Code reader and method of reading an optical code

Abstract

A method of reading an optical code is provided in which a brightness profile of the code is recorded, light and dark part regions are identified in the brightness profile, and the code content of the optical code is read, First sum measurements for the light quantity in the respective light part regions and second sum measurements for the light quantity lacking for a white level in the respective dark part regions are determined from the brightness profile and the code content is read based on the first and second sum measurements.

Claims

1. A method of reading an optical code in which a brightness profile of the code is recorded, light and dark part regions are identified in the brightness profile, and the code content of the optical code is read, wherein first sum measurements for the light quantity in the respective light part regions and second sum measurements for the light quantity lacking for a white level in the respective dark part regions are determined from the brightness profile and the code content is read based on the first and second sum measurements.

2. The method in accordance with claim 1, wherein the respective brightness profile is summed between two of its minima for the first sum measurements.

3. The method in accordance with claim 1, wherein the respective brightness profile is summed between two of its maxima for the second sum measurements.

4. The method in accordance with claim 1, wherein the brightness profile is inverted and is compensated by an offset corresponding to the white level to determine the second sum measurement.

5. The method in accordance with claim 1, wherein the first sum measurements and/or the second sum measurements are determined by integration.

6. The method in accordance with claim 1, wherein the code content is determined from an alternating sequence of the first sum measurements and the second sum measurements.

7. The method in accordance with claim 1, wherein the first sum measurements and the second sum measurements are classified.

8. The method in accordance with claim 7, wherein the first sum measurements and the second sum measurements are classified with reference to a reference value estimated from at least one finest sum measurement.

9. The method in accordance with claim 1, wherein the first sum measurements and the second sum measurements are divided by a reference value and then discretized.

10. The method in accordance with claim 9, wherein the reference value is estimated from at least one finest sum measurement.

11. The method in accordance with claim 1, wherein the optical code is a 2D code.

12. The method in accordance with claim 11, wherein the first sum measurements and the second measurements are determined in an environment of minima or maxima up to a brightness boundary determined by a threshold value.

13. The method in accordance with claim 1, wherein the brightness profile is divided into subsections in which respective first sum measurements and second sum measurements are determined.

14. The method in accordance with claim 13, wherein the subsections comprise rectangular subsections, rows, or columns.

15. The method in accordance with claim 13, wherein the brightness profile is divided multiple times into different subsections to determine first and second sum measurements multiple times; and wherein the results are offset or compared with one another.

16. The method in accordance with claim 15, wherein the results are offset or compared with one another by a logical link.

17. A code reader for reading optical codes that has a light receiver for detecting a brightness profile of the optical code and a control and evaluation unit that is configured to identify light part regions and dark part regions in the brightness profile and to read the code content of the optical code, wherein the control and evaluation unit is further configured to determine first sum measurements for the light quantity in the respective light part regions in the respective light part regions and second sum measurements for the light quantity lacking for a white level in the respective dark part regions from the brightness profile and to read the code content based on the first and second sum measurements.

Description

[0026] The invention will be explained in more detail in the following also with respect to further features and advantages by way of example with reference to embodiments and to the enclosed drawing. The Figures of the drawing show in:

[0027] FIG. 1 a schematic three-dimensional overview representation of the exemplary installation of a code reader above a conveyor belt on which objects having codes to be read are conveyed:

[0028] FIG. 2a an example of a barcode recorded in washed out form;

[0029] FIG. 2b an example of a barcode recorded with low resolution;

[0030] FIG. 3 an exemplary gray scale value profile for the washed out barcode in accordance with FIG. 2a;

[0031] FIG. 4 a detail of a gray scale value profile;

[0032] FIG. 5 another representation of the gray scale value profile in accordance with FIG. 4, now with first and second sum measurements entered for the respective light and dark part regions of the optical code;

[0033] FIG. 6a a further example of a barcode recorded in washed out form;

[0034] FIG. 6b the reconstruction of the barcode in accordance with 6a using the method in accordance with the invention;

[0035] FIG. 7 an example of a 2D code with some light and some dark part regions for which sum measurements are determined up to a brightness boundary;

[0036] FIG. 8 a further example of a 2D code as a starting point for the further Figures;

[0037] FIG. 9a a reconstruction of the 2D code in accordance with FIG. 8 from sum measurements after a column-wise subdivision;

[0038] FIG. 9b a reconstruction of the 2D code in accordance with FIG. 8 from sum measurements after a row-wise subdivision;

[0039] FIG. 10a the logical AND link of the reconstructions in accordance with FIGS. 9a and 9b; and

[0040] FIG. 10b the logical OR link of the reconstructions in accordance with FIGS. 9a and 9b.

