Abstract:
The invention concerns a method for processing a sequence of source pictures to generate artifacts due to aliasing when these source pictures are captured by a video capturing device. This method comprises a first time modulation step for modulating temporally at a first modulation frequency the brightness of pixels of each picture of the sequence around a brightness to be displayed for said picture. According to an important feature of the invention, the method further comprises a second time modulation step for modulating temporally at a second modulation frequency different from the first modulation frequency the brightness of pixels of each picture of the sequence around a brightness value to be displayed for said picture, the first and second modulation frequencies being determined in order not to be visible to the human eye and contributing to generate aliasing artifact at a predetermined aliasing frequency.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is in the field of content protection, most particularly in cinema venues where a camcorder acquisition followed by immediate illegal distribution creates important revenue losses for the content owners. More particularly, the invention relates to a method and a device for processing a sequence of source pictures to generate artifacts due to aliasing when these pictures are captured by a video capturing device. This method modifies the screened cinema pictures and defeat camcording opportunities in movie theaters without any incidence for the viewing human audience. 
       BACKGROUND OF THE INVENTION 
       [0002]    Within this context, the patent application WO 05/027529 aims to combat the copying of source pictures by means of a camera while they are being displayed, for example using a camcorder in a movie theatre. In this document, it is proposed to generate, from each source picture of the sequence to be displayed, at least two successive processed pictures, in which the colour of at least one pixel in the processed pictures is modulated temporally around the colour of the pixel in the source picture and to display these processed pictures. The pixels whose colour is modified represent an anti-piracy pattern, for example the text “ILLEGAL COPY”. The processed pictures are displayed at a high frequency that makes the pattern invisible to the human eye but visible in the sequence filmed by the camcorder. Such a solution requires a modulation of the colour of the pixels at a frequency higher than the colour flicker frequency, which is of around 10/20 Hz, and is applied to projection systems having a refresh frequency of at least 100 Hz. It is also possible to modulate the luminance or the brightness of the pixels instead of their colour and to use modulation frequencies which are not half of the refresh frequency of the projection system but the modulation frequency should be higher than the color flicker frequency (for a modulation in colour) or the luminance flicker frequency (for a modulation in luminance). 
         [0003]    One drawback of such a method is that it requires a high modulation frequency in order to make the modulation effect invisible for legal audiences. So this modulation tends to be removed by the shutter integration of the video capturing device. 
       SUMMARY OF THE INVENTION 
       [0004]    The principle of the invention is based on human eye&#39;s sensitivity to flicker and the way it relates to both flicker frequency and signal energy. It consists in modulating a video source in amplitude with at least two carrier waves to generate temporal aliasing artifacts on camcorder copies without disturbing direct vision for a human eye. 
         [0005]    More particularly, the invention concerns a method for processing a sequence of source pictures to generate artifacts due to aliasing when said source pictures are captured by a video capturing device, said method comprising a first time modulation step for modulating temporally at a first modulation frequency the brightness of pixels of each picture of the sequence around a brightness to be displayed for said picture. According to the invention, it further comprises a second time modulation step for modulating temporally at a second modulation frequency different from the first modulation frequency the brightness of pixels of each picture of the sequence around a brightness value to be displayed for said picture, said second modulation frequency not being a multiple of the first modulation frequency and said first and second modulation frequencies being determined in order not to be visible to the human eye and contributing to generate aliasing artifact at a predetermined aliasing frequency. 
         [0006]    The pixels whose brightness is modulated are pixels of the source pictures that used for displaying an anti-copy pattern. 
         [0007]    In a first embodiment, all pixels of the anti-copy pattern are modulated by the first modulation frequency and the second modulation frequency. 
         [0008]    In a second embodiment, a first part of the pixels of the anti-copy pattern are modulated by the first modulation frequency and a second part is modulated by the second modulation frequency. 
         [0009]    In a third embodiment, the brightness of the pixels of the source pictures comprising a first component and a second component, the first component of pixels of source pictures is modulated at the first modulation frequency and the second component of pixels of source pictures is modulated at the second frequency. The first component is for example luminance and the second component is chrominance. 
         [0010]    The invention concerns also a device for processing a sequence of source pictures to generate artifacts due to aliasing when said source pictures are captured by a video capturing device, said source pictures being received at a first refresh frequency and delivered at a second refresh frequency, comprising
       a frame duplicator for generating K pictures for each source picture, with K being the ratio between the second refresh frequency and the first refresh frequency,   a first generator for generating carrier coefficients at the second frequency, said carrier coefficients following a sine curve having a first modulation frequency,   a first multiplier circuit for multiplying a first non-zero modulation index with a first part of the carrier coefficients delivered by the first generator and zero with the other part of the carrier coefficients,   a second generator for generating carrier coefficients at the second frequency, said carrier coefficients following a sine curve having a second modulation frequency, said second modulation frequency not being a multiple of the first modulation frequency,   a second multiplier circuit for multiplying a second non-zero modulation index with a first part of the carrier coefficients delivered by the second generator and zero with the other part of the carrier coefficients,   an adder circuit for adding together the values delivered by the first and second multiplier circuits and the value  1 ; and   a third multiplier circuit for multiplying the value delivered by the adder circuit with the value of pixels of the duplicated pictures delivered by the frame duplicator.       
 
