MEANS FOR MEASURING PICTURE QUALITY USING OPTICAL PATTERN RECOGNITION

Abstract: The invention relates to means for measuring picture quality of various types of visual pictures and documents using optical pattern recognition. The means comprises an optical recognition equipment with adapted recognition sensitivity and an algorithm for calculation and analysis of error frequencies, together with a test pattern. The invention enables measuring an unknown test object, a picture or a document, using e.g. an OCR program with adapted recognition sensitivity in a reliable way with respect to readability and resolution.

Description: APPLICANT: TELEVERKET TITLE OF INVENTION: MEANS FOR MEASURING PICTURE QUALITY USING OPTICAL PATTERN RECOGNITION FIELD OF THE INVENTION The present invention relates to means for objective measurement of picture quality and, more specifically, means for characterizing picture quality of various types of visual pictures and documents using optical pattern recognition. The expression characterization of picture quality means that the picture quality is described with objective measures, such as the likelihood of a person confusing an in¬ tended character with another character within a character set system, the degree of accuracy in reproducing a grey scale gradient, the signal/noise ratio, MTFA (Modulation Transfer Function Area) value, the likelihood of considering the overall picture quality of a given application accept¬ able, etc. The term visual pictures and documents relates to visual information presented on information carriers in the form of e.g. paper documents, films, electrooptical displays. The term optical pattern recognition relates to an electrooptical system designed to determine automatically the presence of specific picture elements. STATE OF THE ART Examples of known prior art of this type is optical character recognition (OCR) , which is designed to recognize automatically lphanumerical characters (combinations of horizontal, vertical and slanted lines, closed and open curves, etc.). If the character in question is not alpha- numerical (but e.g. consists of EAN codes) this method is usually called Optical Mark Recognition (OMR) . In certain applications the pattern recognition may be "absolute" in that the pattern in question is standardized, which is the case in traditional OCR for alphanumerical characters. In other applications it may be required that the pattern recognition is "relative", which means that the pattern recognition is relative to an ideal test picture specially designed for the application. The OCR and OMR technique is based on a computer program designed to optimize the likelihood of classifying correctly an input character. This automatic character recognition re¬ quires that the input characters are of a certain miniumum quality. They are not intended for operation with texts of lower quality, since it has been found that if the recogni¬ tion level is lower than approximately 90-95 % a too great manual supplementary work is required and, therefore, it is better to perform the whole character recognition (reading) manually. So far, no one has used this technique of optical recog¬ nition to develop this for a measuring device for picture quality. No one has used the recognition degree, which may vary between 0 and 100 %, as an objective measure of picture quality. No one has used a test pattern containing a text and a graphic picture, as a basis for OCR/OMR measurement of picture quality. SUMMARY OF THE INVENTION The present invention provides means for objective measurement of picture quality comprising, on one hand, an optical recognition equipment, and on the other hand a test pattern. The recognition equipment (e.g. a scanner with OCR/OMR software) has a calibrated recognition sensitivity and an algorithm for calculation and analysis of error fre- quencies. Further embodiments are set forth in the accom¬ panying claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail with refe- rence to the accompanying drawings, in which: Fig. 1 shows a test pattern for facsimile printouts; Fig. 2 shows an example of a set of test patterns; and Fig. 3 shows an example of an analysis of misinterpreta¬ tions. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION The present invention provides means for objective measurement of picture quality, mainly comprising an optical recognition equipment and test patterns. The optical recogni¬ tion equipment, e.g. a scanner with OCR/OMR software, has a calibrated recognition sensitivity and an algorithm for calculation and analysis of error frequencies. The recognition degree is high when the picture quality is high. The recognition degree should be sufficiently low when the picture quality is low. The test patterns contain information in various directions such that the picture quality can be assessed in its entirety. The following examples illustrate the construction and the use of the invention. Example 1. In microfilming it is common that the first picture frame is a photograph of a standardized test picture. After development the photographed test picture is enlarged and it is examined with respect to its reproduction in an acceptable way in the filming