EP2376866A1

EP2376866A1 - Calibration standard for an image processing system

Info

Publication number: EP2376866A1
Application number: EP09808929A
Authority: EP
Inventors: Peter Schimitzek
Original assignee: CSB System AG
Current assignee: CSB System AG
Priority date: 2008-12-09
Filing date: 2009-12-04
Publication date: 2011-10-19
Also published as: DE112009003324A5; WO2010066236A1; DE202008016206U1; DE102009050604A1

Abstract

A calibration standard for an image processing system as an optical measuring device is described, which ensures high accuracy and the correct determination of brightness and/or tristimulus values for two-dimensional measurements. According to the invention, the calibration standard comprises at least four dimensional circles (4.1; 4.2; 4.3; 4.4) on a dimensionally stable planar surface (1).

Description

Calibration standard for an image processing system

The invention relates to a calibration standard for an image processing system as an optical measuring device.

A conventional image processing system as an optical measuring device essentially comprises an electronic camera, a computer (electronic data processing system), such as a computer with operating system, an imaging system and a software for image acquisition and evaluation.

To obtain accurate measurement results by means of the image processing system, it is necessary to perform a calibration of the imaging system of the EDP.

Various methods for calibrating optical measuring devices with a camera are known from the prior art, which use regularly arranged geometric structures such as strips, dots or checkerboard patterns on flat surfaces or in a three-dimensional arrangement.

From document DE 103 32 161 A1 a calibration body and calibration method for a camera system is known, which uses a calibration body on a flat plate with a regular two-dimensional pattern. The regularity is defined by the intersections (halftone dots) that form in a system of intersecting flocks of parallel lines. Within at least one of these first geometric objects, at least one further two-dimensional geometric object is arranged whose center of gravity does not coincide with the geometric center of gravity of the first geometric object coincides, thus outputting directional information.

The first geometric object is basically a checkerboard pattern and the second object can be circles or rectangles.

Document DE 10 2006 055 758 A1 discloses a method for calibrating cameras and projectors, wherein for geometric calibration a known plane checkerboard pattern is marked, in which at least one of the crossing points is marked with a position indication readable by image processing of the images of the checkerboard pattern Orientation of the location of the crossing point in the form of bright points in dark fields and dark points in bright fields within the checkerboard pattern is used.

The size of the fields and thus the distance of the edges is known and is used for calibration.

Another method for calibrating an image processing system is described in the publication DE 42 18 971 C2, which uses, among other things, a calibration body with sample fields for radiometric, geometric and colorimetric calibration.

For geometrical calibration, pattern fields of lines and / or structure patterns with periodic light / dark structures and colorimetric calibration color patterns with at least four color references are used in the field of view of the image sensor.

The image processing system is calibrated for the optical testing of all or part of existing samples of wood, wherein the calibration body has approximately the cross-sectional shape of the specimens. This method can therefore only be used if the test object has a similar shape as the calibration body.

The object of the invention is to develop a calibration standard for an image processing system as an optical measuring device, which ensures high accuracy and the correct determination of brightness and / or color values for two-dimensional measurements.

The object is solved by the features listed in claim 1. Preferred developments emerge from the subclaims.

A calibration standard according to the invention has at least four dimensional circles and at least two color rows on a dimensionally stable, flat surface.

The color series consist of at least four color fields with different colors.

In addition to the color series or alternatively, the calibration standard includes at least two gray scale wedges.

The color rows and / or gray levels are arranged so that their gradation is opposite.

Furthermore, at least one color field, which corresponds to the color of a background of the optical recording, can be arranged on the surface.

The individual colors of the color series originate from a standardized color system, such as the Pantone Matching System, the RAL Color system, the CIE (Commission International d'Elairage) - standard color system or the RGB color system.

On the basis of the brightness values predefined with the color fields and / or the greyscale wedge, the determination of measurement conditions and the comparison with the imaging system of the EDP, in particular of a monitor, in the sense of a white balance take place during the calibration procedure in order to normalize the measurement conditions.

There is thus an adaptation to the ambient brightness and the illumination devices for the field of optical recording.

The lighting devices are aligned in the course of the adjustments for a uniform, optimal illumination of the recording area.