         [0018]    The invention concerns also a device for processing a sequence of source pictures to generate artifacts due to aliasing when said source pictures are captured by a video capturing device, said source pictures being received at a first refresh frequency and delivered at a second refresh frequency, comprising
       a frame duplicator for generating K pictures for each source picture, with K being the ratio between the second refresh frequency and the first refresh frequency,   a first generator for generating carrier coefficients at the second frequency, said carrier coefficients following a sine curve having a first modulation frequency,   a first multiplier circuit for multiplying a first non-zero modulation index with the carrier coefficients delivered by the first generator,   a second generator for generating carrier coefficients at the second frequency, said carrier coefficients following a sine curve having a second modulation frequency, said second modulation frequency not being a multiple of the first modulation frequency,   a second multiplier circuit for multiplying a second non-zero modulation index with the carrier coefficients delivered by the second generator,   a selector receiving at first input the values delivered by the first multiplier circuit, at a second input the value  0  and at a third input the values delivered by the second multiplier circuit and delivering the values present at its first input if the current pixel belongs to a first predefined set of pixels, the value present at its third input if the current pixel belongs to a second predefined set of pixels different from the first set and otherwise the value  0 ;   an adder circuit for adding together the values delivered by the selector and the value  1 ; and   a third multiplier circuit for multiplying the value delivered by the adder circuit with the value of pixels of the duplicated pictures delivered by the frame duplicator.       
 
         [0027]    The invention concerns also a device for processing a sequence of source pictures to generate artifacts due to aliasing when said source pictures are captured by a video capturing device, said source pictures being received at a first refresh frequency and delivered at a second refresh frequency, characterized in that it comprises
       a frame duplicator for generating K pictures for each source picture, with K being the ratio between the second refresh frequency and the first refresh frequency;   a first generator for generating carrier coefficients at the second frequency, said carrier coefficients following a sine curve having a first modulation frequency;   a first multiplier circuit for multiplying a first non-zero modulation index with a first part of carrier coefficients delivered by the first generator and zero with the other part of the carrier coefficients;   a first adder circuit for adding together the values delivered by the first multiplier circuit and the value  1 ;   a second generator for generating carrier coefficients at the second frequency; said carrier coefficients following a sine curve having a second modulation frequency, said second modulation frequency not being a multiple of the first modulation frequency;   a second multiplier circuit for multiplying a second non-zero modulation index with a first part of carrier coefficients delivered by the second generator and zero with the other part of the carrier coefficients;   a second adder circuit for adding together the values delivered by the second multiplier circuit and the value  1 ;   a third multiplier circuit for multiplying the value delivered by the first adder circuit with the luminance value of the pixels of the pictures delivered by the frame duplicator; and   a fourth multiplier circuit for multiplying the value delivered by the second adder circuit with the chrominance value of the pixels of the pictures delivered by the frame duplicator.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. In the drawings: 
           [0038]      FIG. 1  is a graphical representation of a video signal recorded by a camcorder (equivalent to a function sinc(πfT) with a shutter speed set to 
           [0000]    
         
