Image analysis – Applications – Document or print quality inspection
Reexamination Certificate
1997-03-17
2001-10-09
Chang, Jon (Department: 2623)
Image analysis
Applications
Document or print quality inspection
C382S162000
Reexamination Certificate
active
06301374
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for automatically checking the printing quality of a multicolor image by means of at least one optoelectronic device enabling one signal Si per chromatic channel to be obtained.
PRIOR ART
Methods and installations for automatically checking the printing quality of a multicolor image have, more especially, but not exclusively, been developed for checking the printing quality of security papers, such as bank notes or fiduciary papers. The various methods and installations for automatically checking the printing quality do so by comparing pixel by pixel or a set of pixels of certain characteristic parts of an image with a reference image. The image to be checked is captured by a system of cameras allowing one capture per chromatic channel and these results are compared with the results of the capture of a reference image. Part of an image is considered to be defective when the densitometric value of a pixel in the chromatic components departs from the model which has a certain predetermined value and which essentially depends on the degree of printing quality desired.
The methods and devices for automatically checking the quality of color printing obviously give superior results to the results obtained using monochromatic systems. Nevertheless, the volume of data to be captured and checked is much greater than is the case in monochromatic checking, thereby making the operation expensive. If it is desired to obtain the same speed as achieved when carrying out monochromatic quality checking, the devices used must be powerful, which increases their cost. Thus, for multicolor checking, for example for the three base colors of red, green and blue, the number of channels is multiplied by three and the operations performed are also multiplied by three compared with monochromatic inspection.
SUMMARY OF THE INVENTION
The object of the present invention is to allow automatic quality control of a multicolor image, but by substantially reducing the cost without this decrease impairing the capability of chromatic detection of defects in the image to be checked.
Another object is to increase the defect detection capacity compared to conventional multicolor systems.
The method according to the invention is one in which the signals Si obtained for the same image or part of the image are combined so as to obtain a single signal which will be delivered to a device for automatically checking the printing quality for each image or part of the image with respect to a reference image or part of a reference image, the combination of said signals being a function F, on the one hand, of the values of signals Si from each chromatic channel and, on the other hand, of the value Si
0
from a reference image or a corresponding part of a reference image, said function having the purpose of maximizing the detectability of differences between the checked image and the reference image.
The advantages of the method according to the invention are that, although a multicolor image is being checked, the signal used to do the actual checking, that is to say the comparison with the reference image, uses a single channel since the signal in question consists of a function of each of the chromatic channels, making it possible to amplify the detectability of the differences in each of the values captured with respect to the corresponding value of a reference image.
The method thus defined by the present invention makes it possible, on the one hand, to decrease the cost of processing the multi-channel signal and, on the other hand, not to decrease the detectability of the magnitude of the chromatic defects which might be present in one or other of the chromatic channels by a judicious choice of the function and of the coefficients.
In the same way, by judiciously choosing the coefficients, it is possible to amplify the chromatic response within a more significant band for that part of the image being inspected.
According to one embodiment of the invention, the coefficients are defined automatically, for example during capture of the reference image.
According to another embodiment, the coefficients are determined by the operator.
According to another embodiment, that part of the image to which the matrix of coefficients corresponds may be of the order of one pixel.
According to another embodiment, the chosen function is defined according to an approximation of the human eye's response to differences in color.
According to another embodiment, the function F is decomposed into a set of partial functions applied to some of the chromatic signals.
According to another embodiment, it is possible to define more than one matrix of coefficients for each part to be checked in order to take into account acceptable variations with respect to the reference image.
The invention also relates to an installation for implementing the method.
The installation comprises one capture device per chromatic channel of the image to be checked, a device for storing coefficients in memory, a device for producing the function and a device for processing the single signal resulting from the function F in order to compare it with the signal corresponding to the reference image.
According to a preferred embodiment, the device which makes it possible to produce the function is composed of at least one look-up table.
The image capture device may be either a matrix camera or a linear camera.
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Bugnion S.A.
Chang Jon
De La Rue Giori S. A.
Moetteli John
Wu Jingge
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