This invention relates to a method for identifying objects appearing at random positions, in random orientations, and at random times on an image window and having, on a surface facing the image window, an identification in the form of a field which includes on at least one data track contrasting symbols and a plurality of given contrasting line patterns, or a product identification code (PIC). The latter identifies the position and orientation of the data track(s) and comprises a plurality of lines having a variable line spacing and/or line widths. In use, the image window is opto-electronically scanned line-by-line and a video signal is generated which reflects to the scanned contrast sequence.
In a first method step, the image window is scanned in a fixed or stationary detection raster configuration (hereinafter simply "raster") and, upon identification of the individual contrasting line patterns, their position or orientation within the detection raster is determined. In a second method step, the angle .beta. at which the data tracks(s) and the lines of the detection raster intersect is determined. In a third method step, sometimes referred to as the reading step, the data field is scanned in direction of the data track(s) in a readout raster which is rotated by the angle .beta. to thereby read and decode the symbols or indicia on the data track(s). Alternatively, the the detection raster is rotated in predetermined angular increments .DELTA..alpha. and scanned. The readout raster is then rotated in direction of the data track(s) by the angle .beta. and the angular increments .DELTA..alpha..
Such method and apparatus are already known. The objects to be identified are, for example, commercial products, department store articles or the like which bear machine readable markings. For this purpose, appropriate identifications are applied to the objects by imprinting thereon a desired code, for example the well-known OCR code. The encoded information may relate to the quality, size, price, the number of articles, and the like and is applied to the surface of the goods in any desired manner.
It is difficult to machine read such information since the objects vary in size and since the information is frequently printed on adhesive labels which are applied to the article at random locations. Therefore, it cannot be assumed that the information is present at a specific location with a fixed orientation and at predetermined time intervals. Thus, the reading of such codes cannot be compared with the reading of punched cards or the like, where a card is available in a precisely defined position at precisely fixed times. In the present case, the exact opposite applies. The data field on the object appears with only rough approximation at a specific place, and the orientation of the data field is relatively arbitrary.
Such methods and apparatus for the identification of objects are used, for example, at the check-out counters of supermarkets and the like in order to identify the price and/or the number of articles which a customer wishes to buy and which he has brought to the counter for this purpose. The articles, such as boxes of varying shapes and sizes, bottles, cartons, cans, and the like, are then placed individually over an image window with the surface bearing the data field directed toward the window. The data fields on the various objects thus appear in variable orientations at differing locations within the image window. The data fields also do not appear at the scanning station at fixed time intervals. Thus, the scanning station must be able to search for the data field and, once found, must read the data track symbols in the direction of the data tracks of the field. The read symbols can then be fed to the cash register in the form of electric impulses so that the register can print out on the receipt the price, the number of the articles, its classification, etc.
The data field applied to the article includes a contrasting line pattern or product identification code ("PIC") which is formed by a plurality of parallel lines of varying spacing and/or line width. The contrasting line pattern reliably and clearly distinguishes the data field, for example the printed label, from other indicia or line patterns which may be present on the object in the vicinity of the data field. Further, within the data field the contrasting line patterns have given positions and orientations which can be used to ascertain the position and orientation of the data tracks relative to the raster lines in order to subsequently generate a raster in direction of the data tracks so that the symbols on the data tracks can be read.
German Offenlegungsschrift No. 2,338,561 discloses a method and an apparatus of the above described type wherein the identification of the contrasting line pattern occurs only when the lines of the pattern are oriented substantially perpendicularly to the scanning direction and the resulting pulse sequence of a video signal generated thereby equals a predetermined pulse sequence which corresponds to the contrasting line pattern used. Thus, the disclosed method is a correlation method. In the method and the corresponding apparatus described in the German Offenlegungsschrift the line deflection signal is a constantly and linearly ascending saw tooth signal which results in a constant scanning of the individual raster lines. The relative position of the contrasting line pattern--and thereby of the data tracks--to the raster lines is determined by storing the amplitudes of the line ramp generator and the line advance ramp generator for the point of intersection between a raster line and the contrasting line pattern, when the latter has been recognized by the PIC decoder. The detection operation is continued until at least two points of intersection A, B appear between the raster lines and the contrasting line pattern. The relative angular position between the contrasting line pattern and the raster lines can be calculated on an analog basis from the amplitude coordinates of the points of intersection.
Storage of the analog amplitude signals for the points of intersection between the raster lines and the PIC for the subsequent analog calculation of the relative position of the PIC--and of the data tracks--is cumbersome. It is also disadvantageous because in addition to the line pulse counters for identifying the PIC, suitable analog circuits are required. Finally, the stored analog amplitude values are subject to time and heat drift, and there is no assurance for an accurate correlation between the line deflection voltage, which constantly varies in time, with specific points of intersection within the line raster. Thus, the determination of an angle between the raster lines and the contrasting line pattern or the data tracks lacks accuracy.