Abstract:
A method for displaying a video image on a display screen by a scanning of the display screen along lines inclined by a first angle with respect to a reference direction by at least one electron beam modulated by a modulation signal. The method includes the steps of storing at least partly the successive initial digital video data associated with the image to be displayed in a memory; transmitting new successive digital video data corresponding to the image which would be displayed based on the stored initial digital video data for a scanning of the display screen along lines inclined with respect to the reference direction by a second angle opposite to the first angle; and providing the modulation signal based on the new transmitted successive digital video data.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and a device for correcting the rotation of a video display. 
   2. Discussion of the Related Art 
   Generally, a video image is displayed on the screen of a display terminal by exciting phosphors arranged on the screen by means of one or several electron beams, emitted by electron guns. In the case of a color screen, a cathode-ray tube with three electron beams, each exciting a type of phosphor respectively emitting a red, green, or blue light, is generally used. The electron beams are modulated in intensity by modulation signals representative of the image to be displayed on screen. 
   Generally, the electron beams are focused at one point of the screen and are deviated together to scan screen lines. The electron beams scan the screen lines from the right to the left of the screen, returning to the left of the screen after the scanning of each line. The screen scanning is performed from the upper horizontal edge to the lower horizontal edge. 
   The electron beam deviations are obtained by two deflection coils, one horizontal deflection coil which controls the scanning of each screen line, and one vertical deflection coil which controls the deviations of the electron beams in the vertical direction. 
   The deflection coils may have certain defects of manufacturing and/or assembly on the display terminal. The magnetic fields of deviation of the electron beams induced by the deflection coils may then be such that the screen lines scanned by the electron beams are not perfectly horizontal while remaining parallel to one another. The image is then displayed on screen with a rotation angle equal to the inclination of the scanned lines. 
   A similar phenomenon occurs when the display terminal is placed in an external magnetic field, for example, the terrestrial magnetic field. The external magnetic field may disturb the electron beam deviation so that the scanned screen lines are slightly inclined with respect to the horizontal direction while remaining parallel to one another. The image is then displayed on screen with a rotation angle equal to the inclination of the scanned lines. The inclination angle of the lines depends on the orientation of the magnetic field and may vary if the display terminal is moved. 
   A conventional solution to this problem is to place a correction coil between the deflection terminals and the rear surface of the cathode-ray tube. The magnetic field induced by the correction coil is adjusted so that the scanned lines of the screen are horizontal. In other words, the correction field induced by the correction coil rotates the displayed image by a correction rotation angle, the sign and the amplitude of which are defined by the polarity and the intensity of the current flowing through the correction coil. The display terminal may include means for setting the current polarity and intensity to enable correcting the rotation defect once the display terminal has been installed. 
   The correction coil and the coil supply system amount to a non-negligible cost of the display terminal. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing a device and a method for displaying a video image enabling correction of an unwanted rotation of the displayed image, due to the inclination of the screen lines scanned by the electron beams, without adding any correction coil. 
   For this purpose, the invention provides a method for correcting the rotation of a video image display on a display screen by a scanning of the display screen according to lines inclined by a first angle with respect to a reference direction, the screen being scanned by at least one electron beam modulated by a modulation signal provided by a modulation system based on successive digital video data, comprising the steps of: 
   a) storing successive initial digital video data associated with the image to be displayed; and 
   b) transmitting, to the modulation system, new successive digital video data determined from the successive initial digital video data, corresponding to the image which would be displayed based on the initial digital video data for a scanning of the display screen according to lines inclined with respect to the reference direction by a second angle opposite to the first angle. 
