CA1123595A - Flight simulator visual display apparatus - Google Patents

Abstract

Improvements in or relating to Visual Display Apparatus Abstract The invention provides head-coupled area-of-interest visual display apparatus particularly for ground-based craft-flight simulators. An image generator of the computer generated image type is used. Line scanning apparatus is cockpit-mounted; transmission of line image is by fibre optic guide ribbon and frame scan apparatus is helmet-mounted. Sensing means detect head/helmet movement to permit voluntary scanning of a wide angle of simulated view from the craft. Due to the projection mount having the capability of continuous rotation, with the image being computed for discrete angular positions only, together with the computational throughput delay in the image computer, the new image, after head/helmet rotation, is momentarily delayed after the new line of vision is established. Means are provided to de-rotate the projected image to its former position on the screen until such time as the modified image is available.

P.2097/1

Description

_ . .

Descript_on This invention relates to visual display apparatus, particularly for ground-based flight simula-tors and particularly for providing a display covering a wide-angle field of view. The invention may be used in apparatus capable of providing either pseudo-collimated or stereo-scopic viewing for a pilot.
The apparatus is of -the head-coupled area-of-in-terest type, wherein an image projected upon a screen is appropria-tely changed both according to the simulated craft position and angular orientation and according to the viewer's instantaneous line of view and is simultaneously moved on the screen to occupy the viewer's field o~ view.
U,, k' Apparatus o~ this type was described in prior~patent specification Number 1,489,758. Such apparatus provided --an area-of-interest display ~or a sole observer which was pseudo-collimated, -that is, the same image was projected for left and right eyes9 so as to appear at infinity.
The present invention solves a specific problem in such apparatus when the image generator is of the computer generated image type and in which a modified image has to be computed for every instantaneous line of view movement. Thej..a-t present, unavoidable throughput delay results in a perceptible P.2097 interval before the modified i~age is available. The present lnvention avoids the momen~ary display of the old image in the new line of view.
Accordingly, the invention provides in a head-coupled area-of-i.nter-est visual display system for projecting upon a screen at least partly surround-ing a viewer, a scene generated ~y an image generator of the computer-generated image type~ and projected as a raster-scanned light-spot image by optical means mounted for movement with the head of the viewer, said ~aster-scanned light-spot image being built up by a scanning raster involving line-scanning from a pro-jected line start defined by any stationary head position of the viewer and frame-scanning orthogonal to said line scanning, the characteristlcs of which image computer are such that, upon change of the viewer's head orientation from a first head orientation to a new head orientation, the changed scene correspond-ing to the new head orientation is delayed for an lnterval noticeable to the viewer, means responslve to a viewer's head orientation sensor for projecting the sald lmage substantlally at the screen position correspondlng to sald first head orientation for a predetermined time interval not less than the said com-puter delay interval and at the screen position corresponding to said new head orientat.ion after sa.id time interval, the said means controlling both the line-scanning and frame-scanning synchronisation of the scanning raster according to the followlng rules:
i~ Advancing the video signal with respect to the projec~ed line start when the new position is the result of an angular rotation in the dlrection of the line scan;
11~ Delaying the video signal wlth respect to the projected line sta~t when the new posltlon ls the result of an angular 3~

rotation in the direction opposite to that of the line scan;
iii) Advancing the video signal with respect to the projected frame s-tart when the new posltion is the result of an angular rotation in the direction of the frame scan;
iv) Delaying the video signal with respect to the projected frame start when the new posi-tion is the result of an angular rotation in the direction opposite to that of the frame scan; and v) Restoring the original relative time sequence of both line- and frame-scanning synchronising pulses at the end of said time interval.