           
             
               
                 
                   
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               ; 
             
           
         
       
           [0039]      FIG. 2  shows the human observer&#39;s perception of time-varying light emissions depending on retina excitation, flicker frequency and modulation index; 
           [0040]      FIG. 3  illustrates the sensitivity of a human eye to color flicker; 
           [0041]      FIG. 4  represents a first circuit implementation of the invention; 
           [0042]      FIG. 5  represents a second circuit implementation of the invention; and 
           [0043]      FIG. 6  represents a third circuit implementation of the invention. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0044]    First of all, in order to better understand the invention, a mathematical modelization of a camcorder device is proposed. The signal m(t) designates the signal representative of the light-induced charges entering and being integrated by a sensor cell of a CCD/CMOS array of the camcorder device. The integration time of this signal is determined by the electronic shutter of the camcorder, which turns out to be the start/stop signal commanding the integration process of the sensor array. If T is the integration time, the output electrical signal of a sensor cell m shut (t) is: 
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         [0045]    The signal m shut (t) is then sampled for storage purposes. It has to be noted that the shutter command signals&#39; periodicity is most likely synchronized to the sampling process, especially when using CCD sensor arrays, which readout process involves a complete initialization of their photocells. The dual integration-sampling process of the signal x(t) can be modelized as follows: 
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         Where m rec (t) is the signal recorded by the camcorder, and
       T s  is the sampling time (e.g. T s =1/50 second for PAL-interlaced, T s =1/25 second for PAL-progressive, T s =1/60 second for NTSC-interlaced, etc. . . . ).   
     
       
     
         [0048]    Based on this mathematical model of the signal recorded by the camcorder and common knowledge about human vision, to generate disturbing aliasing artifacts on a camcorder acquisition, a video signal has to satisfy the following constraints:
       The bandwidth of the signal to be recorded has to be high enough to violate the Nyquist-Shannon sampling theorem. If F is the signal&#39;s bandwidth and F s  is the sampling frequency of the camcorder (F s =1/T s ), it means that F&gt;F s /2;   Aliasing artifacts have to be visible on the camcorder. If f a  is an aliasing frequency, would it be flickering (f a ≠0) or not (f a =0), and if CFF is the Critical Flicker Frequency threshold above which a flickering light is indistinguishable from a steady, non-flickering light for a human eye), it means that f a &lt;CFF;   The modulated video signal has to look exactly the same as the original one for the legal audience. If f m  is the modulation frequency, it means that f m &gt;CFF;   Modulation frequency should be preferably located outside low gain areas on the shutter spectrum.       
 
         [0053]    Shutter spectrum and aliasing frequencies can be determined using the Fourier transform of the equation (2): 
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         Where term1 represents the cardinal sine low pass filtering operation induced by the shutter integration, and
       term2 is the periodization of the resulting spectrum (period=Fs ) induced by the sampling process.   
     
       
     
         [0056]      FIG. 1  shows a graphical representation of the function sinc(πft) with a shutter speed set to 
         [0000]    
       
         
           
             
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         [0000]    (which is the default mode for PAL-interlaced camcorders). The abrupt truncation of light integration by CCD/CMOS array sensors command signals introduces severe sidelobe effects during the low pass filtering process, meaning, despite the low pass behaviour of this filter, that spectral content over the 
         [0000]    
       
         
           
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         [0000]    cutoff limit can pass through the shutter. Sidelobe peaks of the cardinal sine function can be determined using the following table. 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 Peak location in 
                   
               
               
                 Sidelobe 
                 Peak location 
                 Hz 
                 Peak value 
               
               
                   
               
             
             
               
