EP0047145A2 — A method of producing a half-tone reproduction

Claim 1

Independent Claim 222 words

A method of making a half-tone reproduction of an original image by exposing an output surface (28, 30, 32, 34; figure 24) to a scanning beam (SB1; figure 3) which assumes conditions (01) in accordance with the value of a control signal (4, 5; figure 24), comprising, for the generation of the control signal, the steps of: (a) generating position signals (X, Y) representing the position of the scanning beam within a half-tone dot area (DA, figure 3) or other elemental area of a screen grid (SG), each successive position in the screen grid of the scanning beam being determined by the repeated addition of small increments (ΔX, AY; figure 4) to the previous position; (b) comparing each of the said position signals with a picture signal (PS) 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 which causes the scanning beam to write on the output surface in accordance with the result of the said combination of results; characterized in that in step (a) the position signals (X, Y) are compared, at each successive position of the scanning beam, with a predetermined value (214, figure 19A) to detect the crossing of the border of the dot area or other elemental area:

Claim 2

Independent Claim 222 words

A method of making a half-tone reproduction of an original image by exposing an output surface (28, 30, 32, 34; figure 24) to a scanning beam (SB1; figure 3) which assumes conditions (01) in accordance with the value of a control signal (4, 5; figure 24), comprising, for the generation of the control signal, the steps of: (a) generating position signals (X, Y) representing the position of the scanning beam within a half-tone dot area (DA, figure 3) or other elemental area of a screen grid (SG), each successive position in the screen grid of the scanning beam being determined by the repeated addition of small increments (ΔX, AY; figure 4) to the previous position; (b) comparing each of the said position signals with a picture signal (PS) 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 which causes the scanning beam to write on the output surface in accordance with the result of the said combination of results; characterized in that in step (a) the position signals (X, Y) are compared, at each successive position of the scanning beam, with a predetermined value (214, figure 19A) to detect the crossing of the border of the dot area or other elemental area:

Claim 3

Dependent Claim 71 words

A method in accordance with claim 1, wherein the step (a) includes the step of comparing, at each successive position within the screen grid, the position signals with a predetermined border value, the result of the comparison indicating that the half-tone dot area border is approached by the scanning beam, and, when the border is reached, the step of adding special border increments (AXb, ΔYb) to the position signals (X Y).

Claim 4

Dependent Claim 59 words

A method in accordance with claim 1 or 2, wherein the scanning beam scans through a succession of equally-spaced, parallel, substantially vertical lines (SB, figure 3) over the output surface, and the step (a) includes the step of recognizing that a vertical scan has finished, and then initialising the position signals (215, figure 20) for a new vertical scan.

Claim 5

Independent Claim 111 words

A method in accordance with any preceding claim, wherein the position signals within a half-tone dot area or other elemental area are compared with a signal (PS) representing the width-of a half-tone dot area after the said addition of the small increments, in order to determine whether the scanning beam has approached the border of the half-tone dot area or elemental area of the screen grid, and if so, the signals are adjusted to represent the arrival of the scanning beam at the opposite side (42, figure 4) of the same half-tone dot area (DA) or the same elemental area, causing the scanning beam effectively to scan only one such area.

Claim 6

Independent Claim 33 words

A method in accordance with any preceding claim, for producing a "hard dot" half-tone reproduction, wherein the scanning beam-assumes "on" and "off" conditions in accordance with two possible values of the control signal.

Claim 7

Independent Claim 83 words

A method in accordance with any of claims 1 to 4, for making a "soft dot" half-tone reproduction, by exposing the output surface to the scanning beam which assumes a level of intensity in accordance with the value of the control signal, wherein in step (c) the results of.the said combinations are processed to-generate the control signal which causes the scanning beam to record at a level of intensity (figure 17A) in accordance with the said processing of the results (PS-PV, figure 17A).

Claim 8

Independent Claim 60 words

A method in accordance with any preceding claim, wherein two position signals for each scanning beam represent distances along mutually orthogonal axes parallel to the boundaries of a dot area or elemental area, the condition that the half-tone dot area border is approached by the scanning beam being detected by comparing each position signal (X, Y) with the border value.

Claim 9

Independent Claim 47 words

A method in accordance with any of claims 1 to 6, wherein step (a) includes the step of deriving from the said position signals a single index signal (I) for the scanning beam; and wherein in step (b) the index signal is compared with the picture signal.