P.2097 i Short Descrip-tion of Dra_~
In order that the invention rnay readily be carried into prac-tice, one embodiment will now be described in detail, by way of example, with reference to the accompanying drawings, in which:-Fig. l shows head-coupled area-of-interest visual ~-display apparatus for projecting upon a screen at least partly surrounding a viewer, a view generated by an image generator of the computer-generated image type and projected as a raster-scanned light-spot image by optical means mounted for movement with the head of the viewer, and providing for pseudo-collimated viewing of the display;
Fig. 2 shows a series of four diagrams representing raster-scanned images and explaining the requiremen~ts of image derotation with head movement in the direction of line scan;
Fig. 3 sho~s three diagrams representing raster-scanned images and explaining the requirements of image derotation with head movement in the direction of frame scan;
Fig. 4 is a diagrammatic view of laser source 9 laser beam modulator, line scanning, fibre optic light guide ribbon and frame scanning apparatus;
Fig. 5 is a side view of the frame scanner of Fig. 4; and ~ ig. 6 is a detail view showing an alternative line scanner to that of Fig. 4.

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-c ~ Example The apparatus of Figure 1 will be described first in order to illus-trate the form of apparatus in which the present invention is required.
Figure 1 shows in diagrammatic form apparatus for generating and displaying a pseudo-collimated area-of-interes~ view. A pilot 10 wearing a helmet 12 is seated within a part-spherical shell having a retro-reflective interior surface partially represented in ~igure 1 by the concave retro-reflect-ive screen 14. The pilot's line of vision, for right and left eyes and for distant viewing, intersects the screen at points 16 and 18, respectively. The field of view for each eye is centred on the respective one of these two points.The views displayed are identical for right eye and left eye but are displaced laterally by the distance between the points 16 and 18 so that the pilot 10 seesa pseudo-collimated view, that is to say, the displayed view appears to be at infinity and not at the distance of the screen 14. The combined left eye and right eye views will be referred to as the displayed scene.
The displayed scene depends, in this example, upon the simulated posi-tion of an aircraft during an exercise flight, the attitude of the aircraft, the pilot's seating posi~ion in the aircraft and the pilot's instantaneous line of view as determined by the instantaneous orientation of ~he pilot's head and helmet. The position of points 16 and 18 on the screen 14 and hence the posi-tion of the displayed views on the screen depends only on the pilot's head and helmet orientation.
The image required is generated by an image generator 20 of the computer-generated image type which includes a frame buffer store 20'. The pilot's head orientation is sensed by a head orientation sensor 22, which is fixedly moun-ted within the simulated aircraft cockpit in a mounting 24. The displayed view is projected on-to the screen 14, centred in the appropriate locations as -two raster-scanned images, -the line scan apparatus being cockpit-mounted and -the frame scan appara-tus being mounted on the helmet 12. Line scan may be either across the screen 14 or in a generally vertical direction. In the present example, line scan is such that the projected scan line upon the screen and -the line joining the pilot's two eyes lie in one plane. The frame scan is in a plane orthogonal thereto. Thus, if the pilot is sitting - upri~t, line scan is horizon-tal and frame scan is vertical.
- Referring s-till to Fig. 1, a laser source 30 provides an output laser beam 31 which is directed through a full colour modulator 38 -to provide a modulated laser beam 31'. The modulated beam 31' is directed through beam-splitter and reflector elements 32, 33 to provide two beams 34 and 36 of equal in-tensity. The modula-tor 38 is con-trolled from the image generator 20 according to -the view to be pro~ected.
Both modulated beams 34 and 36 pass to a double line scanner 42 fixedly mounted in -the simulated aircraft cockpit. The -two P.2097 .

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~235i~;

scanners, descrlbed in detail la~ter herein, provide two respective scanned beams 44 and 46 which are respectively scanned over the input ends 48 and 50 of two fibre optic light guide ribbons 52 and 54.
The two fibre optic light guides provide a flexible linkage between the fixed line scanner 42 and -the movable helmet 12. The emergen-t scanned light beams from the respective ends 56 and 58 of the ligh-t guides 52 and 54 are focussed by spherical lenses 62 and 64 onto the screen 14 and directed onto a plane mirror 60. The right eye beams are reflected by the mirror 60 along divergen-t paths to form a scan line, -the centre of which is shown at 66.
Similarly, the left eye beams are reflected by the mirror 60 along divergent paths to form a scan line, the centre of which is shown at 68. The centre line of the respective right eye and left eye views is thereby formed on the screen 14, each line having its respective mid point at 16 and 18 r and being viewed by the pilot 10 in the respective line of view 70 and 72.
The mirror 60 is long in relation to its width and is carried in bearings at i-ts end which are mounted on the helmet 12. These bearings are provided by motors 74 and 76 at the two ends which move the mirror 60 to provide the required frame scan. .
The mirror 60 may be a single plane mirror which is either oscillated or rota~ted by the mo-tors 74, 76 on its P.2097 ~3~

