EP0274196A1 — Generating half-tone representations

Claim 1

Independent Claim 291 words

A method of generating a half-tone representation of an original picture in which an exposing beam is caused to scan a record medium, the condition of the exposing beam being controlled by a two state control signal (CS) in accordance with a picture signal (PS) representing colour component densities of the original picture and half-tone dot information (PV) defining for elemental areas within a dot cell corresponding values representative of colour densities whereby elemental areas of the record medium are exposed or not exposed in accordance with the control signal taking up a first or second state respectively, the control signal being generated by: i) monitoring the position of the exposing beam relative to the record medium and relative to a half-tone dot area within which the beam is positioned; ii) comparing a picture signal (PS) representing a colour component density at that position with a half-tone dot cell value (PV) for the elemental area of a half-tone dot cell corresponding to the elemental area of the record medium at the monitored position to generate a difference signal (PS-PV); iii) determining from the difference signal and a predetermined probability function (58-60) the probability that the control signal (CS) should take up its first or second state, the probability function being defined such that for a given picture signal the average proportion of a dot cell which is exposed over a region of the record medium having a plurality of half-tone dots generated in response to the picture signal will be substantially equal to the colour density represented by the picture signal expressed as a proportion of a maximum colour density; and, iv) causing the control signal (CS) to take up its first or second state in accordance with the determined probability.

Claim 2

Independent Claim 291 words

A method of generating a half-tone representation of an original picture in which an exposing beam is caused to scan a record medium, the condition of the exposing beam being controlled by a two state control signal (CS) in accordance with a picture signal (PS) representing colour component densities of the original picture and half-tone dot information (PV) defining for elemental areas within a dot cell corresponding values representative of colour densities whereby elemental areas of the record medium are exposed or not exposed in accordance with the control signal taking up a first or second state respectively, the control signal being generated by: i) monitoring the position of the exposing beam relative to the record medium and relative to a half-tone dot area within which the beam is positioned; ii) comparing a picture signal (PS) representing a colour component density at that position with a half-tone dot cell value (PV) for the elemental area of a half-tone dot cell corresponding to the elemental area of the record medium at the monitored position to generate a difference signal (PS-PV); iii) determining from the difference signal and a predetermined probability function (58-60) the probability that the control signal (CS) should take up its first or second state, the probability function being defined such that for a given picture signal the average proportion of a dot cell which is exposed over a region of the record medium having a plurality of half-tone dots generated in response to the picture signal will be substantially equal to the colour density represented by the picture signal expressed as a proportion of a maximum colour density; and, iv) causing the control signal (CS) to take up its first or second state in accordance with the determined probability.

Claim 3

Dependent Claim 14 words

A method according to claim 1, wherein the exposing beam comprises a laser beam.

Claim 4

Dependent Claim 40 words

A method according to claim 1 or claim 2, wherein step (iv) comprises randomly generating one of two numbers corresponding to the first and second states of the control function, the random number generation being weighted by the determined probability.

Claim 5

Independent Claim 43 words

A method according to any of the preceding claims for reproducing a number of colour separations in which the angle between the exposing beam and the screen grid is different for each colour separation, wherein the probability function (58-60) differs between scanning angles.

Claim 6

Dependent Claim 338 words

Apparatus for generating a half-tone representation of an original picture, the apparatus comprising an exposing beam generator (8); a record medium support (11); means (10) for causing an exposing beam generated by the beam generator to scan a record medium mounted on the support (11); exposing beam control means (5,7) for generating a two state control signal (CS) to control the condition of the exposing beam, the control signal being generated in accordance with a picture signal (PS) representing colour component densities of the original picture and half-tone dot information (PV) defining for elemental areas within a dot cell corresponding values representative of colour densities whereby elemental areas of the record medium are exposed or not exposed in use according to the control signal taking up a first or second state respectively, the exposing beam control means including monitoring means for monitoring the position of the exposing beam relatively to the record medium and relative to a half-tone dot area within which the beam is positioned, half-tone dot cell value generating means (1,2,3) responsive to the position of the exposing beam relative to a half-tone dot area to generate a half-tone dot cell value for the corresponding elemental area, comparison means (4) for comparing a picture signal representing a colour component density at that position with the half-tone dot cell value generated to generate a difference signal (PS-PV); and control signal generating means (5) for generating the control signal in one of its first and second states by determining from the difference signal and a predetermined probability function (58-60) the probability that the control signal (CS) should take up its first or second state, the probability function being defined such that for a given picture signal the average proportion of a dot cell which is exposed over a region of the record medium having a plurality of half-tone dots generated in response to a picture signal will be substantially equal to the colour density represented by the picture signal expressed as a proportion of a maximum colour density.

Claim 7

Dependent Claim 338 words

Apparatus for generating a half-tone representation of an original picture, the apparatus comprising an exposing beam generator (8); a record medium support (11); means (10) for causing an exposing beam generated by the beam generator to scan a record medium mounted on the support (11); exposing beam control means (5,7) for generating a two state control signal (CS) to control the condition of the exposing beam, the control signal being generated in accordance with a picture signal (PS) representing colour component densities of the original picture and half-tone dot information (PV) defining for elemental areas within a dot cell corresponding values representative of colour densities whereby elemental areas of the record medium are exposed or not exposed in use according to the control signal taking up a first or second state respectively, the exposing beam control means including monitoring means for monitoring the position of the exposing beam relatively to the record medium and relative to a half-tone dot area within which the beam is positioned, half-tone dot cell value generating means (1,2,3) responsive to the position of the exposing beam relative to a half-tone dot area to generate a half-tone dot cell value for the corresponding elemental area, comparison means (4) for comparing a picture signal representing a colour component density at that position with the half-tone dot cell value generated to generate a difference signal (PS-PV); and control signal generating means (5) for generating the control signal in one of its first and second states by determining from the difference signal and a predetermined probability function (58-60) the probability that the control signal (CS) should take up its first or second state, the probability function being defined such that for a given picture signal the average proportion of a dot cell which is exposed over a region of the record medium having a plurality of half-tone dots generated in response to a picture signal will be substantially equal to the colour density represented by the picture signal expressed as a proportion of a maximum colour density.

Claim 8

Dependent Claim 32 words

Apparatus according to claim 5, wherein the half-tone dot cell value generating means comprises a store (1) containing values representative of colour densities for the elemental areas of a half-tone dot cell.

Claim 9

Dependent Claim 26 words

Apparatus according to claim 5 or claim 6, wherein the monitoring means, comparison means, and control signal generating means are provided by a suitably programmed computer.

Claim 10

Dependent Claim 103 words

Apparatus according to any of claims 5 to 7, wherein the control signal generating means comprises a probability function look-up table (12) addressed by the difference signal; a random number generator (14) for generating signals representing random numbers having a maximum number of digits which is one less than the maximum number of digits generated by the look-up table (12); and an adder circuit (15) for adding the signals from the look-up table (12) and the random number generator (14) and for providing an output constituted by the most significant bit of the result of the addition to constitute the control signal (CS).

Generating half-tone representations

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