This invention relates to a sharpness emphasizing method of a reproduction picture for use in a picture reproducing machine such as a color scanner, a color facsimile, or the like.
In a conventional picture reproducing machine, a sharpness emphasizing method of a reproduction picture is carried out by using picture signals to be processed or circumferential information of the picture signals.
The former case has a disadvantage, that is, the sharpness emphasis effect is obtained only in the scanning direction of the cylinder's circumference, and hence such a method cannot be applied independently to a color scanner for plate-making, or the like.
In the latter case, as shown in FIG. 1, a light beam 1 obtained by scanning an original picture optically, is divided into two components by a half mirror 2. Each light beam propagates along a light axis 3a or 3b through an aperture 5a or 5b having an opening distance d.sub.1 or d.sub.2, which is formed in a mask 4a or 4b.
Each light beam through the opening 5a or 5b is incident to a photoelectric element 6a or 6b which converts the light beam into a signal. Thus the obtained signal is then converted into a density signal in a logarithmic amplifier 7a or 7b including a preamplifier, thereby obtaining a picture signal A or B.
The picture signal A through the smaller aperture 5a is a so-called sharp signal whose sharpness is to be emphasized. The picture signal B through the larger aperture 5b is usually called an unsharp signal which includes the circumferential information of the sharp signal A.
However, in the embodiment shown in FIG. 1, in fact, the light beam through the aperture 5a is color-separated into red, green and blue colors by half mirrors to obtain three picture signals A.sub.R, A.sub.G and A.sub.B, and the picture signal A.sub.G is selected as the picture signal A.
In order to emphasize the sharpness of a recording signal by using the sharp signal A (A.sub.G) and the unsharp signal B, the unsharp signal B is subtracted from the sharp signal A to obtain an unsharp masking signal C in a differential amplifier 8. Then, the unsharp masking signal C is added to the sharp signal A (A.sub.R, A.sub.G or A.sub.B) in a summing amplifier 9 which outputs a sharpness-emphasized picture signal D (D.sub.R, D.sub.G or D.sub.B), as shown in FIG. 2.
This sharpness emphasizing method of the picture signal is now widely used. However, this method requires a special system having an optical system and an electric operational circuit, for obtaining the unsharp signal B, which is inconvenient and high cost. Further, in this method, by varying the shape of the aperture 5b of the mask 4b the sharpness in certain directions may be emphasized, and by varying the opening distance d.sub.2 of the aperture 5b the sharpness range may be varied. However, various masks are required for these variations.
The opening distance d.sub.1 of the aperture 5a of the mask 4a for the sharp signal A is determined depending on the resolving power of the sharp signal A. When the distance d.sub.1 of the aperture 5a is varied according to a reproducing magnification and so forth, the distance d.sub.2 of the aperture 5b must be changed, and accordingly sets of masks 4a and 4b must be prepared depending on the resolving power of the sharp signal A.
The sharpness emphasizing operation will be explained theoretically in connection with space frequency-response curves shown in FIG. 3. There are shown two slit functions f(A) and f(B) having widths d.sub.1 ' and d.sub.2 ', corresponding to the aperture sizes d.sub.1 and d.sub.2 of the apertures 5a and 5b are shown in FIG. 3a. In order to obtain the space frequency-response curves, or the spectrum distributions F(A) and F(B), which includes a space frequency range sensible to the eyes of human beings, the slit functions f(A) and f(B) are performed by Fourier transformation, as shown in FIG. 3b wherein F(u) means a spectrum value corresponding to a space frequency u.
As shown in FIG. 2, the unsharp masking signal C is A-B, and the sharpness-emphasized picture signal D is A+C. Thus, the spectrum distributions F(C) and F(D) corresponding to the unsharp masking signal C and the sharpness-emphasized picture signal D are expressed as F(A)-F(B) and F(A)+F(B), as shown in FIG. 3b.
Such spectrum distributions are shown in the X axis direction, as shown in FIG. 3a, and the same spectrum distributions are obtained in the Y axis direction. In the X=Y axis direction, the similar spectrum distribution is obtained, as shown in FIG. 3c wherein the aperture size d may be d.sub.1 ' or d.sub.2 ', but the points wherein F(u) equals naught, are different. In practice, in this case, the distribution may be considered as almost the same as the one in the X or the Y direction.
Therefore, on the contrary, the slit functions f(x) may be obtained from the spectrum distribution corresponding to the desired sharpness emphasizing characteristics in the reverse manner.
Consequently, as shown in FIG. 4, wherein d is the opening size of the aperture or the width of the scanning line, from the spectrum distribution F(D') of the sharpness-emphasized picture signal D' the spectrum distribution F(C') of the unsharp masking signal C' is given by F(D')-F(A), and the spectrum distribution F(B') of the ideal unsharp signal B' is obtained by (F(A)-F(C'). Then, the spectrum distribution F(B') is performed by reverse Fourier transformation to obtain the slit function f(B') corresponding to the ideal unsharp signal B'. According to the slit function f(B') obtained the aperture 5b of the mark 4b is formed, and the transmittance of the light through such an aperture 5b is continuously reduced radially from its center according to the slit function f(B').
In FIG. 4 is shown one example of a wave form of such a slit function, and, in practice, the wave form is exactly determined from the slit function.
In this method, a plurality of masks, each having such an aperture, must be prepared in advance, depending on the opening size d.sub.1 of the aperture. However, since the aperture size is minute, and the variation of the light transmittance of the aperture is effected by using the photographic technique, in practice, it is quite difficult to control the light transmittance of the minute aperture area.