FR2987211A1 - Method for displaying stereoscopic images on optically reflective screen, involves producing luminous flux by video projector, projecting polarized flux on optically reflective screen and blocking flux only during optical transition states - Google Patents

Abstract

The method involves producing luminous flux (2) comprising a video sequence of stereoscopic images by a LCD or liquid crystal-on-silicon display video projector, where the sequence includes alternate subsequences of temporally multiplexed images. The luminous flux is polarized (4), so that the subsequences exhibit optical polarization in mutual orthogonal directions. The polarized luminous flux is projected (6) on an optically reflective screen. The polarized luminous flux is blocked (8) only during optical transition states. Independent claims are also included for the following: (1) a display for stereoscopic images (2) a visualization assembly for stereoscopic images.

Description

The invention relates to a method for displaying stereoscopic images. The invention also relates to a stereoscopic image display. The invention finally relates to a stereoscopic image display assembly. A method of displaying stereoscopic images for viewing with passive stereoscopic glasses is known. This method comprises: the production, by a video projector, of a light flux including a video sequence of stereoscopic images comprising an alternation of two sub-sequences of temporally multiplexed images, each of these two sub-sequences being intended to be viewed respectively by a left eye and a right eye of a user with stereoscopic glasses; Polarization of the luminous flux produced so that the two sub-sequences have optical polarizations of mutually orthogonal directions, because of polarization induced by a polarization modulator; the projection, on an optically reflective polarization conservation screen, of the polarized light flux. The user has passive stereoscopic glasses in which filters filter optical polarizations of mutually orthogonal directions, so that a subsequence for one eye is masked to the other eye. In order to obtain stereoscopic images having adequate brightness, it is preferable to produce the luminous flux with a liquid crystal display (LCD) or with a liquid crystal on-silicon projector (LCoS , "Liquid crystal on silicon" in English). Such liquid crystal projectors 30 make it possible to generate a polarized light and thus to induce a very low absorption through a linear polarizer used upstream of the polarization modulator. By LCD liquid crystal projector is meant a video projector in which an image is generated by transmitting a light beam through a liquid crystal device comprising a plurality of pixels. By LCoS liquid crystal projector is meant a video projector in which an image is generated by reflection of a light beam on a liquid crystal device comprising a plurality of pixels. The LCoS projector can for example be such as the video projector marketed by JVC under the reference DLA-RS40.

However, the liquid crystal devices of the LCD or LCoS projectors exhibit an optical transition state when switching between two consecutive images. This optical transition state has a duration typically between 1 ms and 4 ms. This transitional optical state is responsible for the appearance in projected stereoscopic images of visual artifacts, such as ghosting in English. The invention aims to solve this drawback. The invention thus relates to a method for displaying stereoscopic images, comprising: the production, by a liquid crystal video projector or a liquid crystal-on-silicon projector, of a light flux including a video sequence of images stereoscopic images comprising an alternation of two sub-sequences of temporally multiplexed images, these two sub-sequences being intended to be viewed respectively by a left eye and by a right eye of a user equipped with stereoscopic glasses, two immediately consecutive images of the sequence of images being further separated by an optical transition state related to the switching of the video projector; the polarization of the luminous flux produced so that the two sub-sequences have optical polarizations of mutually orthogonal directions; projection, on an optically reflecting polarization-preserving screen, of the polarized luminous flux; characterized in that it further comprises a step of obstruction of the polarized light flux during the transition optical states and only during these transition optical states. According to a variant, the method comprises: the emission, by the video projector, of a synchronization signal synchronized with its switching between the two sub-sequences of temporally multiplexed images; applying a control signal to a shutter to switch it between an optically on state and an optically blocking state when the synchronization signal indicates the switching of the video projector. The invention also relates to a stereoscopic image display 35 comprising: a liquid crystal or liquid crystal projector on silicon, configured to produce a light flux including a video sequence of stereoscopic images comprising an alternation of two multiplexed sub-sequences temporally, these two sub-sequences being intended to be viewed respectively by a left eye and a right eye of a user provided with stereoscopic glasses, two immediately consecutive images of the sequence of images being further separated by an optical state of transition linked to the switching of the video projector; a polarization modulator, receiving the luminous flux from the video projector, configured to apply to the two sub-sequences of images optical polarizations of mutually orthogonal directions; an optically reflecting polarization conservation screen, configured to display the stereoscopic images projected by the video projector. The display comprises a shutter, receiving the luminous flux from the projector, configured to switch to an optically blocking state during transition optical states and only during these transition optical states. According to one variant, the projector includes an output interface configured to deliver a synchronization signal indicating the switching of the video projector. According to another variant, the display comprises a control device including: an interface for receiving a synchronization signal; an output interface configured to deliver a control signal to the shutter; a computer, configured to deliver a control signal switching the shutter between an optically-on state and an optically-blocking state when the synchronization signal indicates a switching of the video projector. According to yet another variant, the computer is configured to deliver a second control signal that switches the shutter from the optically blocking state to the optically-on state after a predefined time after delivering a control signal switching the control. shutter optically to an optically blocking state. According to one variant, the shutter is placed directly upstream of the polarization modulator. According to another variant, the shutter comprises: a liquid crystal cell; two linear polarizers, directly surrounding the liquid crystal cell and whose directions of polarization are mutually orthogonal. According to another variant, the shutter comprises a cholesteric liquid crystal shutter. The invention further relates to a stereoscopic image viewing assembly, comprising: a stereoscopic image display as described above; 2 9 8 7 2 1 1 4 passive stereoscopic glasses having two polarizing filters, each of these filters being configured to block one of the two image sequences produced by the display.

