FR3100897A3 - Differential photographic acquisition process of an authentication device - Google Patents

Abstract

We know the photographic acquisition process known under the name of "killing ambient light" consisting in calculating a difference between two acquisitions of the same scene, particularly when it is applied to the authentication of an authentication device. provided with a so-called marker means making it possible to determine the three-dimensional position in which the photographic acquisition means is located, one under a given lighting and the other with additional lighting. The additional lighting must be switched on and off at the fastest possible rate, so that the acquisition device does not move between the two successive shots, even if it is held in the hand, and that the scene does not did not have time to change either. For this, an image is acquired as soon as the modification of the lighting by switching on or off the additional lighting makes it possible to recognize this marker. Figure for the abstract: Fig 2

Description

Differential photographic acquisition process of an authentication device

The present invention is a method of differential photographic acquisition of an authentication device, which eliminates the effect of ambient light.

The method of taking two photographs with and without flash, to differentiate them in order to “kill the ambient light”, has been known for a long time. Bryan Peterson’s book "Flash Photography", first published in 1952 and republished in 2011, discusses how to "kill ambient light" by subtracting a non-flash photograph from a flash photograph.

Patent US20080158258A1 of December 27, 2006 (David B. Lazarus [US] and John D. Ogden [US] - General Instrument Corporation) discloses a method comprising the acquisition of a plurality of digital images at at least two levels of. different lighting from a controlled light source, and analyzing the plurality of digital images to obtain an image representing the lighting contribution provided by the controlled light source.

Patent US7590344B2 of September 15, 2009 (Georg Petschnigg [US] - Microsoft) describes a process for improving images taken with flash, consisting in acquiring a first image captured with flash and ambient light, then a second image, corresponding to the first image, captured with ambient light, and using the first image and the second image to create a third image with flash only;

We know the patent WO2012136902 of October 12, 2012 by Franck Guigan [FR] and the application PCTFR2019000002 of January 10, 2019 by the same inventor, which allow authentication devices to be authenticated with this method of differential photographic acquisition.

Technical problem

When taking two successive photographs, with a hand-held camera, one of which is taken under ambient lighting and the other with an additional light source, it is difficult to obtain two images representing exactly the same scene. , either because the camera moved, or because the scene changed between the two shots.

With a camera with automatic shutter speed and / or lens aperture adjustment, such as a smartphone usually, you may even have to wait a while for the exposure and / or speed shutter have had time to adapt to the variation in brightness of the scene. This leads to the difference between the two images being disturbed by the fact that they do not exactly represent the same scene and / or the same framing.

The invention will be well understood, and other objects, advantages and characteristics thereof will emerge more clearly on reading the description which follows, which is illustrated by FIGS. 1 and 2 which both represent views of the invention. same authentication device consisting of the superposition of a layer of black ink and a layer of glossy ink which is printed only on a part of the black layer, giving rise to reflections which vary according to the viewing angle and lighting.

is the comparison between - on the left - a photograph taken in ambient light and - on the right - a photograph taken in ambient light with an additional light source which is the flash of the camera.

represents - on the left - the result of the operation of subtracting the image in ambient light from ambient light with an additional light source, after correction of the brightness of the ambient image. This so-called differential image represents the effect of the flash, which can also be understood as the calculation of the image after having "killed the ambient light", and - on the right - the differential image calculated with the same method as that used for calculate the left image, but in the case where the camera has been shifted between taking the two images with and without additional light source. The image on the left is the reference, which could also have been obtained by taking a photograph with only additional lighting, and the one on the right has many defects which have the consequence that it is not representative of this reference and does not therefore does not allow the authentication device to be authenticated.

In both cases, the marker is the periphery of the printed marking, which deforms according to the perspective, and whose analysis makes it possible to know from which location the acquisition device is seen.

The present invention is a photographic acquisition method consisting in making, with a photographic acquisition means provided with a computer, two photographic acquisitions of an authentication device provided with a so-called marker means allowing said computer to determine the three-dimensional position in which said photographic acquisition means is located by reference to said authentication device, one of the acquisitions being made under ambient lighting and the other being made with additional additional lighting, and in make the difference to obtain an image called differential image representing the scene illuminated by the only additional lighting, characterized in that a photographic acquisition is carried out less than a second after said computer has succeeded in recognizing said marker after a modification of the 'lighting by switching on or off said additional lighting

Detailed description of the invention

We know the photographic acquisition process known under the name of "killing the ambient light" consisting in calculating a difference between two acquisitions of an authentication device provided with a means called a marker means making it possible to determine the position in three. dimensions in which said photographic acquisition means is located, one under a given lighting and the other with additional additional lighting.

Different types of markers are described in the aforementioned patent WO2012136902. To take a few examples, it can be any visible element of the scene such as for example a graphic marking whose appearance in perspective varies according to the position of the photographic acquisition means, or a reflecting surface provided with 'a target with which we must make the flash reflection coincide.

This process is implemented in patent WO2012136902 and in the aforementioned application PCTFR2019000002, the photographic acquisition means being advantageously provided with a computer capable of recognizing the means called marker and of deducing therefrom the three-dimensional position in which finds the photographic acquisition means by reference to said authentication device, one of the acquisitions being made under a given lighting and the other being made with additional additional lighting, and to make the difference to obtain a so-called image differential image representing the scene illuminated by additional lighting alone.