   According to an embodiment of the present invention, step b) comprises the steps of: 
   d) determining each of the new digital video data based on initial digital video data associated with distinct scanned lines of the screen; and 
   e) successively transmitting to the modulation system, for each scanned line of the screen, the new digital video data associated with the scanning of a line of the display screen at the end of a dwell after the beginning of the line scanning. 
   According to an embodiment of the present invention, step d) comprises the repeating of the steps of extracting, from among the successive initial digital video data, first and second groups of successive initial digital video data comprising a determined number of successive initial digital video data and located at the same position in the sequence of successive initial digital video data associated with two successive scanned lines of the screen; and determining the determined number of new digital video data, each of the new digital video data corresponding to the weighting of one of the initial digital video data of the first group and of one of the initial digital video data of the second group. 
   According to an embodiment of the present invention, the number of successive initial digital video data of each group of successive initial digital video data is a function of the second angle. 
   According to an embodiment of the present invention, at step e), the dwell is a function of the scanned line and of the second angle. 
   The present invention also provides a device for displaying a video image on a display screen by a scanning of the display screen along lines inclined by a first angle with respect to a reference direction, the screen being scanned by at least one electron beam modulated by a modulation signal provided by a modulation system based on successive digital video data, comprising means for storing successive initial digital video data; means for providing the modulation system with new successive digital video data determined based on the successive initial digital video data, corresponding to the image which would be displayed based on the initial digital video data for a scanning of the display screen along lines inclined with respect to the reference direction by a second angle opposite to the first angle. 
   According to an embodiment of the present invention, the means for providing the new successive digital video data comprises means for calculating the new digital video data based on initial digital video data stored in the storage means and associated with distinct scanned lines of the screen; and means for successively providing the modulation system, for each scanned line of the screen, with the new digital video data associated with the scanning of a line of the display screen upon completion of a dwell after the beginning of the line scanning. 
   According to an embodiment of the present invention, the device comprises means for setting the second angle. 
   The foregoing objects, features, and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows an example of the forming of a device according to the present invention providing pixel data to an electron beam modulation system; 
       FIG. 2  schematically shows an example of a network of pixel data received by the device of  FIG. 1 ; 
       FIG. 3  schematically shows a first step of the display correction method according to the present invention; and 
       FIG. 4  schematically shows a second step of the correction method according to the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention comprises modifying the content of the pixel data associated with the displayed image rather than correcting the inclination of the scanned lines of the screen. In other words, the image is still displayed with an unwanted rotation angle. However, the displayed image no longer corresponds to the initial image received by the display terminal, but to a new image determined from the initial image to be displayed so that the new image, displayed with an angle rotation, appears for a spectator as would appear the initial image with a non-defective scanning. 
     FIG. 1  shows, in the form of a block-diagram, an example of the forming of a device  10  according to the present invention equipping a display terminal and transmitting control signals to a modulation system (not shown). The modulation system provides signals for modulating the intensity of electron beams emitted by electron guns and scanning a screen of the display terminal. 
   Device  10  comprises a controller  11  receiving an angle signal α indicating the angle correction to be made. This angle is equal to the opposite of the inclination angle of the lines scanned on screen with respect to the horizontal direction. The inclination of the scanned lines with respect to the horizontal direction can be determined by any type of adapted sensor which provides the value of angle α to controller  11 . The value of angle α may also be set by the user of the display terminal by means of a setting control. Controller  11  receives a square pulse synchronization signal V SYNC , each rising edge of which corresponds to the beginning of the scanning of the first screen line. Controller  11  receives a square pulse synchronization signal H SYNC , each rising edge of which corresponds to the beginning of a line scanning. 