[0041] FIG. 1 shows an optoelectronic code reader 10 in a preferred situation of use mounted above a conveyor belt 12 that conveys objects 14, as indicated by the arrow 16, through the detection region 18 of the code reader 10. The objects 14 bear optical codes 20 on their outer surfaces which are detected and evaluated by the code reader 10. These optical codes 20 can only be recognized by the code reader 10 when they are affixed to the upper side or at least in a manner visible from above. Differing from the representation in FIG. 1, a plurality of code readers 10 can be installed from different directions for the reading of an optical code 22 affixed somewhat to the side or to the bottom in order to permit a so-called omnireading from all directions. The arrangement of the plurality of code readers 10 to form a reading system mostly takes place as a reading tunnel in practice. This stationary use of the code reader 10 at a conveyor belt is very common in practice. The invention, however, first relates to the code reader 10 itself and to the method implemented therein for the decoding of codes so that this example may not be understood as restrictive.

[0042] The code reader 10 detects the conveyed objects 14 and their optical codes using a light receiver 24 and the corresponding brightness profiles or gray scale value profiles are further processed in a control and evaluation unit 26 to read the code content. It is not the specific detection process that is important for the invention so that the code reader 10 can be set up in accordance with any principle known per se. For example, only one row is detected in each case, whether by means of a linear image sensor or in a scanning process, with a simple light receiver such as photodiode being sufficient in the latter case. A direct attempt can be made to read the code 20 from a linear detection or the control and evaluation unit 26 assembles the rows detected in the course of the conveying movement. A larger region can already be detected in a recording using a matrix-like image sensor, with the assembly of recordings here also being possible both in the conveying direction and transversely thereto.

[0043] The code reader 10 can output information such as read codes or image data via an interface 28. It is also conceivable that the control and evaluation unit 26 is not arranged in the actual code reader 10, that is the camera shown in FIG. 1, but is rather connected as a separate control device to one or more code readers 10. The interface 28 then also serves as a connection between an internal and external control and evaluation. The control and evaluation functionality can be distributed practically as desired over internal and external modules, with the external modules also being able to be connected via a network or cloud. No further distinction is made of all this here and the control and evaluation unit 26 is understood as part of the code reader 10 independently of the specific implementation.

[0044] FIGS. 2a and 2b respectively show a recording of a problematic optical barcode. In FIG. 2a, the barcode is washed out, the boundaries between the light and dark bars, lines, or part regions are poorly defined. In FIG. 2b, the barcode has insufficient resolution and therefore appears as coarse-grained. As described in the introduction, such codes can create problems in a conventional decoding process; they are possibly not read due to the poor detection quality. The method in accordance with the invention now to be described is more robust and in many cases nevertheless able to decode the code content. The explanation initially remains with the example of barcodes, but the method in accordance with the invention can analogously be transferred to 2D codes which will then subsequently be looked at with reference to FIGS. 7-10b.

[0045] FIG. 3 shows a brightness profile or gray scale value profile of the washed out barcode in accordance with FIG. 2a. In this brightness profile, the respective brightnesses, for example the values from 0 to 255 in an 8 bit encoding of the brightness or of the gray scale value, are applied transversely to the barcode with respect to the position on a line. The consideration of a brightness profile or gray scale value profile is not a real restriction. Optical codes comprise regions in two modalities, frequently black and white colors sometimes structures or characteristics, but in any case the code information can be detected by gray scale value images. It is also not important for the understanding of the invention whether the gray scale value profile is detected as a scan of a barcode scanner, an image row of a linear image sensor, or a line drawn through a planar image of the code 20.

