Masked Cross-Correlation for Security Document Authentication

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Solution Overview

Problem

Conventional image template matching techniques struggle to authenticate security documents with strong retroreflective virtual images, as these features produce stronger signals that dominate the analysis and lead to incorrect authentication.

Innovation Solution

The implementation of a modified normalized cross-correlation analysis that masks out strong signals from additional security features, allowing for efficient template matching by using a masking image generated from the captured image, thereby isolating the background image pattern for accurate authentication.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional image template matching techniques are used to authenticate security documents, then the authentication process can be performed, but the strong signals from retroreflective virtual images dominate the analysis and lead to incorrect authentication results

Engineering Contradiction:

Improveauthentication accuracyVSAvoidsignal dominance from retroreflective features

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent extracts and removes the harmful strong signals from retroreflective virtual images and other occluding features from the captured image before performing template matching. This is achieved through signal processing techniques that identify and eliminate dominant signals that would otherwise interfere with the authentication process, allowing the weaker but authentic background patterns to be properly analyzed.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The patent introduces an intermediary processing step between image capture and template matching. This intermediary stage involves generating a modified captured image by removing or masking the strong signals from retroreflective features, thereby creating a cleaner image that allows accurate template matching without the interference of dominant occluding signals.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If multiple security features including retroreflective virtual images are included in the security document, then enhanced security is provided, but the additional features create stronger signals that occlude the background image pattern and complicate authentication

Engineering Contradiction:

Improvesecurity validation reliabilityVSAvoidimage pattern detection difficulty

Core Design Contradiction:

ReliabilityVSDifficulty of detecting and measuring

Solution Approach 1:

The patent converts the harmful effect of strong retroreflective signals into a beneficial process by using these signals as indicators to guide the removal process. The strong signals that initially cause interference are identified and systematically eliminated, transforming the problem of signal dominance into a structured solution where the presence of these signals actually facilitates their removal and improves overall authentication accuracy.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent performs preliminary signal processing to remove or mask the strong signals from retroreflective features before the actual template matching operation. This preliminary action prepares the image data by eliminating potential sources of interference in advance, ensuring that the subsequent authentication process operates on clean, unobstructed image patterns.

Inventive Principle:

Principle #10Preliminary action

3Productivity

If template matching is performed on the captured image without masking, then the process is simple and fast, but the strong signals from occluding objects lead to incorrect match results

Engineering Contradiction:

Improveauthentication speedVSAvoidmatch accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The patent performs a preliminary masking operation on the captured image before template matching. This preliminary step creates a modified image where strong signals from retroreflective features are removed or suppressed. By preparing the image in advance, the actual template matching process remains fast while operating on pre-processed data that yields accurate results without requiring complex iterative corrections.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS8485559B2Document authentication using template matching with fast masked normalized cross-correlation

Publication Date: 2013.07.16 THALES DIS FRANCE SA

AI summary

Techniques are described for authenticating security documents having security images that incorporate multiple security features. The techniques may produce robust template matching results in situations of lighting unevenness, even with stronger occluding objects. The techniques may be particularly useful in validating a security document having a security image composed of one or more “virtual” retroreflective images formed over a background of a repeating retroreflective image. The virtual retroreflective images within the security image provide strong signals that may dominate analysis and validation of the background retroreflective image, thereby resulting incorrect authentication. The techniques provide a modified normalized cross-correlation analysis that masks out the strong signals contributed by the additional security features, such as the retroreflective virtual images, while still allowing for fast and efficient template matching to be performed with respect to the background image.