                 Main lobe 
                 x = 0 
                 f = 0 
                 |sinc(x)| = 1 
               
               
                   
               
               
                 1 st   
                 x = ±4.4934095 
                 
                   
                     
                       
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                         = 
                         
                           ± 
                           
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                 |sinc(x)| = 0.2172336 
               
               
                   
               
               
                 2 nd   
                 x = ±7.7252518 
                 
                   
                     
                       
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                         = 
                         
                           ± 
                           
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                 |sinc(x)| = 0.1283746 
               
               
                   
               
               
                 3 rd   
                 x = ±10.904122 
                 
                   
                     
                       
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                         = 
                         
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         [0057]    The alias generating frequencies satisfying the constraints given previously are identified in the  FIG. 1  by black areas. These frequencies are located around the sidelobe peaks (highest shutter gain areas) in the [25,∞[Hz frequency band (Shannon-Nyquist theorem violation). 
         [0058]    An alias-generating video signal, noted m MOD (t), can be obtained by adding one harmonic to the original video signal m(t) which frequency f m  is over the Shannon limit and inside non-null areas of the shutter spectrum: 
         [0000]      m MOD ( t )= m ( t )+ A  cos(2 πf   m   t )  (4) 
         [0059]    Of course the equation (4) has to be modified to deal with physical constraints, such as non-negative light emissions: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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         [0060]    In other words, to generate the maximum aliasing effects physically possible, the signal m MOD (t) is defined as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0061]    The equation (6) is the expression of an amplitude-modulated (AM) signal, with the original signal m(t) being added to the modulated spectrum m(t)cos(2πf m t), to be opposed to classical AM schemes in transmission where only the carrier wave cos(2πf m t) can be added to the modulated signal to avoid carrier regeneration on reception before the demodulation process. 
         [0062]    Despite the fact that the amplitude modulation process depicted by equation (6) has been used in many methods to counter piracy attempts in movie theatres, it appears to be very restrictive. In order to make the modulation effect invisible for legal audiences, such methods usually propose modulation frequencies being so high that they tend to be completely removed by shutter integration. An alternative is to use lower frequencies with lower modulation indices or to use chrominance-only modulation to make the effect invisible at such frequencies, alas the resulting artefacts tend to be almost invisible as well. 
         [0063]    According to the invention, in order to have a modulation effect which is strong enough to generate aliasing artefacts yet invisible enough for the audience not to be disturbed, it is proposed to use a multi-carrier modulation scheme, based on at least two modulation frequencies f m1  and f m2 , instead of using a single-carrier modulation scheme as used up to now. The first frequency f m1  (the lowest frequency) allows to introduce as much effect as possible for a low flicker rate and goes through significant shutter gains (for example, over −12 dB). The second frequency f m2  allows generating even more disturbance with the remaining bits of signal energy. The alias-generating video signal is now: 
         [0000]        m   MOD ( t )= m ( t )+ A 1 cos(2 πf   m1   t )+ A 2 cos(2 πf   m2   t )  (7) 
         [0064]    With the non-negative light emissions constraint, it leads to the following equation (8): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
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         [0000]    where αε[0,1] is a constant that allows to split the modulation energy between the two frequencies f m1  and f m2 . 
         [0065]    The two modulation frequencies f m1  and f m2  are selected to contribute to generate aliasing at a predetermined frequency f a . 
         [0066]    In a first embodiment, all pixels of the anti-copy pattern to be introduced in the source pictures are modulated by two modulation frequencies f m1  and f m2 . 
         [0067]    In a second embodiment, a first part of the pixels of the anti-copy pattern to be introduced in the source pictures are modulated by the modulation frequency f m1  and a second part of the pixels of the anti-copy pattern (different from the first part) are modulated by the modulation frequency f m2 . 
         [0068]    In a third embodiment, the luminance of the pixels of the anti-copy pattern to be introduced in the source pictures is modulated at the first modulation frequency and the chrominance of these pixels is modulated at the second frequency. 
         [0069]    Different ways are proposed hereinafter to select the modulation frequencies to be used and the corresponding modulation indices 
         [0070]    In a first way, it is proposed to split the global light energy signal around two carrier waves by adjusting modulation indices.  FIG. 2  shows Kelly&#39;s Temporal Contrast Sensitivity (TCS) function for various adapting fields. As it can be seen on this figure, a human observer&#39;s perception of time-varying light emissions highly depends on specific conditions regarding the retina excitation (expressed in trolands), the flicker frequency or light variation cycles per second (horizontal axis) and the modulation index (vertical axis) of the signal. For a given retina excitation, the time-varying light emissions are perceived in the domain located under the corresponding curve. This figure is used to determine the modulation index to be applied to the lowest modulation frequency f m1 . 
         [0071]    Based on the equation (3), the harmonic frequencies f m1  and f m2  generating aliasing artifacts on the frequency f a  can be written as f m1 =∥f a +n 1 F s ∥ and f m2 =∥f a +n 2 F s ∥, where n 1  and n 2  are integers. For example, for generating a visible 15 Hz alias/flicker effect over a PAL-interlaced camcorder with a 1/50 shutter speed, the good candidates could be: 
         [0000]        f   m1 =35 Hz( n   1 =−1) 
         [0000]        f   m2 =65 Hz( n   2 =+1) 
         [0072]    These frequencies are not filtered out by shutter integration (see  FIG. 1 ). So, the modulated signal can be written as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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                         35 
                          