Other characteristics and advantages of the invention will emerge clearly from the description which is given hereinafter, by way of indication and in no way limiting, with reference to the appended drawings, in which: FIG. 1 illustrates a display assembly of FIG. stereoscopic images; - Figure 2 schematically illustrates a section of a liquid crystal cell; FIG. 3 schematically illustrates a shutter of the image display assembly of FIG. 1; FIG. 4 illustrates the control device of the image display assembly of FIG. 1; FIG. 5 schematizes, as a function of time, examples of control and synchronization signals of the control device, accompanying a video sequence fragment of stereoscopic images displayed by the set of FIG. 1; FIG. 6 is a flowchart of a method for displaying stereoscopic images; FIG. 7 illustrates another embodiment of the shutter of FIG.

The invention provides a method of displaying stereoscopic images for viewing stereoscopic images with increased brightness and a reduced number of visual artifacts. Figure 1 illustrates a set 50 for viewing stereoscopic images. This set 50 comprises: a display 20 of stereoscopic images; passive stereoscopic glasses 52. The display 20 comprises: a video projector 22; a polarization modulator 24; a screen 26; a shutter 28; a control device 30. The video projector 22 is configured to produce a luminous flux 23 including a video sequence of stereoscopic images. This video sequence 40 comprises an alternation of two sub-sequences temporally multiplexed. Each of these two sub-sequences is intended to be viewed respectively by a left eye and a right eye of a user provided with glasses 52. This projector 22 is a liquid crystal projector or a liquid crystal projector on silicon. Also, two immediately consecutive images of the image sequence are separated by an optical transition state. This optical transition state is related to the switching of the video projector 22, in particular internal liquid crystal cells. This optical transition state has a duration typically between 1 ms and 4 ms. The video projector 22 advantageously comprises an output interface 25, configured to output a synchronization signal indicating the switching of the video projector 22. This interface 25 is, for example, an electrical connector. The polarization modulator 24 is placed at the output of the projector 22 so as to receive the luminous flux 23 produced by the projector 22. This modulator 24 is here configured to apply optical polarizations of mutually orthogonal directions to the two sub-sequences of images. . For example, the modulator 24 is a liquid crystal optical polarization modulator, well known to those skilled in the art. The screen 26 is intended to receive the luminous flux 23 produced by the video projector 22. This screen 26 is optically reflective and maintains the optical polarization of the projected images. The shutter 28 is placed at the output of the video projector 22, so as to receive the luminous flux 23 produced by the video projector 22. The shutter 28 is configured to be in an optically blocking state during the transition optical states, and only during these periods. transitions optical states. By optically blocking it is meant that the optical transmission is less than 10%. Thus, the luminous flux 23 produced by the video projector 22 is blocked during the transitions optical states of said projector 22. The number of visual artifacts related to this switching and perceived by a user of the glasses 52 is thus reduced. The shutter 28 comprises a liquid crystal cell 40, illustrated in FIG. 2. The cell 40 contains a layer 41 of liquid crystals interposed between two electrodes 43, 45. The distance between these electrodes 43, 45 is advantageously at least equal to at 1pm and preferably at least 2pm. This distance is advantageously at most equal to 10pm. The liquid crystals 41 are twisted-nematic liquid crystals here, for example, the liquid crystals marketed by Merck KGaA under the reference MDA-7030. Such a liquid crystal can be combined with an anti-parallel friction of the substrates of the electrodes 43 and 45. In this example, illustrated in FIG. 3, the shutter 28 also advantageously comprises two linear polarizers 42, 44, the 2 9 8 Optical polarization directions are mutually orthogonal, the cell 40 being directly interposed between these two linear polarizers 42, 44. Such a configuration ensures better extinction during the blocking optical state. The polarizer 42 is placed upstream of the polarizer 44, with respect to the direction of propagation of the luminous flux. The polarizer 42 is arranged so that its optical polarization direction is parallel to the direction of polarization of the light emitted by the projector 22. It is considered here that all the chromatic components of the luminous flux 23 emitted by the video projector 22 generally have a same optical polarization.