The measuring circles can be configured as full circles of defined diameter with lines drawn over the diameter vertically, horizontally and over both diagonals, the center of the measuring circles resulting from the center of gravity of the full circle.

Alternatively, the graduated circles consist of an arrangement of points whose position corresponds to the points of intersection of the lines in the full circle, which are drawn vertically over the diameter in the full circle, horizontally and over both diagonals.

The center of the circle has a marker. During calibration, the lengths of the vertical, horizontal and both diagonal lines are determined as the diameter of the circle.

In the alternative design of the measurement circles from an arrangement of points, the distance between two diametrically arranged points in vertical, horizontal and diagonal alignment is determined during calibration.

The high symmetry of the points of the respective measuring circle makes it possible to determine the distances with an accuracy in the subpixel range with simple statistical methods.

By measuring all lines or the corresponding distances of the points during calibration, potentially occurring measurement errors are further reduced, whereby the center point of the dimensional circles results from the center of gravity of all points on the dimensional circle.

Thus, the parameters of the camera system are directly included in the calibration, whereby optical distortions in the recording are also taken into account by means of the known distances of diagonals of the dimensional circles and of the diagonals between the centers of the dimensional circles.

The diameter of the measuring circles is chosen so that it corresponds to a maximum distance between two boundary lines, such as the thickness of a layer to be measured.

The position of the measuring circles is selected so that their distance from each other is an integer multiple of their diameter. The distances of the dimensional circles to each other are chosen so that they are greater than in the recording area maximum to be measured distances.

During calibration, the distances between two neighboring dimensional circles are determined in each case in a horizontal, orthogonal and diagonal direction.

In one variant, at least two color rows, at least two gray scale wedges, a color field corresponding to the background color and at least four dimensional circles are arranged on the dimensionally stable planar surface.

The color rows are preferably horizontal and the gray scale wedges vertically and arranged in opposite directions.

The calibration standard is preferably realized by means of screen or digital printing on known multilayer compact disks with an aluminum core, as a dimensionally stable, flat surface with a low coefficient of thermal expansion.

These compact plates can additionally be equipped with an antibacterial coating, so that the use in food processing plants is hygienically safe.

For calibration of the image processing system as an optical measuring device, the calibration standard is positioned at the pickup position instead of an object with details to be measured.

In a known manner, the calibration is carried out by making an optical recording of the calibration standard, this on the An imaging system of the EDP, in particular a monitor reproduced on the monitor, the pixel spacing of the imaged dimensional circles in horizontal, vertical and diagonal direction to each other and the lengths of the drawn lines or the distances of the corresponding points with the known defined lengths and distances is determined and this be correlated with each other.

Furthermore, there is a common adjustment of both the brightness and the colors displayed on the monitor with the existing on the calibration standard.

The calibration standard can be used in particular for the calibration of systems for automatic evaluation and classification as well as the determination of qualities and quantities of carcasses by means of optical image processing.

Typically, this is done by an optical recording in the cleavage plane, as in the region of the ham and lumbar region of the hanging on a hook carcass half, with the help of an electronic camera.

The created digital image is subjected in a known manner with the aid of a computerized image analysis, in which contours of meat and fat tissue and bone are detected. On the basis of the contour curves, individual distances are measured photogrammetrically, distances and areas averaged over contour areas, and optionally angles.

In addition, brightness and / or color values are obtained for quality rating. For calibrating the image processing system as an optical measuring device, the calibration standard is positioned at the receiving position instead of the slaughter animal half hanging from the hook on a tube track.

In the simplest case, this is done by means of hooks on the compact plate, which are mounted in the tube path and there existing guide tube.

With the calibration standard, measurement results are obtained whose deviations in the arithmetic mean and standard deviations of the individual deviations are below the values required in the calibration order.

The invention will be described as an embodiment of hand

Fig. 1 as a schematic representation of a calibration standard with four measuring circuits and

Fig. 2 as a schematic representation of a calibration standard with eight dimensional circuits explained in more detail.

1 shows a calibration standard on a dimensionally stable flat surface 1, preferably with a matt white surface, two color rows 2 (2.1, 2.2), two gray scale wedges 3 (3.1, 3.2) and four dimension circles 4 (4.1, 4.2, 4.3, 4.4). on.