                         
                             
                         
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                         Hz 
                          
                         
                             
                         
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                         and 
                          
                         
                             
                         
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                            
                           
                               
                           
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                           Hz 
                            
                           
                               
                           
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                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
         [0073]    The phase opposition (+π) on the carrier 2 allows to cancel the e −jπfT -induced phase opposition between odd and even lobes on the shutter spectrum (see equation (3)). The modulation indices α 1  and α 2  are carefully chosen in order to match with  FIG. 2  to ensure invisibility to the audience. As an example, for a retina excitation of 7.1 trolands, the coefficients are 
         [0000]      α 1 ≈0.2 
         [0000]      α 2 =1−α 1 ≈0.8 
         [0074]    Although these will most likely be set exploiting actual psychophysical tests, since  FIG. 2  is unlikely to be representative of human perception in a movie theater, which has different vision conditions than those used during flicker sensitivity tests. 
         [0075]    In this way, this multi-carrier modulation scheme is to be applied to either one or several components of the video signal in the XYZ color space, as long as modulation indices ensure that every modulated value is located inside the gamut of the display device. 
         [0076]    Another possibility to split modulation signal energy into two different carrier waves can be a dual component modulation, for example a chrominance-luminance modulation. It has been demonstrated that human perception of flickering light highly depends on whether this light is stimulating all retinal cones or just a selected range as illustrated by  FIG. 3 .  FIG. 3  shows the sensitivity of a human eye to color flicker. More particularly,  FIG. 3  shows the threshold modulation indices for two retina excitations (4,5 trolands and 45 trolands), for 4 colors (blue 455 nm, green 512 nm, red 641 nm, red 689 nm) and for a modulation frequency range [1 Hz, 20 Hz]. For a given retina excitation, a given color and a given modulation frequency, the modulation index must be above the corresponding sensitivity curve (the modulation is lower than the modulation index given by the curve) to ensure invisibility of the flicker in the movie theater. 
         [0077]    As a result, an alternative to the first embodiment is a dual chrominance-luminance modulation system, where a chrominance (X and Z components in the XYZ color space) flicker effect is generated using the first carrier frequency f m1 =35 Hz, and the remaining luminance dynamic (Y component) is modulated using the second carrier frequency f m2 =65 Hz. 
         [0078]    In this case, we have 
         [0000]    
       
         
           
             
               
                 m 
                 MOD 
               
                
               
                 ( 
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                 ) 
               
