| 摘要 |
A method is disclosed for making a half-tone reproduction of an original image by exposing an output surface (OS), for example a film wrapped around a rotating drum, to a scanning beam e.g. a modulated, multiple-head laser beam. The modulation of each scanning beam, of which there may be several (SB,SB1), is determined by a control signal generated by:
(a) generating digital position signals representing the position of the scanning beam (SB1) within a half-tone dot area (DA) of an imaginary, "electronic", screen grid (SG) inclined at a screen angle ( beta ) to a horizontal line on the output surface (OS), each successive position (x) in the screen grid being computed by the repeated addition of small increments to the previous position signals, and, at each successive position, the position signals being compared with a predetermined value to detect the crossing of the border of the dot area (DA);
(b) comparing each of the said position signals with a picture signal representing the picture density at the corresponding point of the original image to be reproduced; and
(c) combining the results of the said comparisons to generate the control signal.
<??>The picture signal determines the size of the dot required, and hence the position in the screen grid of the dot border (DB). Its comparison with the position signals thus indicates the required output modulation. For simple "hard" dots, as shown, this is an "on/off" modulation, the control signal being on ("1") or off ("0"), at successive points (x) of the scan. The computer may however derive a multiple-level output modulation control signal, to give "soft" dots of any shape.
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