[0046] For better clarity, FIG. 4 again shows a smaller section of a brightness profile of a barcode with washed out modules. In the ideal case, this would be a step function that in each case abruptly changes from a minimal level to a maximum level, or vice versa, which is here marked in simplified form as black and white and is reproduced, for example, by gray scale values 0 and 255. The objective of the method in accordance with the invention is ultimately to restore this ideal state or to read the code as if this ideal state were present.

[0047] The washed out detection of the barcode could be compensated at least in part by methods known from image processing, for instance by algorithmic resharpening or an image processing filter. This is conceivable in accordance with the invention as a pre-processing step. The resharpening or filtering could, however, on the one hand, change transitions in a manner that would ultimately lead to an incorrect reading. On the other hand, artifacts would also remain after such a pre-processing as a rule.

[0048] FIG. 5 uses the brightness profile of FIG. 4 to illustrate its evaluation with the aid of sum measurements or integrals in an embodiment of the invention. Viewed energetically, it can be stated as the starting point that the same remitted light quantity is always detected by the light receiver 24 from the light part regions, light bars, or gaps of a barcode independently of the resolution or of the pixel density. In the case of artifacts such as insufficient focusing, too low a sampling rate, or motion artifacts, this light quantity is distributed as required to a plurality of pixels and is superposed with the remitted light of different part regions of the barcode. The total light quantity of the light part regions remains the same here and this means that the integral over the brightness profile does not depend on whether and to what extent the barcode is recorded as washed out or with too low a resolution. Under the assumption that the dark part regions do not remit any light, the named integral corresponds to the sum of the light of all the light part regions.

[0049] It may now further be assumed that the artifacts remain local at least to the extent that light of a light part region only illuminates the adjacent part regions, but not more remote part regions. However, then the integral over the brightness profile in the region from a dark part region to the next dark part region that comprises exactly to one light part region has to correspond exactly to the light quantity remitted by the light part region. The trick here is that this is a summary observation and the integral of specific brightness extents such as flatter or steeper flanks or of other irregularities is only influenced a little.

[0050] Corresponding first sum measurements for the light part regions are entered at the bottom in FIG. 5 by way of example. In this respect, integration took place in each case from minimum to minimum of the brightness profile. The respective area corresponding to the integral is shaded dark, with two different gray shades being used purely for illustration. It is not necessary to integrate exactly between two minima since the starting and end values of the integration are anyway in a dark part region and therefore hardly contribute to the integral.

[0051] The same idea can be transferred to the dark part regions in that the inverted brightness profile is looked at. Inversion can be understood as a mirroring at the X axis or by calculation as a multiplication by −1. To avoid the negative values that thereby arise, the maximum level can subsequently be added everywhere to the inverted brightness profile, that is, for example, the value 255 with an 8 bit encoding of the brightness values. The integration limits are in turn the minima in the inverted brightness profile and accordingly the maxima in the non-inverted brightness profile. Corresponding sum measurements for the dark part regions are entered at the top in FIG. 5. The respective area corresponding to the integral is shaded bright, with two different gray shades also being used here purely for illustration.

[0052] The assumption that light remitted from a light part region only influences the directly adjacent dark part regions can also be relaxed. A matching light portion can then also be assigned to every light part region and the principle can still be used analogously, correspondingly for the dark part regions. The integral boundaries do not therefore necessarily have to be fixed to directly adjacent minima or maxima.

[0053] The result of the described evaluation is a plurality of first sum measurements and second sum measurements, in each case a first sum measurement per light part region and a second sum measurement per dark part region of the barcode. These sum measurements can now be alternatingly combined to form a sequence. With a homogeneous illumination of the barcode, the sum measurements for each part region having an extent corresponding to a specific multiple proportional to the module size It must be remembered here that the module size describes the extent of the smallest bar of the barcode and the extent of all the bars amounts to a respective multiple of the module size is the same. Decoding can be understood as a classification that seeks these multiples since the code information is actually located therein. Instead of measuring the width of the bars as customary, in accordance with the invention the sum measurements or integrals are used that likewise include the information required for a decoding in accordance with the above statements.

