EP2022037B1 - Methods for sequential color display by modulation of pulses - Google Patents

Abstract

Each color image is broken down into at least one series of at least three successive primary images of different primary colors which are displayed successively by modulating the activation pulse of the pixels of an imaging device; the invention requires that the spread of the pulse phase of the pixels be shortened over the three successive sub-frames: the pulse periods of the pixels of the first primary image are displaced to the end of the sub-frame of the first image, and the pulse period of the pixels of the third primary image are displaced in the sub-frame of the third primary image towards the beginning of the sub-frame of this third image. This has the beneficial effect of reducing color break defects.

Description

The invention relates to a method for displaying a sequence of color images using an imager having a two-dimensional array of activatable pixels; each of the images is decomposable into at least one series of at least three primary images of different primary colors; for displaying any color image of this sequence, the pixels of the three primary images of this series are successively displayed by modulating the activation duration of said corresponding pixels of the imager. The documents US6392656 , US6972736 and US6756956 describe such a method.

In this method, the beginning of the activation of the pixels of each primary image generally occurs at the beginning of the display of this image; the time difference that is then generated between the display of these primary images generates color break-up defects.

An object of the invention is to limit this type of defect.

• d_1-Pij, d_2-Pij, d_3-Pij comme les durées d'activation dudit pixel pour l'affichage, respectivement, de la première, de la deuxième et de la troisième image primaire de ladite série,
• d_Off-12-Pij comme l'intervalle de temps entre la fin t_Off-1-Pij de l'activation dudit pixel P_ij pour afficher ladite première image primaire de ladite série et le début t_On-2-Pij de l'activation du même pixel P_ij pour afficher ladite deuxième image primaire de la même série,
• d_Off-23-Pij comme l'intervalle de temps entre la fin t_Off-2-Pij de l'activation dudit pixel P_ij pour afficher ladite deuxième image primaire de ladite série et le début t_On-3-Pij de l'activation du même pixel P_ij pour afficher ladite troisième image primaire de la même série,
  > on a, quel que soit ledit pixel P_ij de l'imageur et pour chaque série, la relation : $$\left({\mathrm{d}}_{\mathrm{Off}\mathrm{-}\mathrm{12}\mathrm{-}\mathrm{Pij}}\mathrm{+}{\mathrm{d}}_{\mathrm{Off}\mathrm{-}\mathrm{23}\mathrm{-}\mathrm{Pij}}\right)\mathrm{<}\left[{\mathrm{T}}_{\mathrm{R}}\mathrm{-}\left({\mathrm{d}}_{\mathrm{1}\mathrm{-}\mathrm{Pij}}\mathrm{+}{\mathrm{d}}_{\mathrm{2}\mathrm{-}\mathrm{Pij}}\mathrm{+}{\mathrm{d}}_{\mathrm{3}\mathrm{-}\mathrm{Pij}}\right)\right]/2.$$
  

To this end, the subject of the invention is a method for displaying a sequence of color images using an imager equipped with a two-dimensional array of activatable pixels, in which, for the display of at least one color image of said sequence, said image being decomposed into at least one series of at least three successive primary primary color images, the pixels of said three primary images of the at least one series are successively displayed by modulating the duration of activation of said corresponding pixels of the imager, in which process, for each series having a duration T _R , if one defines, for each pixel P _ij of said color image:

• d _{1 -Pij} , d _{2 -Pij} , d _{3 -Pij} as the activation times of said pixel for the display, respectively, of the first, second and third primary images of said series,
• d _Off-12-Pij as the time interval between the end t _Off-1-Pij of the activation of said pixel P _ij to display said first primary image of said series and the start t _On-2-Pij of the activating the same pixel P _ij to display said second primary image of the same series,
• d _Off-23-Pij as the time interval between the end t _Off-2-Pij of the activation of said pixel P _ij to display said second primary image of said series and the start t _On-3-Pij of the activating the same pixel P _ij to display said third primary image of the same series,
  we have, whatever said pixel P _ij of the imager and for each series, the relation: $$\left({\mathrm{d}}_{\mathrm{off}\mathrm{-}\mathrm{12}\mathrm{-}\mathrm{Pij}}\mathrm{+}{\mathrm{d}}_{\mathrm{off}\mathrm{-}\mathrm{23}\mathrm{-}\mathrm{Pij}}\right)\mathrm{<}\left[{\mathrm{T}}_{\mathrm{R}}\mathrm{-}\left({\mathrm{d}}_{\mathrm{1}\mathrm{-}\mathrm{Pij}}\mathrm{+}{\mathrm{d}}_{\mathrm{2}\mathrm{-}\mathrm{Pij}}\mathrm{+}{\mathrm{d}}_{\mathrm{3}\mathrm{-}\mathrm{Pij}}\right)\right]/2.$$
  

