EP0236594A1 — Method of and device for recognizing half-tone im…

Claim 1

Independent Claim 146 words

A method of recognizing half-tone image information, the local frequency of which is within predetermined limits, within an information area that may also contain other kinds of image information, the image information area being scanned line by line to generate a video signal which is subjected to a recognition process to determine, for each part of the signal, whether the same belongs to half-tone image information, characterised in that a) the grey level maxima above a predetermined high threshold value are determined in the video signal, b) the distance between each two successive maxima is determined, the distance being compared with a predetermined fixed distance, c) for the part of the video signal between two successive maxima a positive decision is made (belongs to half-tone image information) if the difference found in b) between the distance as determined and the fixed distance lies within predetermined limits.

Claim 2

Independent Claim 146 words

A method of recognizing half-tone image information, the local frequency of which is within predetermined limits, within an information area that may also contain other kinds of image information, the image information area being scanned line by line to generate a video signal which is subjected to a recognition process to determine, for each part of the signal, whether the same belongs to half-tone image information, characterised in that a) the grey level maxima above a predetermined high threshold value are determined in the video signal, b) the distance between each two successive maxima is determined, the distance being compared with a predetermined fixed distance, c) for the part of the video signal between two successive maxima a positive decision is made (belongs to half-tone image information) if the difference found in b) between the distance as determined and the fixed distance lies within predetermined limits.

Claim 3

Dependent Claim 55 words

A method according to claim 1, characterised in that in step b) the last determined distance at any time is also compared with the last-but-one determined distance and in that step c) a positive decision is taken only if the difference between the last determined distance and the last-but-one determined distance lies within predetermined limits.

Claim 4

Dependent Claim 46 words

A method according to claim 1 or 2, characterised in that in step a) a following maximum can be determined only if, after the determination of a maximum, the grey level in the video signal has fallen at least once below a predetermined low threshold value.

Claim 5

Independent Claim 72 words

A method according to any one of the preceding claims, characterised in that in step a) the absolute maximum grey level and the absolute minimum grey level are also determined from the video signal processed until then and in that the high threshold value is made equal to the absolute minimum grey level plus a predetermined percentage of the difference between the absolute maximum grey level and the absolute minimum grey level.

Claim 6

Independent Claim 72 words

A method according to any one of the preceding claims, characterised in that in step a) the aboslute maximum grey level and the absolute minimum grey level are also determined from the video signal processed until then and in that the low threshold value is made equal to the absolute minimum grey level plus a predetermined percentage of the difference between the absolute maximum grey level and the absolute minimum grey level.

Claim 7

Dependent Claim 124 words

A method according to any one of claim 1 to 3, characterised in that after a high threshold value has been established this value is reduced at a predetermined speed, in that in step a) the absolute maximum grey level and the absolute minimum grey level are determined from the video signal processed after the last increase of the high threshold value, and in that a reference value is calculated which is equal to the absolute minimum grey level plus a predetermined percentage of the difference between the absolute maximum grey level and the absolute minimum grey level, and in that the high threshold value is made equal to said reference value if the instantaneous high threshold value is lower than this reference value.

Claim 8

Independent Claim 124 words

A method according to any one of claims 1 to 3, characterised in that after a low threshold value has been established this value is increased at a predetermined speed, in that in step a) the absolute maximum grey level and the absolute minimum grey level are determined from the video signal processed after the last reduction of the low threshold value, and in that a reference value is calculated which is equal to the absolute minimum grey level plus a predetermined percentage of the difference between the absolute maximum grey level and the absolute minimum grey level, and in that the low threshold value is made equal to said reference value if the instantaneous low threshold value is higher than this reference value.

Claim 9

Independent Claim 32 words

A method according to any one of claims 4 to 7, characterised in that a predetermined minimum distance is maintained between the instantaneous maximum grey level and the instantaneous minimum grey level.

Claim 10

Independent Claim 93 words

A method according to any one of the preceding claims, characterised in that the recognition decision for a part of the pixel stream belonging to a scanning line is compared with the decision for the next adjacent scanning line, in that the recognition decision of the next line is maintained if it is equal to the preceding decision, in that the decision is changed to "not unambiguously recognizable" if it is unequal to the preceding decision, and in that the decision is maintained if the preceding decision was a "not unambiguously recognizable" decision.

