Evaluation of projection pattern for transitions in pattern...

Radiant energy – Photocells; circuits and apparatus – With circuit for evaluating a web – strand – strip – or sheet

Reexamination Certificate

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C356S610000

Reexamination Certificate

active

06495848

ABSTRACT:

The present invention concerns a process for detecting the spatial structure of a three-dimensional surface by projection of a pattern on to the surface along a projection direction which defines a first axis, and by pixel-wise detection of at least one region of the pattern projected on to the surface, by means of one or more sensors in a viewing direction of the sensor or sensors, which defines a second axis, wherein the first and the second axes (or a straight line parallel to the second axis) intersect at an angle different from 0° so that the first and the second axes (or the straight line parallel thereto) define a triangulation plane, wherein the pattern is defined at least upon projection into a plane perpendicularly to the first axis by a varying physical parameter which can be detected by the sensor (sensors), and wherein the pattern is such that the difference in the physically measurable parameter, measured between predeterminable image pixels or pixel groups, along a predeterminable pixel row which is preferably parallel to the triangulation plane, assumes at least two different values.
Such a process is known for example from U.S. Pat. No. 5 615 003.
The pattern which is specifically disclosed in that patent specification has a very high degree of similarity to bar codes as have already long been in use for example in the retail trade for identifying products and prices, wherein the sequence of lines or bars of different thicknesses defines a series of binary numbers which is clearly associated for example with a product and its price. For the purposes of detecting the spatial structure of a three-dimensional surface however the bar widths and the spacings between bars of that kind are distorted according to the respective specific structure of the surface involved and the known process is distinguished by devices and measures which, in spite of the distortion effect, permit identification of given regions of the bar pattern so that the first distorted pattern can be adapted to the projected pattern, wherein the parameters of such adaptation afford the structure of the surface which produces the above-mentioned distortion effects.
Another similar process which uses an encoded pattern is known from German patent application No 196 38 727, in which respect that process is not limited to encoding of a pattern only in one direction but is composed of individual distinguishable pattern elements each of similar respective structure and size, wherein the pattern elements also differ alternately from each other in mutually perpendicular directions. An encoded pattern is characterised in that at least portions of the pattern which are of a given minimum size are not repeated at any point in the entire pattern surface. Even if that requirement is not satisfied in the strict sense, this means that repetitions of pattern structures can certainly also occur as long as those repetitions are only sufficiently far apart on the pattern surface that, upon evaluation of an imaged pattern structure, there is practically no possibility of confusion between two randomly identical regions of the pattern.
All previously known processes however basically operate on the principle of pattern recognition, that is to say imaging, implemented by calculation, of the flat pattern on to the distorted pattern of which the image is formed from the viewing direction on a non-flat surface. Problems always occur in that respect however when the information content of the projected pattern element can vary in the imaging procedure, depending on the spatial extent or other configuration of the surface. If for example the physical parameter which is to be measured and which varies and by which the pattern structure is defined includes the color of the pattern or of individual pattern elements, then interpretation of color information in the projected pattern can be interfered with or even made impossible, by virtue of the surface being colored in nature. When using different line widths for a pattern the line width in the image depends not only on inclinations of the surface in a direction parallel to the triangulation plane but also inclinations of the object or the surface with respect to a line normal to the triangulation plane. This means that, upon the projection of lines of different width, by virtue of the spatial extent of the surface being of a disadvantageous configuration, for example a narrow line in the image is represented in a width which is of the order of magnitude of the image of a line which is wide in the projected pattern, and accordingly can result in an incorrect item of information in the evaluation procedure.
In addition the evaluation of encoded patterns involving a differing geometrical configuration of pattern elements imposes very high demands on the evaluating data processing procedure as the geometry of the projected pattern elements can recur in severely deformed condition in the image.
In comparison with the above-depicted state of the art, the object of the present invention is to provide a process for detecting the spatial structure of a three-dimensional surface, which is substantially independent of different inclinations with respect to a line normal to the triangulation plane and also independent of the other surface properties which independently of the spatial extent of the surface can influence the physical parameter to be measured.
That object is attained in that, for the purposes of evaluation of the imaged pattern structure only changes in the physical parameter between the predetermined pixels or predetermined pixel groups of one or more pixel rows which are preferably parallel to the triangulation plane are detected and converted into spatial co-ordinates of the surface.
In that respect the changes in the physical parameter are detected at least in the component thereof parallel to the triangulation plane and converted into spatial co-ordinates.
The fact that only changes in the physical measurement parameter of the pattern, which is to be detected, are detected, preferably between adjacent pixels or adjacent pixel groups, means that specific surface properties are so-to-speak eliminated as the surface regions which are imaged or reproduced in particular by adjacent pixels generally have the same properties. The inclination of the surface with respect to a line normal to the triangulation plane also has no effect on detection and possibly identification of the pattern structure as it is not the pattern that is detected in respect of its specific dimensions and shapes and compared to the original projection pattern and adapted thereto, but only the values of the physical parameter which is of interest, being measured in predetermined preferably adjacent pixels or pixel groups, are compared to each other, more specifically by forming the difference thereof. In that case the specifically imaged pattern is then produced simply on the basis of characteristic changes in the physical parameter from given pixels to other predetermined pixels, in particular their adjacent pixels, or from given pixel groups to other predetermined and preferably adjacent pixel groups, or along another suitable sequence of pixels which are to be compared together.
As in accordance with the invention the change in the values, which are detected or reproduced in the image pixels, of the specifically evaluated physical parameter, can assume at least two different values, those differing values in respect of the change, which can occur from one pixel to another or from one pixel group to another, make it possible to define and recognise structures without the specific properties of the surface playing a part in that respect.
Desirably the pattern is so oriented that, in a predetermined measurement direction along which pixels or pixel groups are compared to each other, there are change values which are at a maximum in respect of amount at least in part while the other change values which occur are comparatively small in respect of amount or are of the value zero.
In a preferred variant of

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