axis parallel to the plane in which the line scan is pro-jected, or the mirror 60 rnay be a multi-faceted mirror rod of, for example, octagonal cross-sec-tion which is continuously rotated by the motors 74, 76. In the present example, the mirror 60 is a single plane mirror and is oscillated for frame scan.
As the pilo-t's head moves, so does the displayed view move over the screen, so as to be in the pilot's new line of view and the view itself is changed according to the simu-lated real world view in the direction of the line of view.
To this end, the visual system receives data from the host fligh-t computer on lines 80 arld 81. Position data defining -the simulated aircraft position -throughout a simulatecl flight exercise is supplied to the image generator 20 on line 80. Attitude data, defining -the simulated aircraf`-t instantaneous at-titude, is supplied on line 81 to a vector summing unit 82 together with head orientation data, defining the pilot's actual instantaneous line of view~ on line 84. ~`
The summed output is supplied to the image generator 20 on line 86. A throughput delay error signal obtained by su~tracting the attitude input to the image generator one throughput delay period ago f`rom the current head attitude position is supplied to -the throughput error control 100 on line 119.
The duplicated image, respec-tively for the right eye and left eye views, in accordance with the inputted data, and ., .

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allowing for the l~nown seating position of the pilot in the simulated aircraft type, are supplied to the respective modulators 38 and 40 on lines 88 and 90.
It will be appreciated tha-t the change of the displayed image with simulated aircraft posi-tion is relatively slow.
However, the change of the displayed image with head orientation is complete and relatively very rapid, The image generator is unable to compute an entirely new image immediately a new line of view is established due to the throughput delay of the image generator computer. To over-come this limitation the residual old displayed view is derota-ted to its former screen posi-tion un-til -the computed new displayed view is available.
The requirement of image derotation for viewer's head movement in the direction o~ line scan is illustrated in ~ig. 2.
~ ig. 2 shows at (A) the rectangular boundary 200 of a raster-scanned image upon the screen 14. It is assumed that the viewer's head is upright, so that in Fig, 2(A) the line scan 202 is horizontal and the frame scan 203 is vertical, the directions shown being relative to the viewer's viewpoint, The boundary of the s-tart of scanning lines is shown by the ver-tical line 204. Similarly, -the frame start is shown by the horizontal line 205. The rectangle 200 thus represents -the boundary of a projected scene at any instant.

P.2097 ~23~

Fig. 2 shows at (B) -the condition for a stationary head position corresponding to a first line of view for a period greater than -the throughput delay. The computed lmage 201 then exactly coincides with the scene within the scanned area 200.
Fig. 2 shows at (C) -the condi-tion at the ins-tant immediately following head movement in the direction of line scan. Because -the image is projected from the viewer's helmet, the projected scene 200' is moved to the right, as is the viewer's new ins~tantaneous line of view.
Due to the throughpu~t delay, the computed image 201 corresponds -to the scene of ~the viewer's old line of view, as shown by Fig. 2(B). Thus, the image projected at this instant corresponds not -to -the area 200' but to the area 201.
That is~ the old scene needs to be dero-tated to the left, which is baclcwards along the scan lines, so as to coincide with the area 201.
Such image derota-tion is e~fected by the apparatus of ~ ~-the present invention for the period of the throughput delay.
Fig. 2 shows at tD) the condition after head position is held s-teady for a period longer than the throughput delay.
The new computed image 201' now corresponds to -the scene within the area 200'. Image derotation is no longer required and the computed image 201' and projected image 200' are again coincident, as they were for Fig. 2(B), but now in an area of screen 14 shifted to the right relatively -to Fig. 2(B), ~J