Claim 11

Independent Claim 201 words

A device for recognizing half-tone image information whose local frequency lies within predetermined limits, within an information area which may also contain other kinds of image information, the image information area being scanned line by line by a scanner or camera to generate a video signal which in a recognition unit is subjected to a recognition process to determine, for each part of the signal, whether the same belongs to half-tone image information, characterised in that the recognition unit is provided with a maximum detector by means of which maxima in the video signal are determined, a first comparator with which each maximum is compared with a high threshold value stored in a memory, which comparator, on the occurrence of a maximum above the threshold value, delivers a signal to a counter controlled by clock pulses at a fixed frequency, by means of which signal the counter is stopped whereafter its contents is compared in a second comparator with a reference value stored in a memory, whereafter the counter is reset and starts counting again, which second comparator delivers an output signal to the output of the recognition unit if the contents lies within a predetermined area around the reference value.

Claim 12

Independent Claim 201 words

A device for recognizing half-tone image information whose local frequency lies within predetermined limits, within an information area which may also contain other kinds of image information, the image information area being scanned line by line by a scanner or camera to generate a video signal which in a recognition unit is subjected to a recognition process to determine, for each part of the signal, whether the same belongs to half-tone image information, characterised in that the recognition unit is provided with a maximum detector by means of which maxima in the video signal are determined, a first comparator with which each maximum is compared with a high threshold value stored in a memory, which comparator, on the occurrence of a maximum above the threshold value, delivers a signal to a counter controlled by clock pulses at a fixed frequency, by means of which signal the counter is stopped whereafter its contents is compared in a second comparator with a reference value stored in a memory, whereafter the counter is reset and starts counting again, which second comparator delivers an output signal to the output of the recognition unit if the contents lies within a predetermined area around the reference value.

Claim 13

Dependent Claim 76 words

A device according to claim 10, characterised in that the contents prior to the resetting of the counter is stored in a memory and in that if the second comparator delivers an output signal the contents is compared in a third comparator with the previous contents stored in the said memory, the third comparator delivering an output signal to the output of the recognition unit if the difference between the two contents is within predetermined limits.

Claim 14

Dependent Claim 148 words

A device according to claim 10 or 11, characterised in that the recognition unit is provided with a minimum detector by means of which minima in the video signal are determined, a fourth comparator with which each minimum is compared with a low threshold value stored in a memory, which fourth comparator, on the occurrence of a minimum below the threshold value, delivers a signal to a flip-flop to bring the flip-flop into a first position in which the flip-flop provides a connection between the input of the recognition unit and the said maximum detector, while on the occurrence of a maximum above the level of the high threshold value the said first comparator delivers a signal to the flip-flop in order to bring it into a second position in which the flip-flop establishes a connection between the input of the recogni tion unit and the minimum detector.

Claim 15

Independent Claim 117 words

A device according to any of claims 10 to 12, characterised in that the recognition unit is also provided with a fifth comparator with which a maximum above the high threshold value as determined by the first comparator is compared with an absolute maximum value stored in a memory, which fifth comparator, on the occurrence of a maximum above the absolute maximum value, feeds said maximum to the memory in order to replace the absolute maximum value present therein until then, and also feeds the maximum value to a calculator unit in which, on the basis of the new absolute maximum, a new high threshold value is calculated, which is stored in the high threshold value memory.

Claim 16

Independent Claim 118 words

A device according to any one of claims 10 to 13, characterised in that the recognition unit is also provided with a sixth comparator with which a minimum above the high threshold value as determined by the fourth comparator is compared with an absolute minimum value stored in a memory, which sixth comparator, on the occurrence of a minimum below the absolute minimum value, feeds said minimum to the memory in order to replace the absolute minimum value present therein until then and also feeds the minimum value to a calculator unit in which, on the basis of the new absolute minimum, a new low threshold value is calculated which is stored in the low threshold value memory.

Method of and device for recognizing half-tone image information

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