1. Field of the invention
The present invention is related to a method of and apparatus for simulating an object image to be printed by showing the image on a display, such as a CRT, prior to production of the separation negatives or positives by an image scanner, such as a color scanner, thereby setting color separation conditions (or reproduction conditions), especially color and gradation conditions which are preferable for reproducing an original image by printing process.
2. Description of the Prior Art
The reproduction conditions are set in an image scanner, such as a color scanner or a color page make-up system, to produce a reproduced image in accordance with requirements for color and gradation conditions. Appearance of the reproduced image concerning color and gradation will be referred to as "color tone" in this specification. The color tone depends on color reproducibilities of respective colors in the original, an overall color balance, etc., and gradation reproducibilities of a highlight and shadow density, characteristics of tone reproduction, contrast, etc.. For black and white printing, the color tone denotes the above stated color and gradation conditions except those concerning chromatic color.
FIG. 1 is a block diagram showing the structure of an image scanner in the art, which is provided with an image simulator for establishing the reproduction conditions. The image scanner 110 comprises an image reading unit 111 for photoelectrically scanning an original to generate an image signal I, a condition setting unit 112 for setting the reproduction conditions for the image signal I, an image processing unit 113 for processing and modifying the image signal I under the reproduction conditions to generate a modified image signal J, and an image recording unit 114 for exposing a photosensitive material by scanning light beam or beams as a function of the modified image signal J to produce separation negatives (or separation positives). Incidentally, printing plates, which are usually four plates for yellow, magenta, cyan and black printers, are produced with the separation negatives, a color printed sheet then is produced by loading the printing plates in a printing machine.
The image simulator 120 comprises a console panel 121, a host computer 122 and a monitor unit 125. The monitor unit 125 comprises a modification circuit 126 for further modifying the modified image signal J to generate first image signals J.sub.1 with yellow (Y), magenta (M), cyan (C) and black (K) components, a matrix circuit 127 for converting the first image signals J.sub.1 into second image signals J.sub.2 with blue (B), green (G) and red (R) components suitable for a CRT display 130, a CRT control circuit 129 for controlling the CRT display 130, and the CRT display 130. The image simulator 120 displays an image simulating the color printed sheet on the CRT display 130 as a function of the modified image signal J.
Because he modification circuit 126 and the YMCK/BGR matrix circuit 127 are digital circuits, the modified image signal J supplied to the monitor unit 125 is firstly A-D converted by an A-D convertor (not shown) provided in the monitor unit 125 if the signal J is an analog signal. On the other hand, if the signal J is a digital signal, it is directly supplied to the modification circuit 126. In either case, a D-A converter not shown) is provided between the YMCK/BGR matrix circuit 127 and the CRT control circuit 129 because the CRT control circuit 129 is an usually analog circuit.
FIG. 2 is a flow chart showing the procedure of reproducing the separation negatives P (with components Y, M, C and K).
At a step S.sub.11, a seemingly relevant reproduction conditions are set in the condition setting unit 112 on the basis of experience of an operator.
At the next step S.sub.12, a scanning head not shown) provided in the image reading unit 111 is operated in a quick scan mode. In this mode, the scanning head moves faster than usual, whereby it reads an original to produce an image signal I with respect to fewer pixels than usual which are enough to make an image on the CRT display 130. The image signal I is converted into the modified image signal J by the image processing unit 113 under the reproduction conditions set at the step S.sub.11. The modified image signal J is supplied to the modification circuit 126, to thereby be stored in an inner memory (not shown) of the modification circuit 126. As shown in FIG. 1, the image signal I (B, G, R) generated by the image reading unit 111 consists of color components of blue (B), green (G) and red (R). Similarly, the modified image signal J (Y, M, C, K) generated by the image processing unit 113 consists of color components of yellow (Y), magenta (M), cyan (C) and black (K).
At a step S.sub.13, the monitor unit 125 displays a proof image on the CRT display 130 as a function of the modified image signal J. The operator watches the proof image and inputs modification data for the reproduction conditions by means of the console panel 121 into the modification circuit 126 in order to obtain an optimum proof image.
At a step S.sub.14, the modification data obtained at the step S.sub.13 are supplied to the condition setting unit 112 to modify the reproduction conditions previously set in the unit 112. The operation of this step S.sub.14 is referred to as "production setup". If necessary, the proof image is shown again on the CRT display 130 through the steps S.sub.12 (a quick scan mode) and S.sub.13 (a proof mode) before the next step S.sub.15, in order to check whether the production setup is proper or not. The reproduction conditions may be further modified, if necessary.
At the step S.sub.15, the image scanner 110 operates under the reproduction conditions set at the production setup, and the image recording unit 114 exposes color separation images on photosensitive material. The photosensitive material is developed to form the separation negatives P (Y, M, C, K).
An image memory 131 drawn by a dash dotted line in FIG. 1 is also known in the art. If the image memory unit 131 is provided in the image simulator 120, the image data I (B, G, R) obtained at the quick scan mode are stored in the image memory unit 131. The proof image is shown on the CRT display as a function of the image data I (B, G, R) stored in the image memory unit 131 while the image data I (B, G, R) are processed by the image processing unit 113 under the reproduction conditions stored in the condition setting unit 112. Accordingly, the proof image can be repeatedly shown without the quick scanning of the original.
Further, the operator may directly modify the reproduction conditions in the condition setting unit 112 rather than inputting the modification data into the modification circuit 126, in order to obtain the optimum proof image (or optimum reproduction conditions). Consequently, the modification circuit 126 can be omitted. Moreover, the modification circuit 126 can be also omitted, and the production setup (S.sub.14) may be simultaneously executed in the proof mode (S.sub.13).
The operator can confirm the color tone of the object image by means of the CRT display 13 shown in FIG. 1 before the image recording unit 114 exposes the object images. Accordingly, the operator is given an opportunity to set the reproduction conditions while predicting the color tone of the object image, whereby a mistake on setting he reproduction conditions is prevented.
However, the apparatus in the art has the following issues: First, although the color tone of the proof image shown on the CRT display simulates that of the printed sheet as much as possible, the color tone of the proof image and that of the printed sheet do not coincide with each other perfectly. It is because there is a difference in reproducible color gamut between the CRT and color inks. Therefore, the operator cannot always obtain the optimum reproduction conditions. Second, it is difficult to systematically control the color tone for each original image to be treated in sequence. A printed sheet often includes plural object images reproduced from plural original images. Therefore, even if the color tone of each object image is well controlled separately, the color tones within the entire region of the printed sheet can be hardly balanced. Namely, the color tones of the plural object images on the printed sheet are liable to fluctuate to some extent. Accordingly, the reproduction conditions are required to be set in consideration of the difference in the color tone between the printed sheet and the proof image on CRT by a skilled operator. Finally, when an operator continuously watches the CRT display, his sense of the color tone is somewhat paralyzed. Accordingly, it is sometimes difficult even for the skilled operator to establish relevant reproduction conditions because he cannot precisely recognize the color tone.