Advantageously, said relation is applicable for each of the images to be displayed of said sequence. In the case where the images of the video sequence are decomposed a plurality of series of at least three successive primary images, the invention applies to each of these series; the series may have identical or different durations.

The display of an image of this sequence is obtained by the successive display of three subframes of different primary colors, generally red, green and blue. In the prior art, the activation phases of the pixels are generally positioned in the same way regardless of the primary image to be displayed, for example at the beginning of the subframe or in the middle of the subframe; the identical positioning of the activation phases implies the following relation: (d _Off-12-Pij + d _Off-23-Pij ) ≥ [T _R - (d _{1 -Pij} + d ₂ - _Pij + d _3-Pij )] / 2. According to the invention, in order to reduce the color-breaking defects, the distribution of the activation phases of the pixels in three successive sub-frames of the same series is tightened compared to the prior art: the periods of activating the pixels of the first primary image towards the end of the subframe of this first image, and, during the subframe of the third primary image, shifting the activation periods of the pixels of this third primary image. towards the beginning of the subframe of this third image. This advantageously reduces the color break-up defects in the display of the video sequence.

• l'affichage des pixels n'est pas réalisé par modulation de la durée d'activation de ces pixels comme dans l'invention, mais par modulation de l'amplitude du rétro-éclairage (« backlight ») ;
• les périodes « noires » d_Off-12-Pij, d_Off-23-Pij entre l'activation d'un même pixel d'images primaires différentes d'une même série (c'est-à-dire d'une même trame) sont identiques (voir les distances d_Off-12-Pij entre R et G successifs d'une part, et d_Off-23-Pij entre G et B successifs d'autre part, sur les figures 4C,10C, 12C) ;
• les périodes noires en fin de trame sur les figures 10C, 12C ne correspondent pas à une intervalle de temps entre la fin de l'activation d'un pixel pour afficher une image primaire et le début de l'activation du même pixel pour afficher une autre image primaire de la même série (ou même trame), mais d'une autre série (ou autre trame).

Note that in the document US6570554 :

• the display of the pixels is not achieved by modulating the activation time of these pixels as in the invention, but by modulation of the backlight amplitude ("backlight");
• the "black" periods of _Off-12-Pij , _Off-23-Pij between the activation of the same pixel of different primary images of the same series (that is to say, of the same frame) are identical (see the distances of _Off-12-Pij between R and G successive on the one hand, and _Off-23-Pij between G and B successive on the other hand, on Figures 4C , 10C, 12C);
• the black periods at the end of the frame in FIGS. 10C, 12C do not correspond to a time interval between the end of the activation of a pixel to display a primary image and the beginning of the activation of the same pixel to display a another primary image of the same series (or same frame), but of another series (or other frame).

Also note that the term "pixel activation" here induces the emission of this pixel (upstream of a liquid crystal cell for example) and can not, as in US6570554 , designate the activation of the backlight of a liquid crystal cell.

Preferably, if T _S2 is the maximum permissible duration of activation of the pixels of said imager during the display of the second primary image, regardless of said pixel P _ij , the relation is: (d _Off-12-Pij + d _Off-23-Pij + d _2-Pij ) ≤ T _S2 ; advantageously, said relation is applicable for each of the images to be displayed of said sequence.

According to this advantageous variant, during the subframe of the first primary image, all the activation pulses of the pixels preferably end at the end of this subframe, and during the subframe of the third image. primary, all the pixel activation pulses preferably all start at the beginning of this subframe. Thus, even more significantly, the color break-up defects in the display of the video sequence are reduced.