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as shown by the corner referenced 200/201 in Fig. 2(D).
Fig. 3 shows in three diagrams, referenced B, C and D, -the corresponding requirement for head movement in the direction of frarne scan. The three diagrams are again referred -to the area of raster scan 200 of Fig. 2(A).
Fig. 3 shows at (B) the condition for stationary head posi-tion with a first line of view for a period greater than the throughput delay. The computed image 201 exactly coincides with the scene within the scanned area 200.
Fig. 3 shows at (C) -the condition at the instant immediately following head movement in -the direction of frame scan. Because the image is projec-ted from the ~iewer's helmet, the projected scene 200' is moved downwardly, as is the viewer's new instantaneous line of view.
Due to -the throughpu-t delay, -t;he compu-ted image 201 still corresponds to -the scene of t;he viewer's old line of view, as shown at Fig. 3(B). Thus, -the image projected at this i~stant corresponds not -to the area 200' but to the area . .
201. That is, the old scene needs to be derotated upwardly, backwards relatively to the direction of frame scan, so as to occupy the area 201.
This image derotation is effected throughout the period of throughput delay~
Fig. 3 shows at (D) the condi-tion after head rotation is held stead~ for a period longer than -the throughput delay.
The new computed image 201' now corresponds to the scene ....
P.2097 '.

within the scanned area 200'. Image derotation is no longer required and the computed image 201' and projected image 200' are again coincident, as they were for Fig. 3(B), but now in an area of screen 14 shifted downwardly relatively to Fig. 3(B), as shown by the corner referenced 200/201.
When head movement occurs involving movements in the directions of line scan and frame scan simultaneously, both modes of image derotation are requiredO
The required image dero-tation can be effected by controlling the relationship between -the video signal and the line scan and frame scan positions. This control can be produced in a number of ways.
The line scanner is typically a continuously ro-tating polygon which sweeps the input laser beam or beams through an arc to produce a line scan, as in the example of Fig. l~.
Three alternatives are available:
(i) If the video signal is produced at a constant rate then the line scan drive may be phase modulated to maintain -the correct line in space -to produce an image with the correct spatial orientation. If the line projection system is capable of transmi-tting only the displayed field of view, then the image size will only be that part which is common to both the computed and projected images. This area is shown hatched in Fig. 2(C)~ If the fibre optic ribbon and the projec-tion system is capable of projecting more than the P.2097 ~3~
-required field of view in the line scan direc-tion -then the field of view obtained may be held constan-tO
(ii) The video signal may be produced at a constant ra-te and the line scanner rotated at a cons-tant ra-te. The required angular shift may then be in-troduced with a supplementary mirror. Line scanning apparatus J alterna-tive -to that of Fig. 4 and including such a supplementary mirror is described la-ter herein wi-th reference to Fig. 6.
(iii) The polygon mirror may be run at a constant angular velocity and -the video signal -timing adjusted by altering the ~time a~t which the video signal is read out of -the frame store 20' of ~the image genera-tor 20.
This ensures that the video signal corresponding to a point in space is produced at the prede-termined time tha-t the scanner points the light beam at tha~t par-t of the screen representing the required poin-t in space.
Of these three methods described above, method (i) involves the phase modulation o~ a mechanical system rotating at high speed and has the disadvantages associated with the inertia and response -times of such a system. Method (ii) overcomes some of -these problems by using a supplementary mirror. This mirror does not rotate at high speed but nevertheless has inertia inherent in any mechanical system and so it will have some response time. Method (iii) requires only the ability -to read out a memory at controlled times.
Since a memory is not a mechanical system, i-t has no inertia P.2097 ~3 and can be read out in a discontinuous manner if required.
Accordingly, method (iii) is the preferred method for line scan synchronisation in the present invention.
The frame scanner of Fig. 1 does no-t offer the same options as does the line scanner due to the difficulties of implementa-tion. The alternative methods corresponding to those described for the line scanner are as follows:
(i) If the video signal is produced at a constant rate then the frame scan drive may be controlled to give the required pointing direction. In this case the frame scanner will be a posi-tion servomechanism driven by a sawtooth waveform .in which the s-tarting point of -the ramp may vary in a con-trolled manner and the slope of the ramp may vary in a controlled manner in order to give a constant angular sweep in free space when the projector mount is bein~
s-lbjected to angular shifts.
(ii) The use of a supplementary mirror is impractical in the frame scanner of Fig. 1.
(iii) I~ the frame scanner is driven with a sawtooth of constant period, s-tart point and slope, then the read out times from the frame store 20' may be adjus-ted to produce -the video signal when the scanner is at the required orienta-tion in free space.
Of these -three me-thods, method (i) requires adjustments to the period and rate o~ a mechanical system which, due to its construction,has a very low inertia. Hence, the settling . .