   Controller  11  controls a memory  12 , in which are stored digital pixel data PIX_DATA. As an example, each of digital pixel data PIX_DATA is representative of an intensity value of a red, green, or blue component of a pixel of the image to be displayed. The pixel data PIX_DATA stored in memory  12  correspond to all or part of the pixel data necessary to display an image on screen. In the case where the display terminal is a television set, the pixel data PIX_DATA are determined based on the image signal received by the television set. 
   Device  10  comprises a first correction unit (V_COR)  14  which is adapted to reading from memory  12 , according to control signals transmitted by controller  11 , specific pixel data from the set of pixel data stored in memory  12 . First corrector  14  determines from the selected specific pixel data and from coefficients transmitted by controller  11  new digital pixel data NEW_PIX_DATA and stores them in a memory area. 
   Device  10  comprises a second correction unit (H_COR)  18  which is adapted to reading from first correction unit  14  the new digital pixel data NEW_PIX_DATA and to transmitting the read pixel data to a digital-to-analog converter (ADC)  19  according to a determined sequence based on coefficients transmitted by controller  11 . 
   Digital-to-analog converter  19  converts the new digital pixel data NEW_PIX_DATA into analog signals PIX. Analog signals PIX are then transmitted to the modulation system which determines, therefrom, the signals of modulation of the electron beam intensity. 
   The steps of the display correction method according to the present invention will now be described in detail. 
     FIG. 2  shows a network  20  of points P i,j , i ranging from 0 to N, and j ranging from 0 to M, where N and M respectively represent the number of lines and columns of network  20 . A point P i,j  of network  20  represents a pixel datum, more specifically the necessary datum enabling generation of a modulation signal for the display of a color component of a pixel on screen. There exists a network similar to that of  FIG. 2  for each color component. Hereafter, pixel data of the same color component only will be considered. Each line of network  20  corresponds to a line of the image to be displayed upon scanning of a screen line by the electron beams. In the example of  FIG. 2 , network  20  comprises pixel data for the display of five lines, with seven pixels per line. As a comparison, a screen generally comprises more than 300 lines and more than 700 pixels per line, only half of the screen lines being generally scanned at once in a screen scanning. 
   Call (H, V) an orthonormated referential, the axes of which are parallel, respectively, to the lines and to the columns of network  20  and centered on the screen center. In this referential, a point P i,j  has coordinates P i,j (H_P i,j ; V_P i,j ). It will be considered hereafter that the lines and columns of network  20  are separated by one unit in referential (H, V). 
     FIG. 3  shows a network  24  of squares Q k,l , k varying from 0 to R, and l varying from 0 to N, where R and N respectively represent the number of lines and columns of network  24 . Network  24  has the same number N of columns as network  20  and has a higher number of lines R. Network  24  very schematically symbolizes the network of new pixel data NEW_PIX_DATA generated by first corrector  14 . A square Q k,l  of network  24  has coordinates Q k,l (H_Q k,l ; V_Q k,l ) in referential (H, V). 
   In  FIG. 3 , a network  28  of points P′ i,j  is shown as being superposed to network  24  of squares Q k,l . Network  28  of points P′ i,j  represents the theoretical deformation which is desired to be applied to network  20  of  FIG. 2 . This theoretical deformation consists of shifting each column j of network  20  along direction V by a different distance for each column so that each line of network  20  appears to be inclined by an angle α, expressed in radian, with respect to direction H. Straight dotted line  30  represents the desired direction of a line of network  28 . 
   Controller  11  determines the new theoretical coordinates of points P′ i,j  of network  28  based on the coordinates of the corresponding points P i,j  of network  20 . The coordinates of a point P′ i,j  of network  28  transformed from point P i,j  of network  20  are P′ i,j (H_P i,j ; V_P i,j +H_P i,j *α) for small angles α. 
   For each point P′ i,j , controller  11  determines points Q SUPk+1,j  and Q INFk,j  of the same column j on either side of point P′ i,j  or, if such is the case, point Q k,j  superposed to point P′ i,j . The coordinates of points Q SUP  and Q INF  are:
 