According to a first preferred variant, there is furthermore the relation: d _Off-12-Pij = d _Off-23-Pij ; this relation then applies to each pixel Pij of the color images of said sequence, for each series of at least three primary images intended to display each of these images. This relation implies that, for each series, the pixel activation phases for the display of the second primary image are centered with respect to the activation phases of the pixels for the display of the first and third primary images of the pixels. this series.

According to another preferred variant, there is furthermore the relation: d _Off-12-Pij = 0 and / or _Off-23-Pij = 0; this relation then applies to each pixel Pij of the color images of said sequence, for each series of at least three primary images intended to display each of these images. This relation implies that, for each series, the activation phases of the pixels for the display of the second primary image are contiguous to the activation phase of the pixels for the display of the first or third primary image of this image. series.

Preferably, the primary color associated with said second primary image is green; the other primary colors, that of the first image and that of the second image, are preferably red and blue; thus, according to the invention, it is the subframes red and blue which are close to the green subframe, to reduce, in particular, the defects of color break. The subject of the invention is also an image display system comprising a matrix imager provided with a two-dimensional array of activatable pixels and means for activating said pixels that are adapted to apply the method according to the invention.

Preferably, the activatable pixels of said imager are formed by electro-optical valves, and the system further comprises means for successively illuminating said imager with each primary color. For the display of each primary image, the imager is then illuminated by the corresponding primary color from the illumination means. The duration of illumination of each primary color is then the maximum permissible duration of activation of the pixels of the imager during the display of the primary image corresponding to this primary color.

Preferably, said illumination means comprise a light source emitting in said three primary colors, optical means for directing the light emitted by said source onto the electro-optical valve network of said imager and a colored wheel which is interposed on the path this light between said source and said imager and which comprises segments of color filters, each filter being adapted to transmit one of the different primary colors emitted by the source. The rotation of the wheel coloring allows to illuminate successively the imager by each primary color.

• la figure 1 illustre schématiquement un mode de réalisation d'un système d'affichage d'images permettant d'utiliser le procédé selon l'invention ;
• la figure 2 représente un circuit de commande de pixel de l'imageur du système d'affichage d'images de la figure 1 ;
• la figure 3 représente la roue colorée du système d'affichage d'images de la figure 1 et le découpage de la durée T_F d'une trame d'image en deux périodes T_R de rotation de cette roue, elles-mêmes subdivisées en trois phases 1, 2 et 3 d'illumination par différentes couleurs primaires, de durées respectives T_S1, T_S2 et T_S3 ;
• la figure 4 représente, pour un même pixel d'une image couleur à afficher par un premier mode de mise en oeuvre du procédé selon l'invention, les chronogrammes suivants : signal vidéo V_VIDEO et signal de référence V_RAMP, tension V_MIR appliquée à l'électrode inférieure de la valve optique correspondant à ce pixel, tension V_ITO appliquée à l'électrode supérieure de cette même valve optique, différence de potentiel entre les électrodes de cette valve, échelonnement des phases d'activation de cette valve qui en résulte, et échelonnement des phases d'illumination de cette valve conformément à la figure 3 ;
• les figures 5 et 6 représentent les mêmes chronogrammes, pour un même pixel d'une image couleur à afficher respectivement par un deuxième et un troisième mode de mise en oeuvre du procédé selon l'invention.

Preferably, the system comprises a projection lens adapted and positioned to produce the image of said imager on a projection zone. This projection zone is generally formed by a projection screen which, optionally, can be integrated into the system (in the case of backlights). The invention will be better understood on reading the description which follows, given by way of nonlimiting example, and with reference to the appended figures in which:

• the figure 1 schematically illustrates an embodiment of an image display system for using the method according to the invention;
• the figure 2 represents a pixel control circuit of the imager of the image display system of the figure 1 ;
• the figure 3 represents the colorful wheel of the image display system of the figure 1 and cutting the duration T _F of an image frame into two rotational periods T _R of this wheel, themselves subdivided into three phases 1, 2 and 3 of illumination by different primary colors, of respective durations T _S1 , T _S2 and T _S3 ;
• the figure 4 represents, for the same pixel of a color image to be displayed by a first mode of implementation of the method according to the invention, the following chronograms: video signal V _VIDEO and reference signal V _RAMP , voltage V _MIR applied to the lower electrode of the optical valve corresponding to this pixel, V _ITO voltage applied to the upper electrode of this same optical valve, potential difference between the electrodes of this valve, staggering of the activation phases of this valve which results, and phasing of the illumination phases of this valve according to the figure 3 ;
• the figures 5 and 6 represent the same timing diagrams, for the same pixel of a color image to be displayed respectively by a second and a third mode of implementation of the method according to the invention.