process. Using the present invention the quality control may be performed automatically and in an objective way. Example 2. Scanning a document for optical storage re¬ quires that the scanning equipment continuously is adjusted to enable optimal recording of e.g. lead pencil writing mixed with ballpoint pen writing and typewritten text. Using the present invention the quality control can be performed directly and automatically control the scanning equipment setting parameters to obtain optimal picture quality. Example 3. In transmitting still pictures over the telephone network a picture telephone user using the present invention can establish if the picture quality of the document display equipment will be acceptable with ISDN coded transmission. The transmitting party can print using a video printer before a transmission a test pattern reproduction "frozen" in the document camera. Then the printout is input in the pattern recognition equipment in which it is scanned and characterized in accordance with the software selected, e.g. "57 % recognition degree in ISDN transmission of text fac X r siz " Example 4. Telefax printouts may have different resolu¬ tion in the horizontal and vertical direction. In order to characterize the resolution of telefax printouts an appli¬ cable test pattern is needed containing spatial information in both directions, as is shown in Fig. 1. To characterize readability of telefax printouts, on one hand, an adapted recognition sensitivity is required, and, on the other hand, an algorithm for prognostic calculation and analysis of mis¬ interpreted elements. Calibration of sensitivity and recogni- tion degree is in this case performed by means of a set of test patterns with well-defined physical test results, as is shown in Fig. 2. The test patterns made with precision in laboratory en¬ vironment, consist of a series of pictures having one and the same subject, e.g. a picture or a text, having varying picture quality. Each test pattern of the series is characte¬ rized, on one hand, by an objective picture quality measure, e.g. MTFA (Modulation Transfer Function Area), and on the other hand, by a subjective measure expressed as quality judgements as e.g. "excellent", "good", "acceptable", "poor" or "very poor". By means of a series of such test patterns a picture produced by means of any reproduction medium can be graded with respect to objective as well as subjective picture quality. The prognostic analysis of the misinterpretation may appear as in Fig. 3, showing error frequencies in a read¬ ability test of a standard picture (Tele Nova Compis; normal face) and a test picture (IBM PC Color; bold face) . The total average includes data of all 70 characters, but in the figure only values are shown for characters having an average error frequency greater than 3 % (100 % = 25 errors) . Example 5. The picture quality of a closed picture com¬ munication system can be monitored using the present inven¬ tion and tested by the transmission side periodically trans- mitting a test picture known to the receiving side, which picture is analyzed there providing quality judgements to the receiving side. The same principle can be used in networks by several users and in open systems provided that the user collective has an internal standardized test pattern and shake" is obtained if the picture quality analysis is com¬ bined with automatic choice of e.g. channel band width in the ISDN network. Thus, the present invention provides means automatically achieving an objective measurement of the quality of a picture or text. Specifically, values of the quality para¬ meters readability and resolution are obtained. A measurement of the likelihood of misinterpreting a picture is obtained. A person skilled in the art appreciates that the inven- tion may be applied in several different ways. The examples above are not a complete recitation. The invention is only limited by the claims below.

Claims: CLAIMS 1. Means for objective measurement of picture quality of reproduction media using optical pattern recognition, characterized in that a pattern recognition equipment to- gether with a test pattern are used to objectively measure and read the quality parameters readability and resolution of a picture or text. 2. Means according to claim 1, characterized in that measurement and characterization of the quality parameters readability and resolution of a picture or text are performed by prognostic calculation and analysis of misinterpreted ele¬ ments. 3. Means according to claim 1, characterized in that the pattern recognition equipment with the test pattern, in dependence of the field of application, is adapted and cali¬ brated with respect to recognition sensitivity and degree. 4. Means according to claim 1, characterized in that adaptation and calibration of the recognition sensitivity and degree is applied to a series of physical or electronical test patterns containing pictures and/or texts with varying and physically well-defined test results and the corre¬ sponding subjective expressions.

Inventors: FENG YINGDUO, OESTBERG OLOV, FENG, YINGDUO, OESTBERG, OLOV
Applicants: TELEVERKET
Application year: 1992
Main classification: G06F15
IPC codes: G06K9, G06T1, G06T7