Claim 10

Dependent Claim 69 words

A method in accordance with claim 8, wherein the result of the combination of the index signal and the picture signal (PS-PV; figure 17A) is a signal representing the proximity in the screen grid of the scanning beam to a dot edge, the position of the dot edge being a function of the picture signal and being a contour (DB, figure 4) of equal intensity in the required dot.

Claim 11

Dependent Claim 63 words

A method in accordance with claim 9, wherein the said signal representing the proximity of the scanning beam to a dot edge is processed in step (c) to generate a control signal which varies as a linear function of the said signal, between zero and a predetermined constant value causing zero and maximum exposure respectively of the output surface by the scanning beam.

Claim 12

Dependent Claim 70 words

A method in accordance with claim 6 -or any claim appendant thereto,wherein the variation in intensity of the effective exposure, of a film constituting the output surface, across edges of dots is sufficiently steep that minor variations in a film threshold value (750, figure 16) of the said film do not seriously affect the position of the dot edge produced, yet is sufficiently gentle that the dots are easily etchable.

Claim 13

Dependent Claim 26 words

A method in accordance with claim 11, wherein the exposure at the centre of each dot is to at least 200% of the film threshold value.

Claim 14

Dependent Claim 20 words

A method in accordance with claim 12, wherein the said exposure is up to 300% of the film threshold value.

Claim 15

Independent Claim 225 words

A method of making a half-tone reproduction of an original image by exposing an output surface to a plurality of scanning beams (701 to 706, figure 16) each of which is produced according to a method in accordance with any of claims 7 to 13, wherein each of the scanning beams assumes conditions in accordance with the value of a respective control signal, the method comprising, for the generation of each control signal, the steps of; (a) generating position signals representing the position of the corresponding scanning beam within a half-tone dot area or other elemental area of a screen grid, each successive position in the screen grid of the scanning beam being determined by the repeated addition of small increments to the previous position, 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 or other elemental area; (b) deriving from the said position signals an index signal (I); (c) Comparing the index signal with a picture signal/representing the picture density at the point of the original image to be reproduced corresponding to the position represented by the said position signals; and (d) according to the result of the said comparison, generating the said control signal which causes its respective scanning beam to write accordingly on the output surface.

Claim 16

Dependent Claim 91 words

A method in accordance with claim 14, for making a "soft dot" half-tone reproduction, by exposing the output surface to the scanning beams which assume levels of intensity in accordance with the values of the corresponding control signals, wherein in step (c) the said index signal is combined with the picture signal; and wherein in step (d) the result of the said combination is processed to generate the control signal which causes the scanning beam to record at a level of intensity in accordance with the said processing of the result.

Claim 17

Independent Claim 99 words

A method in accordance with any preceding claim, of making a half-tone reproduction by exposing the output surface to a plurality of scanning beams, wherein each scanning beam scans through a succession of parallel, substantially vertical lines over the output surface with a first predetermined equal spacing between successive lines, the scan progressing steadily in a horizontal direction, the scanning beams being arranged across the output surface with a second predetermined horizontal spacing between each scanning beam that is much smaller than the said first spacing, so that there is substantially no overlap between successively-scanned groups of scanning lines.

Claim 18

Dependent Claim 78 words

A method in accordance with claim 16, wherein the said plurality of scanning beams overlap, the said second predetermined spacing being defined as the horizontal distance between points of maximum intensity in adjacent beams, the effective exposure of the output surface being equal to the sum (770, figure 16) of the intensities of the overlapping beams and varying smoothly across the screen grid between a minimum in regions outside dots to a maximum in central regions of dots.

Claim 19

Independent Claim 72 words

A method in accordance with any preceding claim, wherein position signals are generated for each scanning beam and for each of two or more different colour separations (28, 30, 32, 34; figure 24) which require different screen angles, the position signals corresponding to the different values of the screen angle being computed either simultaneously, or sequentially according to the rotation of each successive colour separation output surface past a recording head (26).

Claim 20

Dependent Claim 74 words

A method.in accordance with claim 18, wherein at least two output surfaces (28, 30),to form two colour separations (y,c) after exposure, are wrapped around a drum so that each passes the same exposing head in each revolution of the drum, and the extraction from a store of picture signals (PS) for each successive colour separation is controlled synchronously with the changing of the screen angle (β) to be used in the position signal computation.