The figures representing chronograms do not take into account a scale of values in order to better reveal certain details that would not be clearly visible if the proportions had been respected.

• un imageur matriciel 1 comprenant un réseau bidimensionnel de pixels activables P_ij, ici des valves à cristaux liquides ; ces pixels sont répartis en colonnes i et en lignes j ; comme illustré à la figure 2, chaque valve comprend une cellule à cristaux liquides LC intercalée entre une électrode supérieure transparente en ITO et une électrode inférieure réfléchissante MIR ; l'électrode supérieure transparente est commune à toutes les valves de l'imageur ;
• des moyens d'illumination de cet imageur comprenant une source lumineuse 2 alimentée par une alimentation électrique 8 et émettant dans trois couleurs primaires répertoriées C1 pour la couleur rouge, C2 pour la couleur verte et C3 pour la couleur bleue, des moyens optiques non représentés adaptés pour diriger la lumière émise par cette source sur le réseau de valves à cristaux liquides de l'imageur 1, et une roue colorée 3 interposée sur le trajet du faisceau de la source éclairant l'imageur ; la roue colorée 3 comprend trois segments S₁, S₂, S₃ de filtres colorés laissant passer respectivement la première (rouge), la deuxième (verte) et la troisième (bleue) couleur primaire d'émission de la source 2 ; cette roue colorée est entraînée par un moteur 7 de manière à pouvoir éclairer successivement l'imageur 3 par chaque couleur primaire lors d'une rotation de cette roue ; la période de rotation de cette roue est nommée T_R ;
• un objectif de projection 4 adapté et positionné pour réaliser l'image de l'imageur 1 sur une zone de projection non représentée ;
• des moyens de contrôle du système 5 qui, associés à des moyens d'activation de chaque valve optique C_ij représentés à la figure 2, permettent de contrôler l'activation des pixels P_ij de l'imageur, de la source lumineuse 2 via son alimentation 8, et la rotation de la roue colorée 3 via son moteur d'entraînement 7;
• une interface d'entrée 6 apte à recevoir des signaux vidéo représentatifs des images d'une séquence vidéo, et à décomposer chaque image en deux séries de trois images primaires, une première image primaire de couleur rouge, une deuxième image primaire de couleur verte, et une troisième image de couleur bleue.

We will now describe, with reference to the figure 1 an embodiment of the image display system according to the invention; this system includes:

• a matrix imager 1 comprising a two-dimensional array of activatable pixels P _ij , here liquid crystal valves; these pixels are divided into columns i and lines j; as shown in the figure 2 each valve comprises an LC liquid crystal cell interposed between a top transparent ITO electrode and a lower reflective MIR electrode; the upper transparent electrode is common to all the valves of the imager;
• illumination means of this imager comprising a light source 2 fed by a power supply 8 and emitting in three primary colors listed C1 for the red color, C2 for the green color and C3 for the blue color, optical means not shown adapted for directing the light emitted by this source onto the array of liquid crystal valves of the imager 1, and a colored wheel 3 interposed on the beam path of the source illuminating the imager; the colored wheel 3 comprises three segments S ₁ , S ₂ , S ₃ of color filters passing respectively the first (red), second (green) and third (blue) primary emission color of the source 2; this colored wheel is driven by a motor 7 so as to be able to illuminate successively the imager 3 with each primary color during a rotation of this wheel; the rotation period of this wheel is named T _R ;
• a projection lens 4 adapted and positioned to make the image of the imager 1 on a projection area not shown;
• system control means 5 which, associated with activation means of each optical valve C _ij shown in _FIG. figure 2 , allow to control the activation of the pixels P _ij of the imager, the light source 2 via its power supply 8, and the rotation of the color wheel 3 via its drive motor 7;
• an input interface 6 adapted to receive video signals representative of the images of a video sequence, and to decompose each image into two series of three primary images, a first primary image of red color, a second primary image of green color, and a third blue color image.

