, and then correcting for blurring. Of the histogram obtained from the image signal for the projection image that does not include the original in the histogram obtained from the image signal for the projection image that includes the original. Detect mountains against the background with a histogram excluding the mountain output range,
An image signal processing method, wherein the background noise cut level is set in the vicinity of a skirt on a signal level side including an image in the ridges on the left and right of the 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 obtained by smoothing the image signal, compressing the smoothed signal, and further performing level shift. Image signal processing method. 4. The image signal processing method according to claim 3, wherein the smoothing is performed using a median filter. 5. The projection image is obtained by a negative film, and the background noise cut level is set in the vicinity of the skirt of the high output level side of the humangram mountain with respect to the background. The image signal processing method according to item 1. 6. The projection image is obtained by a positive film, and the background noise cut level is set in the vicinity of the skirt of the low output level side of the mountain of the histogram with respect to the background. The image signal processing method described in the item.