The additional lighting must be switched on and off at the fastest possible rate, so that the acquisition device does not move between the two successive shots, even if it is held in the hand, and that the scene does not did not have time to change either.

This provides two advantages:
- on the one hand, the correction to be made to one of the two images before making the difference is weaker than between an image with a large aperture and / or a long pause (that without additional lighting) and that made earlier or later at low aperture and / or short pause (the one with this additional lighting), because the exposure has not had time to change too much,
- and on the other hand the position of the camera and the scene photographed have not changed too much either.

The rate of photographic acquisitions is however limited by the fact that the computer must have had time to recognize the marker in order to subsequently be able to calculate the position of the acquisition means with respect to the authentication device.

The principle of the present invention consists in carrying out a photographic acquisition as quickly as possible after a modification of the lighting by switching on or off said additional lighting, advantageously less than a second after the computer has succeeded in recognizing the marker, or , even better, as soon as said computer has succeeded in recognizing the marker.

It is recommended that you choose a shutter speed and lens aperture such that shots with and without additional lighting both have sufficient contrast.

In case the camera has automatic adjustment of the shutter speed and / or the aperture of the lens, which is the case with most modern cameras and smartphones, there is a third The advantage is that the camera does not have time to modify its settings too strongly between views with and without additional lighting.

In a particular embodiment, video is recorded while the additional lighting is turned on and off at a rate greater than one cycle per second, and this video is then analyzed at a slower rate, for example frame by frame. , so that the computer has time to recognize the marker after changing the lighting by switching on or off the additional lighting. This allows the additional lighting to be switched on and off at a rate greater than five cycles per second.

It is advantageous in this case to start switching on the additional lighting intermittently before taking the first of the two shots, to prevent the automatic setting from being able to correspond to the light conditions corresponding to a first shot with or without lighting. additional, but is already done as a function of an average value between the light conditions with and without the additional lighting.

The two images do not need to be taken, one when the additional lighting is fully on and the other when it is fully off. It suffices that there is a difference in the intensity of this additional lighting between the two shots.

An improved method is to turn on the additional lighting intermittently and take a series of photographs during the same period. You can then choose which pair of photographs is used to calculate the differential image, that is, the most suitable for calculating the difference, the one that gives the best results.

The present invention is also an authentication method consisting in establishing, according to one or other of the methods which have been described above, a description called a subsequent 2D description of an authentication device and in comparing it with a description. said reference 2D description of the authentication device, the subsequent 2D description being established from a point of view close to that used to establish the reference 2D description. It thus makes it possible to improve the methods described in the aforementioned patent WO2012136902.

When the reference image has been acquired under lighting similar to that of the additional lighting, the differential image can be compared directly with this reference image.

It is also possible to establish several reference descriptions made from different locations, and compare the subsequent description with the reference 2D description drawn up from the point of view closest to that used to establish the subsequent 2D description.

It is particularly advantageous, for on-line identification, for example, to ask the user of the photographic acquisition device to place it in one or more different locations that he could not foresee in advance.

This therefore makes it possible to improve the methods described in the aforementioned patent application PCTFR2019000002.

Applications

A particularly advantageous use from an economic point of view is that in which one compares the already recorded description of all or part of a product or of a document, for example of an authentication device, under a known light or obtained with the present method, with a new description, to see if the object or document is indeed the one that was originally photographed and recorded. There are therefore many applications in the fight against counterfeiting and false documents, copying works of art and identifying people face to face or online.

Claims (9)

Photographic acquisition process consisting in making, with a photographic acquisition means provided with a computer, two photographic acquisitions of an authentication device provided with a so-called marker means allowing said computer to determine the three-dimensional position in which is said photographic acquisition means by reference to said authentication device, one of the acquisitions being made under ambient lighting and the other being made with additional additional lighting, and to make the difference to obtain a so-called differential image representing the scene illuminated by the only additional lighting, characterized in that a photographic acquisition is carried out less than one second after said computer has succeeded in recognizing said marker after a modification of the lighting by switching on or switching off said additional lighting. Photographic acquisition method according to Claim 1, characterized in that a photographic acquisition is carried out as soon as said computer has succeeded in recognizing said marker after a modification of the lighting by switching on or switching off said additional lighting. Photographic acquisition method according to any one of claims 1 or 2 characterized in that said additional lighting is a flash associated with the photographic acquisition means. Photographic acquisition method according to any one of claims 1 to 3, characterized in that a video is recorded while said additional lighting is switched on and off at a rate greater than one cycle per second, and that said video is then analyzed at a slower speed. Photographic acquisition process according to Claim 4, characterized in that the said additional lighting is switched on and off at a rate greater than five cycles per second. A method of photographic acquisition according to any one of claims 1 to 5 characterized in that the additional lighting is switched on intermittently before making the first photographic acquisition. Authentication method consisting in establishing a description called a subsequent 2D description of an authentication device and comparing it with a description called a reference 2D description of said authentication device, said subsequent 2D description being established from a close point of view of that used to establish said reference 2D description, characterized in that a reference 2D description and / or a subsequent 2D description is obtained by a method according to any one of claims 1 to 6. Authentication method according to claim 7 comprising establishing several reference descriptions made from different locations, and comparing said subsequent description with said reference 2D description established from a point of view close to that used to establish said subsequent 2D description. Authentication method according to claim 7 or claim 8 consisting in asking the user of said photographic acquisition device to place it in one or more different locations that he could not foresee in advance.