             
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                           ( 
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         [0000]    where A1, A2 and A3 are modulation indices that are carefully selected to make sure that all modulated vectors are in the display device gamut. For a given modulation frequency, a given color and a given retina excitation, a valid modulation index A is located over the corresponding sensitivity curve of the  FIG. 3 . Furthermore, another condition is to make sure that all selected XYZ vectors from X(1−A1) to X(1+A1), Y(1−A2) to Y(1+A2) and Z(1−A3) to Z(1+A3) are located inside the display gamut. 
         [0079]      FIGS. 4 ,  5  and  6  are block diagrams of three exemplary circuit implementations of the inventive method. 
         [0080]      FIG. 4  shows a device  100  for implementing the inventive method where all the pixels of the anti-copy pattern (for example ILLEGAL COPY) are modulated by both modulation frequencies f m1  and f m2 . The device  100  receives source pictures and delivers output pictures to a video projector  400  working at a refresh frequency F r ′ (=144 Hz for example). The source pictures are received at a refresh frequency F r  (=24 Hz for example). The device comprises:
       a frame duplicator  110  for generating K pictures for each source picture, with K=F r ′/F r ,   a first generator  120 A for generating carrier coefficients at a frequency F r ′; these carrier coefficients of type cos(2πf m1 t) are computed previously and stored in a look-up table;   a first multiplier circuit  130 A for multiplying the modulation index α 1  with each carrier coefficient delivered by the generator  120 A; the modulation index α 1  is a value predefined as mentioned previously and stored in a memory circuit; this non-zero value α 1  is used for the pixels of the source pictures that must be modulated (=pixels of the anti-copy pattern) and the value  0  is used for the other pixels (=no modulation);   a second generator  120 B for generating carrier coefficients at a frequency F r ′; these carrier coefficients of type cos(2πf m2 t) are computed previously and stored in a look-up table;   a second multiplier circuit  130 B for multiplying the modulation index α 2  with each carrier coefficient delivered by the generator  120 B; the modulation index α 2  is a value predefined as mentioned previously and stored in a memory circuit; this non-zero value α 2  is used for the pixels of the source pictures that must be modulated (=pixels of the anti-copy pattern) and the value  0  is used for the other pixels (=no modulation);   an adder circuit  140  for adding together the values delivered by the multiplier circuits  130 A and  130 B and the value  1 ; and   a third multiplier circuit  150  for multiplying the value delivered by the adder circuit  140  with the value of the pixels of the duplicated pictures delivered by the frame duplicator  110 ; the part of the pixels for which the modulation index α 1  or α 2  is not zero (=pixels of the anti-copy pattern) are modulated at the modulation frequencies f m1  and f m2 ; the other pixels are not modulated; the output pictures are provided to the video projector  400 .         
         [0088]      FIG. 5  shows a device  200  for implementing the inventive method where a part of pixels of the anti-copy pattern, the pixels belonging to a set E 1 , are modulated at the modulation frequency f m1  and the remaining pixels of the anti-copy pattern are modulated at the modulation frequency f m2 . The device  200  receives source pictures and delivers output pictures to a video projector  400  working at a refresh frequency F r ′ (=144 Hz for example). The source pictures are received at a refresh frequency F r . The device comprises:
       a frame duplicator  210  for generating K pictures for each source picture, with K=F r ′/F r ,   a first generator  220 A for generating carrier coefficients at a frequency F r ′; these carrier coefficients of type cos(2πf m1 t) are computed previously and stored in a look-up table;   a first multiplier circuit  230 A for multiplying the modulation index α 1  with each carrier coefficient delivered by the generator  220 A; the modulation index α 1  is a value predefined as mentioned previously and stored in a memory circuit; this non-zero value α 1  is used for a first part E 1  of the pixels of the source pictures that must be modulated (=a first part of the pixels of the anti-copy pattern);   a second generator  220 B for generating carrier coefficients at a frequency F r ′; these carrier coefficients of type cos(2πf m2 t) are computed previously and stored in a look-up table;   a second multiplier circuit  230 B for multiplying the modulation index α 