The angular widths of the colored filter segments S ₁ , S ₂ , S ₃ of the colored wheel 3 are preferably adapted in a manner known per se so that, during each rotation of this wheel, the illumination times T _S1, T _S2 , T _S3 of the imager in each primary color are adapted so that the fusion of the resulting illuminations forms a white tint; this white color generally corresponds to a target color temperature .; this arrangement advantageously makes it possible to make the most of the luminous flux emitted by the source 2; for convenience, T _S1 = T _S2 = T _S3 was chosen here.

• deux mémoires MA, MB adaptées pour stocker une donnée vidéo V_vidéo représentative du pixel correspondant d'une image primaire à afficher ;
• un multiplexeur MUX relié aux deux mémoires MA, MB, qui est adapté pour sélectionner le contenu de l'une ou de l'autre mémoire ;
• un comparateur COMP relié à la sortie du multiplexeur MUX et à une entrée de référence RAMP du circuit, adapté pour comparer le contenu V_vidéo de la mémoire sélectionnée par le multiplexeur MUX et le signal V_RAMP appliqué sur l'entrée de référence RAMP, de manière à délivrer un signal de sortie V_MIR de valeur haute V_MIR-H ou de valeur basse V_MIR-L, selon la logique suivante : si V_vidéo > V_RAMP, alors V_MIR = V_MIR-H, sinon V_MIR = V_MIR-L; la sortie de ce comparateur est reliée à l'électrode inférieure réfléchissante MIR du pixel P_ij.

With reference to the figure 2 the matrix imager 1 further comprises an array of control circuits, thereby forming what is called an active matrix; each circuit is for the control of a pixel; each circuit C _ij which controls a pixel P _ij comprises:

• two memories MA, MB adapted to store video _video data V representative of the corresponding pixel of a primary image to be displayed;
• a multiplexer MUX connected to the two memories MA, MB, which is adapted to select the contents of one or the other memory;
• a comparator COMP connected to the output of the multiplexer MUX and to a reference input RAMP of the circuit adapted to compare the _video content V of the memory selected by the multiplexer MUX and the signal V _RAMP applied to the reference input RAMP, in order to deliver an output signal V _MIR of high value V _MIR-H or of low value V _MIR-L , according to the following logic: if V _video > V _RAMP , then V _MIR = V _MIR-H , else V _MIR = V _MIR-L ; the output of this comparator is connected to the lower reflective electrode MIR of the pixel P _ij .

• des entrées mémoires, déjà décrites, reliées à des électrodes de colonnes X_i ;
• des commandes d'accès W_MA et W_MB aux mémoires MA et MB, reliées à des électrodes de lignes, non représentées ; ainsi, tous les circuits de commande C_ij d'une même ligne j partagent ces commandes d'accès ;
• une commande de sélection des mémoires SEL_MA_MB, et une entrée de référence RAMP, déjà décrites, reliées chacune à une électrode commune au panneau ; ainsi, tous les circuits de commande C_ij de l'imageur 1 partagent la même commande de sélection des mémoires et le même signal de référence ;

Each control circuit C _ij thus comprises the following inputs:

• memory inputs, already described, connected to column electrodes X _i ;
• access commands W_MA and W_MB to the memories MA and MB, connected to row electrodes, not shown; thus, all the control circuits C _ij of the same line j share these access commands;
• a memory selection control SEL_MA_MB, and a reference input RAMP, already described, each connected to an electrode common to the panel; thus, all the control circuits C _ij of the imager 1 share the same memory selection command and the same reference signal;

We will now describe a first embodiment of the method according to the invention for displaying a sequence of images using the image display system which has just been described.