Claim 21

Dependent Claim 253 words

Apparatus for producing a half-tone reproduction of an original image by exposing an output surface (28, 30, 32, 34; figure 24) to a scanning beam which assumes conditions in accordance with the value of a control signal, comprising: a support for the-output surface; an exposing head for producing the scanning beam and exposing the output surface when placed on the support; means for obtaining a relative scanning motion between the exposing head and the output surface; and a screen computer (2) for generating the beam control signal by: (a) generating position signals (X, Y) representing the position of the scanning beam within a half-tone dot area (DA, figure 3) or other elemental , area of a screen grid (SG), each successive position in the screen grid of the scanning beam being determined by the repeated addition of small increments (AX, AY; figure 4) to the previous position; (b) comparing each of the said position signals with a picture signal (PS) 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 which causes the scanning beam to write on the output surface in accordance with the result of the said combination of results; characterized in that in step (a) the position signals (X,Y) are compared, at each successive position of the scanning beam, with a predetermined value (214, figure 19A) to detect the crossing of the border of the dot area or other elemental area.

Claim 22

Dependent Claim 253 words

Apparatus for producing a half-tone reproduction of an original image by exposing an output surface (28, 30, 32, 34; figure 24) to a scanning beam which assumes conditions in accordance with the value of a control signal, comprising: a support for the-output surface; an exposing head for producing the scanning beam and exposing the output surface when placed on the support; means for obtaining a relative scanning motion between the exposing head and the output surface; and a screen computer (2) for generating the beam control signal by: (a) generating position signals (X, Y) representing the position of the scanning beam within a half-tone dot area (DA, figure 3) or other elemental , area of a screen grid (SG), each successive position in the screen grid of the scanning beam being determined by the repeated addition of small increments (AX, AY; figure 4) to the previous position; (b) comparing each of the said position signals with a picture signal (PS) 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 which causes the scanning beam to write on the output surface in accordance with the result of the said combination of results; characterized in that in step (a) the position signals (X,Y) are compared, at each successive position of the scanning beam, with a predetermined value (214, figure 19A) to detect the crossing of the border of the dot area or other elemental area.

Claim 23

Dependent Claim 63 words

. Apparatus in accordance with claim 20, wherein the screen computer compares, at each successive position within the screen grid, the position signals with a predetermined border value, the result of the comparison indicating that the .half-tone dot area border is approached by the scanning beam, and, when the border is reached, adds special border increments (ΔXb, ΔYb) to the position signals (X,Y).

Claim 24

Dependent Claim 52 words

Apparatus in accordance with claim 20 or 21, wherein the scanning beam scans through a succession of equally-spaced, parallel, substantially vertical lines over the output surface, and the step (a) includes the step of recognising that a vertical scan has finished, and then initialising the position signals for a new vertical scan.

Claim 25

Independent Claim 217 words

A method of producing photogravure separations corresponding to aboriginal image by exposing an output surface to a scanning beam (SB1, figure 3) which assumes conditions in accordance with the value of a control signal (4, 5; figure 24), characterized in that it comprises, for the generation of the control signal, the steps of: (a) generating position signals (X,Y) representing the position of the scanning beam within a gravure cell area (DA, figure 25) of a screen grid, each successive position in the screen grid of the scanning beam being determined by the repeated addition of small increments (ΔX, ΔY) to the previous position, and, at each successive position, the position signals being compared with a predetermined value to detect the crossing of the border of the gravure cell area; (b) comparing each of the said position signals with a picture signal (PS) 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 which assumes either a predetermined constant value (Ov, figure 26), to cause the beam to produce part of a gravure cell wall, (251, figure 25), or else a cell depth value (PS, figure 26) to cause the beam to produce part of a gravure cell (250).

Claim 26

Independent Claim 217 words

A method of producing photogravure separations corresponding to aboriginal image by exposing an output surface to a scanning beam (SB1, figure 3) which assumes conditions in accordance with the value of a control signal (4, 5; figure 24), characterized in that it comprises, for the generation of the control signal, the steps of: (a) generating position signals (X,Y) representing the position of the scanning beam within a gravure cell area (DA, figure 25) of a screen grid, each successive position in the screen grid of the scanning beam being determined by the repeated addition of small increments (ΔX, ΔY) to the previous position, and, at each successive position, the position signals being compared with a predetermined value to detect the crossing of the border of the gravure cell area; (b) comparing each of the said position signals with a picture signal (PS) 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 which assumes either a predetermined constant value (Ov, figure 26), to cause the beam to produce part of a gravure cell wall, (251, figure 25), or else a cell depth value (PS, figure 26) to cause the beam to produce part of a gravure cell (250).

Claim 27

Dependent Claim 21 words

A method in accordance with claim 25, wherein the said cell depth value is proportional to the picture signal level (PS).

A method of producing a half-tone reproduction

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