2  with each carrier coefficient delivered by the generator  120 B; the modulation index α 2  is a value predefined as mentioned previously and stored in a memory circuit; this non-zero value α 2  is used for a second part E 2  (different from the first part E 1 ) of the pixels of the source pictures that must be modulated (=a second part of the pixels of the anti-copy pattern);   a selector  260  receiving at an input  0  the values delivered by the multiplier circuit  230 A, at an input  1  the value  0  and at an input  2  the values delivered by the multiplier circuit  230 B; if the current pixel p belongs to the predefined set E 1 , the selector delivers the value present at its input  0 ; if the current pixel p belongs to the predefined set E 2 , the selector delivers the value present at its input  2  and otherwise it delivers the value  0  present at its input  1 ;   an adder circuit  240  for adding together the values delivered by the selector  260  and the value  1 ; and   a third multiplier circuit  250  for multiplying the value delivered by the adder circuit  240  with the value of the pixels of the duplicated pictures delivered by the frame duplicator  210 ; thus the pixels belonging to the set E 1  are modulated at the frequency fm 1 , the pixels belonging to the set E 2  are modulated at the frequency fm 2  and the other pixels are not modulated; the pixels of the output pictures are provided to the video projector  400 .         
         [0097]    In  FIGS. 5 and 6 , the brightness of the anti-copy pattern pixels of the pictures is modulated. It can be either the luminance or the chrominance of the pixels.  FIG. 6  proposes a circuit implementation where the luminance of the anti-copy pattern pixels is modulated at a first frequency and the chrominance of these pixels is modulated at a second modulation frequency. 
         [0098]      FIG. 6  shows a device  300  receiving source pictures and delivering output pictures to a video projector  400  working at a refresh frequency F r ′ (=144 Hz for example). The source pictures are received at a refresh frequency F r . The device comprises:
       a frame duplicator  310  for generating K pictures for each source picture, with K=F r ′/F r ; a first output of the duplicator delivers the luminance signal of these pictures and a second output delivers the chrominance signal of these pictures;   a first generator  320 A for generating carrier coefficients at a frequency F r ′; these carrier coefficients of type cos(2πf m1 t) are computed previously and stored in a look-up table;   a first multiplier circuit  330 A for multiplying the modulation index α 1  with each carrier coefficient delivered by the generator  320 A; the modulation index α 1  is a value predefined as mentioned previously and stored in a memory circuit; this non-zero value α 1  is used for the pixels of the source pictures that must be modulated (=pixels of the anti-copy pattern) and the value  0  is used for the other pixels (=no modulation);   a first adder circuit  340 A for adding together the values delivered by the multiplier circuit  330 A and the value  1 ;   a second generator  320 B for generating carrier coefficients at a frequency F r ′; these carrier coefficients of type cos(2πf m2 t) are computed previously and stored in a look-up table;   a second multiplier circuit  330 B for multiplying the modulation index α 2  with each carrier coefficient delivered by the generator  120 B; the modulation index α 2  is a value predefined as mentioned previously and stored in a memory circuit; this non-zero value α 2  is used for the pixels of the source pictures that must be modulated (=pixels of the anti-copy pattern) and the value  0  is used for the other pixels (=no modulation);   a second adder circuit  340 B for adding together the values delivered by the multiplier circuit  330 B and the value  1 ;   a third multiplier circuit  350 A for multiplying the value delivered by the adder circuit  340 A with the luminance value of the pixels of the duplicated pictures delivered by the frame duplicator  210 ; the resulting signal is provided to the video projector  400 ; and   a fourth multiplier circuit  350 B for multiplying the value delivered by the adder circuit  340 B with the chrominance value of the pixels of the duplicated pictures delivered by the frame duplicator  210 ; the resulting signal is provided to the video projector  400 .         
         [0108]    Of course, the scope of the present invention is not limited to the embodiments described hereinabove. More particularly, other values of refresh frequencies F r ′ and F r ′ or modulation frequencies fm i  can be used. More than two modulation frequencies can be used. The different embodiments can also be combined.