The duration T _F of each image of this sequence, or image frame duration, is divided here into two series of three primary images; each series of three primary images corresponds to a rotation period T _R of the colored wheel. As indicated above, the time allocated to the illumination of the imager by each primary color during a colored wheel revolution is here T _S1 = T _S2 = TS3 ^; we therefore have T _R = T _S1 + T _S2 + T _S3 and T _F = 2 × T _R ; for example, T _F = 20 ms. The input interface 6 delivers to the control means 5 series of three primary images; each primary image is output as a video signal for each pixel of that image to be displayed; with reference to the figure 3 during the display of a first primary image of a series, by means of the control means, the video signals for displaying pixels of the second pixel are loaded into the memories MA or MB of each pixel control circuit; primary image, which is to be displayed immediately after the first being displayed; for this loading, one proceeds for example by selecting each line of pixels of the imager and, a line being selected, using the access control for example W_MA of the memories MA, the access of the memories MA is opened each of the control circuits of the pixels of this line and, using the column electrodes X _i , the values of the video data of the pixels of the corresponding line of the image to be displayed are addressed to these memories; when all the second primary image to be displayed is thus stored in the active matrix of the imager and the illumination duration T _S1 in the first primary color has elapsed, using the SEL_MA_MB memory selection command, one triggers the delivery by the MUX multiplexers of these video signals V _VIDEO to one of the inputs of the comparators COMP; simultaneously, the RAMP reference input of these comparators COMP is sent with a ramp signal V _RAMP = R2 as represented in the upper graph of FIG. figure 4 ; this is a linear signal increasing during the first half of the illumination phase of the imager in the second primary color, then linear decreasing during the second half of this illumination phase; while that the imager is now illuminated by the second primary color, each control circuit comparator C _ij compares the signals V _VIDEO and V _RAMP , and delivers a logic signal V _MIR ; the shape of the ramp signal V _RAMP = R ₂ implies here, as illustrated in the second graph of the figure 4 a centering of the pixel activation phases, of duration d _{2 -Pij} , on the illumination phase of the imager by the second primary color, of duration T _S2 . The third graph of the figure 4 gives the value of the V _ITO potential applied to the transparent upper electrode of the electro-optical valves: this potential is here equal to V _MIR-H . The fourth graph of the figure 4 gives the voltage V _LC applied across the optical valves, which is equal to V _MIR - V _ITO .

• la fin des phases d'activation de tous les pixels de la première image primaire de chaque série coïncide avec la fin de la phase d'illumination de l'imageur par la première image primaire ;
• le début des phases d'activation de tous les pixels de la troisième image primaire de chaque série coïncide avec le début de la phase d'illumination de l'imageur par la troisième image primaire.

For each series of primary images to be displayed, the display of the third and of the first primary image is obtained according to the same method, extrapolated from the method for displaying the second primary image; the top graph of the figure 4 gives the signal V _RAMP = R ₁ applied during the display of the first primary image, which is linear decreasing, and the signal V _RAMP = R ₃ applied during the display of the third primary image, which is linear decreasing; the "activation" chart of the figure 4 gives the phasing of the three pixel activation periods that result: we see that:

• the end of the activation phases of all the pixels of the first primary image of each series coincides with the end of the illumination phase of the imager by the first primary image;
• the beginning of the activation phases of all the pixels of the third primary image of each series coincides with the beginning of the illumination phase of the imager by the third primary image.

• d_1-Pij, d_2-Pij, d_3-Pij comme les durées d'activation de ce pixel pour l'affichage, respectivement, de la première, de la deuxième et de la troisième image primaire de ladite série,
• d_Off-12-Pij comme l'intervalle de temps entre la fin t_Off-1-Pij de l'activation de ce pixel P_iJ pour afficher la première image primaire et le début t_On-2-Pij de l'activation du même pixel P_ij pour afficher cette deuxième image primaire,
• d_Off-23-Pij comme l'intervalle de temps entre la fin t_Off-2-Pij de l'activation de ce pixel P_ij pour afficher la deuxième image primaire et le début t_On-3-Pij de l'activation du même pixel P_ij pour afficher la troisième image primaire,

on constate que :

• on a la relation : d_Off-12-Pij + d_2-Pij + d_Off-23-Pij = T_S2 ;
• du fait du centrage des phases d'activation des pixels pour l'affichage des deuxièmes images primaires de chaque série, on a d_Off-12-_Pij = d_Off-23-Pij.