On the flat surface 1, a color field 5 is additionally arranged, whose color, for example blue, corresponds to that of a background for the optical recording of the object to be measured. The color series 2.1; 2.2 have four color fields whose colors correspond to the essential colors to be distinguished occurring in the image area of the optical image to be measured.

For the quality classification of carcasses, the colors from a bright orange to a dark red are of importance in order to be able to assess the proportions of skin, fat and different muscle tissues.

The gradations of the color series 2 (2.1, 2.2) and the gray scale wedges 3 (3.1, 3.2) are preferably arranged in opposite directions, so that potential color distortions and differences in brightness can be reduced by adjusting the illumination system and distortions are taken into account by the camera system during calibration.

The diameter of the measuring circles 4 (4.1, 4.2, 4.3, 4.4) is selected so that it corresponds to a maximum occurring value of a significant detail to be measured in the image area.

For measurements in the intake of the ham and loin region of half of a pig, the diameter of the circles 4 (4.1, 4.2, 4.3, 4.4) corresponds in particular to the maximum thickness of the fat.

The distance of the rectangularly arranged measuring circles 4 (4.1, 4.2, 4.3, 4.4) from each other is in each case an integer multiple of the diameter.

In Fig. 1, a horizontal distance is shown, the threefold, a vertical distance four times and a di agonal distance is five times the chosen diameter of the measuring circles 4 (4.1, 4.2 / 4.3, 4.4).

The distances of the measuring circles 4 (4.1, 4.2, 4.3, 4.4) from one another are thus greater than the maximum lengths of the measurement to be measured in the recording area of the electronic camera, such as a maximum muscle thickness of the pig.

In the dimensional circles 4 (4.1; 4.2; 4.3; 4.4) shown as full circles, as shown on the measuring circle 4.2, a vertical 6, a horizontal 7 and diagonal 8 (8.1; 8.2) are drawn over the diameter.

The vertical 6, the diagonal 8 (8.1, 8.2) and the horizontal 7 subdivide the measuring circles 4 (4.1, 4.2, 4.3, 4.4) into sections at an angle of 45 degrees.

These angles are used to calibrate the measuring system for angle measurement.

In the illustration of a variant of the calibration standard of FIG. 2 are on the dimensionally stable flat surface 1, the two color series 2.1; 2.2, two grayscale wedges 3.1; 3.2, which are divided in the middle, and in addition to the measuring circles 4.1; 4.2; 4.3; 4.4 according to Fig. 1, two outer Maßkreise 4.5 and 4.6 and two inner Maßkreise 4.7 and 4.8 arranged.

The respective distance of the measuring circuits 4.1; 4.2; 4.5; 4.6; 4.3 and 4.4 to each other is the same and corresponds to an integer multiple of their diameter. The measuring circuits 4.1; 4.2, 4.5 and 4.6 and 4.5; 4.6; 4.3 and 4.4 are arranged like a square.

In the center of each square of the scale circle is 4.7 or 4.8 arranged.

The measuring circles 4 (4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8) are not designed as a full circle, but, as shown again on the measuring circle 4.2, at the points of intersection of the verticals 6 shown in FIG , the horizontal 7 and the diagonal 8 (8.1; 8.2) with the full circle, points arranged.

The center of each circle 4 (4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8) has a marker 9 for its identification.

The calibration standard of FIG. 2 is used to calibrate the image processing system as an optical measuring device when greater lengths and distances of an object or detail from an object are to be determined with high accuracy.

Furthermore, the calibration standard of Figure 2 with the square and arranged in the center of the square, the circles 4.1; 4.2, 4.5; 4.6 and dimension circle 4.7 and 4.5; 4.6; 4.3; 4.4 and 4.8, by adding one or more squares, depending on lengths and distances to be determined, by horizontally mirroring them over the centers of two dimensional circles such as 4.1 and 4.2 or / and 4.3 and 4.4. The calibration standard can also be expanded by means of vertical mirroring via the measuring circles 4.1, 4.5 and 4.3 or / and 4.2, 4.6 and 4.4.

A combination of both aforementioned reflections is also possible.