Q SUPk+l,j (H_P i,j ; V_P i,j +Ent(H_P i,j *α)+1)
 
Q INFk,j (H_P i,j ; V_P i,j +Ent(H_P i,j *α))
 
where Ent(H_P i,j *sin α) is equal to the integral part of H_P i,j *sin α.
 
   Controller  11  controls first corrector  14  to assign the pixel data represented by square Q SUPk+1,j  (respectively Q INFk,j ) a value which corresponds to a weighting of the value of the pixel data represented by point P′ i,j  according to the distance separating point P′ i,j  from square Q SUPk+1,j  (respectively Q INFk,j ). This operation is repeated for all points P′ i,j  to obtain network  24  of new pixel data NEW_PIX_DATA. The value of new pixel data NEW_PIX_DATA is thus equal to a weighted sum of two initial pixel data PIX_DATA, or equal to the value (possibly weighted) of initial pixel data PIX_DATA, or equal to a value corresponding to a zero pixel color component intensity. 
   It is possible not to calculate all the new pixel data NEW_PIX_DATA of network  24 , to reduce the memory space taken up by the new pixel data NEW_PIX_DATA. 
   In practice, first corrector  14  may select from memory  12  groups of pixel data PIX_DATA which correspond to several sets of points P′ i,j  of different lines, each set containing for each line points of same successive columns. First corrector  14  then calculates the values of the new pixel data NEW_PIX_DATA which are associated with the sets of points P′ i,j . 
   Second corrector  18  reads from first corrector  14  each line of new pixel data NEW_PIX_DATA. Upon reception of a rising edge of synchronization signal V SYNC , controller  11  transmits to second corrector  18  dwell values ΔTk associated with each line k of network  28  and which are a function of angle α. The theoretical dwell ΔT i,j  associated with point P′ i,j  is given by the following formula: 
   
     
       
         
           
             Δ 
             ⁢ 
             
                 
             
             ⁢ 
             
               T 
               
                 i 
                 , 
                 
                     
                 
                 ⁢ 
                 j 
               
             
           
           = 
           
             
               - 
               
                 V_P 
                 
                   i 
                   , 
                   
                       
                   
                   ⁢ 
                   j 
                 
               
             
             · 
             α 
             · 
             
               
                 T 
                 Huseful 
               
               
                 2 
                 ⁢ 
                 
                   
                     ( 
                     
                       H_P 
                       
                         i 
                         , 
                         
                             
                         
                         ⁢ 
                         j 
                       
                     
                     ) 
                   
                   MAX 
                 
               
             
           
         
       
     
   
   where T Huseful  is the duration of the visible portion of a line on screen, (H_P i,j ) MAX  is the maximum ordinate of points P i,j . Dwell ΔTk associated with each line k of network  24  may be obtained by interpolating dwells ΔT i,j , with a fixed j. 
   At each rising edge of synchronization signal H SYNC , second corrector  18  waits for the completion of dwell ΔTk before starting the transmission to converter  19  of the new pixel data NEW_PIX_DATA of line k. 
     FIG. 4  shows a network  32  of points  34  symbolizing the pixel distribution on the display screen based on the new pixel data NEW_PIX_DATA transmitted to the modulation system according to the method of the present invention in the case where the screen scanning would have no rotation defect. For a spectator, the image displayed on screen would then substantially correspond to the initial image which would have undergone an angle rotation α. 
   Since the screen scanning submits the displayed images to an angle rotation opposite to angle α, the image displayed on screen, corresponding to the new pixel data NEW_PIX_DATA, actually appear for the spectator similarly to the initial image corresponding to the pixel data PIX_DATA in the absence of a rotation. 
   The present invention has many advantages. 
   First, the present invention avoids adding a coil type compensation system. It thus enables reducing the manufacturing cost of the display terminal. 
   Second, the present invention enables accurate correction of the deformation of the displayed image, since upon construction of network  24  of new pixel data NEW_PIX_DATA from network  20  of initial pixel data PIX_DATA, any shifting may be assigned to any column j of network  20 . Further, upon transmission of the new pixel data NEW_PIX_DATA to converter  19 , any dwell may be assigned to each transmitted line. 
   Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, the present invention may apply to display terminals of computer monitor type, automatic teller machine screen, etc. 
   Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.