Here, the shutter 28 is placed directly upstream of the modulator 24 and is advantageously bonded to an input surface of the modulator 24. Thus, it is not necessary to add a polarizer input to the modulator 24. The cell 40 is advantageously arranged such that the angle between the polarization axis of the polarizer 42 and the alignment direction of the helical nematic liquid crystal molecules measured on the surface of the cell 40 adjacent to the polarizer 42 is equal to 45 ° . The control device 30, illustrated in FIG. 4, comprises: a reception interface 32 of a synchronization signal; an output interface 36 of a control signal on the shutter; A computer 34. The reception interface 32 is here connected directly to the output interface 25 of the video projector 22. The computer 34 is configured to deliver, on the output interface 36, a control signal switching the shutter 28 between an optically on state and an optically blocking state when the synchronization signal, received on the interface 32, indicates a switching of the projector 22. This control signal is, for example, an electric potential difference applied between the two electrodes 43, 45 of the cell 40. The computer 34 is advantageously also configured to deliver a second control signal configured to switch the shutter from the optically blocking state to the optically conducting state, after a predefined delay after delivering a control signal switching the shutter between an optically on state and an optically blocking state. This predefined delay is greater than or equal to the duration of the optical transient state of the video projector 22. This delay is here greater than or equal to 1 ms. In known manner, the device 30 may also comprise an output interface 37 of a control signal of the modulator 24. In known manner, the passive stereoscopic glasses 52 may comprise two linear optical polarizers of mutually orthogonal polarization directions 40. These directions are aligned with the directions of polarization conferred on the luminous flux by the modulator 24.