With the number of measuring circles 4 (4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8) and their diameters, the adaptation to the measuring task is performed to determine the lengths and distances as well as the accuracy.

Used reference signs

1 level surface

2 color series (2.1, 2.2)

3 grayscale wedge (3.1, 3.2)

4 measuring circuit (4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8)

5 color field

6 vertical

7 horizontal

8 diagonal (8.1, 8.2)

9 markers

Claims

Patent claims

1. Calibration standard for an image processing system, essentially comprising an electronic camera, a computer with an operating system, an imaging system, such as a monitor, and software for image capture and evaluation, as an optical measuring device for two-dimensional measurements and for determining brightness and/or or color values of an object, characterized in that the calibration standard has at least four measuring circles (4.1; 4.2; 4.3; 4.4) on a dimensionally stable flat surface (1).

2. Calibration standard according to claim 1, characterized in that the calibration standard additionally has at least two color rows (2.1; 2.2) each with at least four color fields with different colors.

3. Calibration standard according to claim 1, characterized in that the calibration standard additionally has at least two gray scale wedges (3.1; 3.2).

4. Calibration standard according to one of the preceding claims, characterized in that at least one color field (5) with the color of a background for recording the object with the electronic camera is arranged on the dimensionally stable flat surface (1).

5. Calibration standard according to one of the preceding claims, characterized in that two of the gray scale wedges (3.1; 3.2) arranged on the surface (1) or the different colors of color rows (2.1; 2.2) have opposite gradations.

6. Calibration standard according to one of the preceding claims, characterized in that the measuring circles (4.1; 4.2; 4.3; 4.4) are designed as full circles of defined diameter and each have a vertical (6), horizontal (7) and two to these drawn over the diameter has diagonals (8.1; 8.2) arranged at an angle of 45 degrees.

7. Calibration standard according to one of the preceding claims, characterized in that the diameter of the measuring circle (4.1; 4.2; 4.3; 4.4) is selected so that it corresponds to a maximum value that occurs of an essential detail to be measured in the image area.

8. Calibration standard according to one of the preceding claims, characterized in that the measuring circles (4.1; 4.2; 4.3; 4.4) are arranged in a rectangular shape with a distance from one another that is an integer multiple of the diameter.

9. Calibration standard according to claim 1 and one of claims 2 to 8, characterized in that the distances between the measuring circles (4.1; 4.2; 4.3; 4.4) are greater than a maximum distance to be measured in the recording area of the electronic camera.

10. Calibration standard according to one of the preceding claims, characterized in that, in addition to the measuring circles (4.1; 4.2; 4.3; 4.4), two outer measuring circles (4.5; 4.6) and two inner measuring circles (4.7; 4.8) are arranged on the surface (1). are.

11. Calibration standard according to one of the preceding claims, characterized in that the measuring circles (4.1; 4.2, 4.5) and (4.6) and (4.5;

4.6; 4.3) and (4.4) are arranged in a square shape and with a measuring circle at the center of the respective square

(4.7) or (4.8) is arranged.

12. Calibration standard according to one of the preceding claims, characterized in that the measuring circles (4.1; 4.2; 4.3; 4.4; 4.5; 4.6; 4.7; 4.8) consist of an arrangement of points which are at the intersections of the verticals originally placed over the diameter (6), the horizontal (7) and the diagonals (8.1; 8.2) are arranged with the full circle and a center point of each measuring circle (4.1; 4.2; 4.3; 4.4; 4.5; 4.6; 4.7; 4.8) is marked by means of a marker (9 ) is marked.

13. Calibration standard according to one of the preceding claims, characterized in that the calibration standard is expanded by means of horizontal reflection over the center points of two measuring circles, such as (4.1 and 4.2) and/or (4.3 and 4.4).

14. Calibration standard according to one of the preceding claims, characterized in that the calibration standard is expanded by means of vertical mirroring over the measuring circles (4.1, 4.5 and 4.3) and/or (4.2, 4.6 and 4.4).

15. Calibration standard according to one of the preceding claims, characterized in that the adaptation to the measuring task for measuring route lengths and distances with the number of measuring circles (4.1; 4.2; 4.3; 4.4; 4.5; 4.6; 4.7; 4.8) is present and with their Diameter the accuracy is determined.