[0054] In the example of FIG. 5, the following alternating sequence of integrals or sum measurements results, read alternately from the top and the bottom: 1580, 800, 2400, 950, 800, 980, 2500, 2430, 1620, 1700, 900, 2370, 1400, 850, 750, 1600. With a perfect invariance of the integrals, this would be multiples of a largest common divisor that can serve as a reference value and that is in turn proportional to the module size. There is only a proportionality relationship and not any identity with the module size because sum measurements or integrals of the part regions were determined and not widths.

[0055] Any desired classifiers can be used to classify the example sequence. There are recognizably large differences between the sum measurement of bars of different widths so that the classification work becomes a lot easier and more robust to solve than, for example, with widths that are subject to great fluctuation due to washed out edges. The smallest sum measurement could, for example, be selected as the reference value or the i smallest sum measurements are averaged. i here should be small enough to not dilute the average value by an integral from a wider bar.

[0056] The sequence 2, 1, 3, 1, 1, 1, 3, 3, 2, 2, 1, 1, 3, 2, 1, 1, 2 results from the above exemplary sequence using such a classification that can also be understood as a standardization or discretization. This is the decoding result in accordance with the invention that is actually correct for the section of the barcode shown in FIGS. 4 and 5.

[0057] FIGS. 6a-b illustrate this again for a complete exemplary code. FIG. 6a shows the original, washed out recording of the barcode while 6b shows a restored barcode corresponding to the decoding from sum measurements or integrals. It is not important here that the bars are shown as sharp in FIG. 6b, this is due to the restoration, but rather that the bar widths exactly reproduce the washed out barcode in FIG. 6a. In conventional decoding methods, there would here be differences from the real code content due to which the barcode could not possibly be read at all.

[0058] FIG. 7 shows an exemplary 2D code to which the method previously explained for barcodes will now be applied or expanded. The detected gray scale values of the 2D code now form a two-dimensional brightness profile. Two exemplary maxima in light part regions are marked by white dots and two exemplary minima in dark part regions are marked by black dots. A respective associated white line or black line indicates a matching integration boundary. It lies where, starting from a white dot, a brightness threshold is respectively fallen below for the first time or, starting from a black dot, is exceeded for the first time. Two different brightness thresholds can be set for light part regions and dark part regions, the brightness thresholds can in particular be selected as very low or very high and can thus increase the robustness. A maximum is then surrounded by a dark edge sequence and a minimum by a light edge sequence.

[0059] There is also a module size as a smallest extent of a code element in a 2D code, but it now applies in two dimensions and thus defines a least square. A determination can be made via the sum measurements as to how many of these least squares form a light part region or a dark part region respectively. In contrast to a one-dimensional case, this is, however, alone not sufficient for decoding since the arrangement in, for example, a long row, a compact block, or any intermediate shape is not unambiguous without additional spatial information. The summary information on the number of smallest squares per part region can be decisive as an additional information source of whether a code is still read, for example to parameterize or plausibilize a different decoding process.

[0060] To enrich the summary information and provide a greater spatial reference, 2D brightness profiles can be subdivided and an embodiment of the method in accordance with the invention is respectively applied to the subregions of the subdivisions. The divisor results are then subsequently assembled. Examples for such subdivisions are rectangles with n×m pixels, in particular selected in dependence on an estimated module size, and rows, columns, or slanted lines. Especially the subdivision into rows or columns has the advantage that the method described for barcodes can be directly applied thereto. It is conceivable to apply the method in accordance with the invention multiple times with different subdivisions and to link the results.

[0061] FIG. 8 shows a data matrix code recorded as washed out as an arbitrary representative of possible 2D codes. Advantageous embodiments should be illustrated by way of example for this example.

[0062] In FIG. 9a, subdivisions or integrations are carried out column-wise, that is the method explained above with respect to barcodes is applied in every column. In FIG. 9b, subdivision or integration was carried out row-wise. The results differ; they are dependent on the selected subdivision. Respective classification or decoding attempts on both results are one possibility of dealing with this. It is furthermore conceivable to prepare the results by image processing algorithms.

[0063] FIGS. 10a and 10b illustrate a further possibility in which previous results are logically linked. The column integration image of FIG. 9a and the row integral image of FIG. 9b are specifically logically AND linked here by way of example and are logically OR linked in FIG. 10b. The direction dependence can be resolved in this manner and the result further improved.