An example of operation of the display 20 will now be described, with reference to the stereoscopic image display method of FIG. 6 and with the aid of FIG. 5. During a step 2, the video projector 22 produces a luminous flux 23 comprising a sequence of IMG stereoscopic images. This IMG sequence comprises two sub-sequences of temporally multiplexed images. One of these sub-sequences of OD images is intended to be viewed by a right eye of a user of the glasses 52, the other of these sub-sequences of OG images is intended to be viewed by a left eye of said user . Two consecutive OD, OG images of the IMG sequence are separated by a transition optical state 60, related to the switching of the video projector 22. By switching the video projector 22, here is meant the moment when the video projector 22 alternates between the two. OD, OG image sequences. The video projector 22 switches here at a frequency of 144 Hz. Advantageously, during a step 10, the video projector 22 transmits a synchronization signal SYNC on the interface 25. This signal SYNC is synchronized with the switching of the video projector 22 between the two sub-sequences of images OD, OG. Here, the SYNC signal is an electrical voltage. This SYNC signal may have a first value 61 or a second value 62. For example, the first voltage value 61 is zero; the second value 62 is between 1V and 50V. The switching of the sub-sequence of images OD to OG is for example indicated by a rising edge 63 of the signal SYNC of the first value 61 to the second value 62. The switching of the sub-sequence of images OG to OD is for example indicated by a falling edge 64 of the signal SYNC of the second value 62 to the first value 61. This signal SYNC is delivered on the input interface 32 of the controller 30. In a step 4, the luminous flux 23 produces by the projector 22 is biased by the modulator 24, so that the two sub-sequences of images have optical polarizations of mutually orthogonal directions. Herein, first and second mutually orthogonal polarization directions are respectively applied to the OD and OG images. In a known manner, the modulator can be synchronized via the device 30 by means of the signal SYNC. Advantageously, during a step 12, the computer 34 of the control device delivers a control signal CMD when the signal SYNC indicates a switching. Here, the CMD signal is an electrical voltage. This CMD signal may take a first value 71 or a second value 72. For example, the first voltage value 71 is zero; the second value 72 is between 1V and 50V. Here, this CMD signal is delivered by the computer 34 40 on the output interface 36, and is applied between the electrodes 43, 45.

During a step 8, the shutter 28 closes the luminous flux 23, switching from an optically state to an optically blocking state. Here, the switching of the shutter 28 to the optically blocking state is carried out by applying the CMD signal to the second value 72 on the electrodes 41, 43 of the cell 40, so as to apply an electric field in a transverse direction to the liquid crystal layer 41. In the presence of such an electric field, the shutter 28 is in an optically blocking state. The shutter 28 is kept in the optically blocking state during the entire duration of the optical state of transition 60. For this purpose, the switching of the shutter 28 to the optically state passing through the computer 34 only intervenes at the end of a predefined period of time after switching of the shutter 28 in the optically blocking state. This duration d is greater than or equal to the duration of the optical transient state 60 of the video projector 22. The optically blocking state may have a duration d of at least 105% of the duration of the transition state 60, preferably at most equal to 120%. This switching is for example controlled by the computer 34 by applying the first value 71 of the CMD signal. In a step 6, the luminous flux 23 is projected onto the screen 26 after passing through the modulator 24 and the shutter 28. The IMG stereoscopic image sequence can be viewed by a user provided with the glasses 52. many other embodiments are possible. In known manner, the luminous flux 23 produced by the video projector 22 can be broken down into three chromatic components corresponding to the primary colors. Due to the internal operation of the video projector 22, these primary colors may have different optical polarizations. For example, at least two primary colors may have mutually orthogonal linear polarizations. In this case, it is possible to use a delay blade for rotating the polarization of one of the primary colors by 90 °. For example, the delay blade marketed by the company Advisol, under the reference Spar-C-50, can rotate the polarization of green 90 ° without changing that of blue and red. If, at the output of the projector, the polarization of the green is orthogonal to that of blue and red (the most common case), the three primary colors will have the same polarization orientation at the output of the delay plate. SYNC synchronization and CMD control signals may take a different form from that detailed previously. As a variant, the modulator 24 comprises a quarter-wave plate placed at the output of the modulator 24. Thus, the linear polarization of the luminous flux projected on the screen 26 is converted into a circular polarization. The stereoscopic images displayed then have circular polarizations of opposite directions. The mutually orthogonal linear polarizers of the glasses 52 are replaced by circular polarizers in opposite directions allowing the visualization of these stereoscopic images. By using circular rather than linear polarizations, a user of spectacles 52 can tilt his head without the stereoscopic images being viewed having a significant decrease in contrast. The shutter 28 can be placed at the output of the modulator 24. In the case where the modulator 24 comprises a quarter-wave plate, this quarter-wave plate 10 is displaced at the outlet of the shutter 28. The shutter 28 can include more than one cell 40; said cells are directly joined to each other and are serially aligned. In another embodiment, the shutter 28 includes a cholesteric liquid crystal shutter 46. The polarizers 42, 44 are then not included, as illustrated in FIG. 7. The shutter 46 is, for example, the shutter described in the patent application FR1159187. Alternatively, the cell 40 is an optical device known to those skilled in the art as the ECB (Electrically-controlled Birefringence) liquid crystal cell, or OCB ("optically-compensated bend" in English). ), or liquid crystal surface mode device (liquid crystal), or polymer-dispersed liquid crystal (PSLC), or PDLC ("polymer-dispersed liquid crystal" in English). , or the device known as "pi-cell" in English, or a ferroelectric cell, or an anti-ferroelectric cell, or any combination of these devices.