Thus, if one defines, for each pixel P _ij of the color image to be displayed:

• d _1-Pij , d _{2 -Pij} , d _3-Pij as the activation times of this pixel for the display, respectively, of the first, second and third primary images of said series,
• d _Off-12-Pij as the time interval between the end t _Off-1-Pij of the activation of this pixel P _iJ to display the first primary image and the start t _On-2-Pij of the activation of the same pixel P _ij to display this second primary image,
• d _Off-23-Pij as the time interval between the end t _Off-2-Pij of the activation of this pixel P _ij to display the second primary image and the start t _On-3-Pij of the activation of the same pixel P _ij to display the third primary image,

We observe that :

• we have the relation: d _Off-12-Pij + d _{2 -Pij} + d _Off-23-Pij = T _S2 ;
• because of the centering of the activation phases of the pixels for the display of the second primary images of each series, we have _Off-12 - _Pij = d _Off-23-Pij .

The control method which has just been described makes it possible to substantially reduce the color-breaking defects for the video-sequence display.

It should be noted that the use of reference signals in the form of a ramp for controlling the transmission duration of the pixels of an imager is described in the prior art, for example in the document US2001-026261 .

A second embodiment of the method according to the invention will now be described, again for displaying a sequence of images using the image display system which has just been described.

The only difference with the first method which has just been described resides in the form of the reference signal V _RAMP ; here, in each series of three primary images, instead of the previous succession R ₁ , R ₂ , R ₃ , there is, with reference to the figure 5 , the succession R ' ₁ : linear decreasing, R' ₂ : linear decreasing again, and R ' ₃ : linear increasing, so that the time interval of _Off-12-Pij between the end of the activation of each pixel P _ij to display the first primary image, red, and the beginning of the activation of the same pixel P _ij to display the second primary image, green, is always zero; we always have the relation (of _Off-12-Pij = 0) + of _2-Pij + of _Off23-Pij = T _S2 ; in this embodiment, the activation phases of all the pixels of the first primary image, red, are contiguous to the activation phases of all the pixels of the second primary image, green; there is also a significant reduction in color-breaking defects for video-sequence display. A third embodiment of the method according to the invention will now be described, again for displaying a sequence of images using the same image display system. The only difference with the first method is also the shape of the reference signal V _RAMP ; here, in each series of three primary images, instead of the succession R ₁ , R ₂ , R ₃ of the first mode realization, we have, with reference to the figure 6 , the succession R " ₁ = R ' ₁ : linear decreasing, R" ₂ : linear increasing, and R " ₃ = R' ₃ : linear increasing, so that the time interval of" _Off-23-Pij between the end of the activation of each pixel P _ij to display the second primary image, green, and the beginning of the activation of the same pixel P _ij to display the third primary image, blue, is always zero; we always have the relation of _Off-12-Pij + d " _2-Pij + (d" O _ff-23-Pij = 0) = T _S2 , in this embodiment, the activation phases of all the pixels of the second primary image, green, are contiguous to the activation phases of all the pixels of the third primary image, blue, and there is also a significant reduction in the color failure defects for the video sequence display.

If, in all the embodiments presented, we have the relation of _Off-12-Pij + d _{2 -Pij} + d _Off-23-Pij = T _S2 , the invention also includes the cases where we would have _{Off -12-Pij} + d _{2 -Pij} + d _Off-23 - _Pij <T _S2 .

If, in all the embodiments presented, the pixel activation phases of the first primary image of each series always end at the same time as the illumination phase of the imager by this primary color, and the phases of activation of pixels of the third primary image always start at the same time as the illumination phase of the imager by this primary color, the invention includes cases where the end or the beginning of these phases do not coincide, of the moment that the following relation is satisfied: (d _Off-12-Pij + d _Off-23-Pij ) <[T _R - (d _{1 -Pij} + d _{2 -Pij} + d _3-Pij )] / 2. Note that this relationship is obviously satisfied in all the embodiments that have just been presented.

The invention has been described with reference to a decomposition of each image of a video sequence into two sets of three successive primary images of different primary colors; the invention also applies to cases of decomposition of each image into a single series of three primary images, or in addition to two sets of three primary images; the different series may have different durations.

The invention also applies to cases where each series has a number of primary images greater than three, as long as there are three successive in each series to apply the method according to the invention; by extension, among the primary colors, one can even count a color of white tint.

The invention has been described with reference to an image display system in which the sequencing of the primary images is provided by a colored wheel; other modes of sequencing the primary images can be used without departing from the invention.