Claims (10)

REVENDICATIONS1. Method for displaying stereoscopic images, CLAIMS1. A method for displaying stereoscopic images, comprising: -production (2), by a liquid crystal projector or a liquid crystal projector on silicon, of a luminous flux including a video sequence of stereoscopic images comprising an alternation two sub-sequences of temporally multiplexed images, these two sub-sequences being intended to be viewed respectively by a left eye and a right eye of a user provided with stereoscopic glasses, two immediately consecutive images of the sequence of images being in further separated by an optical transition state related to the switching of the video projector; polarization (4) of the luminous flux produced so that the two subsequences have optical polarizations of mutually orthogonal directions; projection (6), on an optically reflecting polarization conservation screen, of the polarized light flux; characterized in that it further comprises a step of obstruction (8) of the polarized light flux during the transition optical states and only during these transition optical states. 2. Method according to claim 1, comprising: the emission (10), by the video projector, of a synchronization signal synchronized with its switching between the two sub-sequences of temporally multiplexed images; the application (12) of a control signal on a shutter to switch it between an optically-on state and an optically-blocking state when the synchronization signal indicates the switching of the video projector. A stereoscopic image display (20) comprising: a liquid crystal or silicon-on-silicon projector (22) configured to produce a light flux including a stereoscopic image video sequence comprising an alternation of two multiplexed sub-sequences temporally, these two sub-sequences being intended to be viewed respectively by a left eye and a right eye of a user equipped with stereoscopic glasses, two immediately consecutive images of the sequence of images being further separated by an optical transition state related to the switching of the projector; a polarization modulator (24), receiving the luminous flux from the projector (22), configured to apply optical polarizations of mutually orthogonal directions to the two sub-sequences of images; an optically reflective polarization conservation screen (26) configured to display the stereoscopic images projected by the video projector (22); characterized in that the display (20) comprises a shutter (28), receiving the luminous flux from the projector, configured to switch to an optically blocking state during transition optical states and only during these transition optical states. 4. Display according to claim 3, wherein the projector (22) comprises an output interface configured to deliver a synchronization signal indicating the switching of the projector. 5. Display according to claims 3 or 4, comprising a control device (30) including: a reception interface (32) of a synchronization signal; an output interface (34) configured to output a control signal to the shutter; a computer (36) configured to provide a control signal switching the shutter (28) between an optically-on state and an optically-blocking state when the sync signal (SYNC) indicates a switch of the projector (22). The display according to claim 5, wherein the computer (36) is configured to output a second control signal switching the shutter from the optically blocking state to the optically-passing state after a predefined time after delivery. a control signal switching the shutter from optically to an optically blocking state. 7. Display according to any one of claims 3 to 6, wherein the shutter (28) is placed directly upstream of the polarization modulator (24). The display of any one of claims 3 to 7, wherein the shutter (28) comprises: - a liquid crystal cell (40); two linear polarizers (42,44), directly bordering the liquid crystal cell (40) and whose polarization directions are mutually orthogonal. The display of any one of claims 3 to 7, wherein the shutter (28) comprises a cholesteric liquid crystal shutter (41). A stereoscopic image display assembly (50), comprising: a stereoscopic image display (20) according to any one of claims 3 to 9; passive stereo glasses (52) comprising two polarizing filters, each of these filters being configured to block one of the two image sequences produced by the display.