The invention has been described with reference to a projection image display system where the activatable pixels of the imager are liquid crystal valves; other activatable pixels can be used without departing from the invention, such as micromirror pixels (DMD) or LED pixels, especially when they are controllable in time modulation in an analog manner, as described for example in the document US6590549 . Note that in the document WO2006 / 003091 , micromirroirs are not controllable analogically.

The invention has been described with reference to a projection image display system; other image display systems may be used to implement the invention.

Claims (8)

1. A method of displaying a sequence of color images using an imaging device (1) equipped with a two-dimensional matrix of active pixels in which, in order to display at least one color image from said sequence, said image being decomposed into at least one series of three successive primary images of different primary colors, the pixels of said three primary images from at least one series are successively displayed by modulating the activation duration of said corresponding pixels of the imaging device, characterized in that, for each series having a duration T_R, if the following definitions are made for each pixel P_ij of said color image:

   - d_1-Pij, d_2-Pij, d_3-Pij as the activation durations of said pixel for the display of the first, second and third primary image, respectively, of said series;

   - d_Off-12-Pij as the time interval between the end t_Off-1-Pij of activation of said pixel P_ij for displaying said first primary image from said series and the beginning t_On-2-Pij of activation of the same pixel P_ij for displaying said second primary image from the same series; and

   - d_Off-23-_Pij as the time interval between the end t_Off-2-Pij of activation of said pixel P_ij for displaying said second primary image from said series and the beginning t_On-3-_Pij of activation of the same pixel P_ij for displaying said third primary image from the same series;

   - T_S2 as the maximum admissible duration of pixel activation of said imaging device during the display of the second primary image,

   then, for any said pixel P_ij of the imaging device and for each series, the following relations hold: $$\left({\mathrm{d}}_{\mathrm{O}\mathrm{f}\mathrm{f}-12-\mathrm{P}\mathrm{i}\mathrm{j}}+{\mathrm{d}}_{\mathrm{O}\mathrm{f}\mathrm{f}-23-\mathrm{P}\mathrm{i}\mathrm{j}}\right)<\left[{\mathrm{T}}_{\mathrm{R}}-\left({\mathrm{d}}_{1-\mathrm{P}\mathrm{i}\mathrm{j}}+{\mathrm{d}}_{2-\mathrm{P}\mathrm{i}\mathrm{j}}+{\mathrm{d}}_{3-\mathrm{P}\mathrm{i}\mathrm{j}}\right)\right]/2,$$

   

   and $$\left({\mathrm{d}}_{\mathrm{O}\mathrm{f}\mathrm{f}-12-\mathrm{P}\mathrm{i}\mathrm{j}}+{\mathrm{d}}_{\mathrm{O}\mathrm{f}\mathrm{f}-23-\mathrm{P}\mathrm{i}\mathrm{j}}+{\mathrm{d}}_{2-\mathrm{P}\mathrm{i}\mathrm{j}}\right)\le {\mathrm{T}}_{\mathrm{S}2}\mathrm{.}$$

   
2. The method as claimed in claim 1, characterized in that it furthermore holds that: d_Off-12-Pij = d_Off-23-Pij.
3. The method as claimed in claim 1, characterized in that it furthermore holds that: d_Off-12-Pij = 0 and/or d_Off-23-Pij = 0.
4. The method as claimed in any one of claims 1 to 3, characterized in that the hue of the primary color associated with said second primary image is green.
5. An image display system comprising a matrix imaging device (1) equipped with a two-dimensional matrix of active pixels and means for activating (5) said pixels, characterized in that they are suitable for applying the method as claimed in any one of the preceding claims.
6. The system as claimed in claim 5, in which the active pixels of said imaging device are formed by electro-optical valves, characterized in that it furthermore comprises means for successively illuminating said imaging device with each primary color.
7. The system as claimed in claim 6, characterized in that said illumination means comprise a light source (2) emitting said three primary colors, optical means for directing the light emitted by this source onto the matrix of electro-optical valves of said imaging device and a color wheel (3) that is placed in the path of this light between said source (2) and said imaging device (1) and which comprises colored filter segments (S1, S2, S3), each filter being suited to transmit one of the various primary colors emitted by the source (2).
8. The system as claimed in either of claims 6 or 7, characterized in that it comprises a projection lens (4) that is suitable and positioned for producing the image of said imaging device (1) on a projection area.

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