P.2097 time following such a dis-turbance may be acceptable. It can preserve the instantaneous field of vie1~ constant through the -throughput delay period. Method (ii) is imprac-tical due to the physical constraints of -the projection lens and frame scanner assembly of ~ig. 1. Method (iii) involves adjus-tmen-t to a system without inertia or the requirements of continui-ty. However method (iii) reduces the virtual field of view during the throughput delay period to that which is common to the areas 200' and 201 of ~ig. 3(C), as is shown by the ha-tched area.
Continui~g with the description of the apparatus of ~ig. 1, a synchronising pulse generator 106 suppli.es pulses on line 108 to -the throughput delay error con-trol uni-t 100.
Line scan control signals are supplied to the line scanners of unit 42 from unit 92 by way of line 9~ rame scan control signals are supplied to the frame scan motors 74, 76 from unit 96 by way of a flexible line 98. Video synchronisa-tion timing pulses are fed to the frame buffer 20 of the C.G.I. image generator 20, from the lmit 100 on line 110. Control of the relative timings between the line scan control 929 the frame scan con-trol 96 and the C.G.I.
image generator frame buffer 20' is effec-ted by the throu~hput delay error compensation circuit 100 by way of lines 102, 104 and 110, respectively.

It will be noted ~that the projection middle lines 66 and P.2097 : :
:~

. , , 68 do not coincide wi-th the lines of view 70 and 72 for the reason that projection is effected from above the pilot's eyes. Projected onto any horizontal plane, the respec-tive lines are coincident but, projected onto any ver-tical plane, -the respective lines diverge away from the screen. The angle of divergence is small bu-t is nevertheless great enough, compared wi-th the apex angle of the half-brilliance cone of reflection of a re-tro-reflective screen material,to result in a viewed scene of much reduced brilliance. It is preferred therefore to use a screen of modified retro-reflective material for which the axis of the half-brilliance cone of reflection is depressed downwardly by the angle between the projec~tion lines 66, 68 and -the line of view lines 70, 72.

P~2097 ~ 17 -The various uni-ts of the apparatus, shown in the block schematic part of Fig. 1, will now be considered in further detail in the following order:
Laser Source.
Laser Beam Modulator.
Line Scanner.
Fibre Optic Light Guide Ribbon.
Frame Scanner.
Helmet-Head Orientation Sensor.
Throughput Delay Error Compensation Unit.

P.2097 ., - ~ . , ' .

~35~5 Laser Source, Laser Beam Modulatorl Line Scanner~ ~ibr~TOptic Li~ht Guide Ribbon and Frame_Scanner One laser source, laser beam modulator, line scanner, fibre optic light guide ribbon and frame scanner elements of the apparatus will be described together with reference to ~ig. 4 and Fig. 5.
~ ig. 4 shows the laser beam source 30 which provides the output laser beam 31 direc-ted through the full colour modula-tor 38. 30th the laser beam source 30 and the modulator 38 are of known form. The full-colour modula-ted beam output is shown at 31' in this figure, in which in-termediate beatn~splitters are not shown. The line scanner ls shown generally at 42.
The line scanner comprises a synchronously-driven polygonal section mirror drum 144 whlch rotates continuously in the direetion shown by the arrow 145 to sweep the beam 31' over the sean path 44. One pass oceurs for -the movement of each mirror facet of the mirror drum 144 past the beam 31'.
A ~lbre optie light gui~e, formed into a flat ribbon 52 over most of its length, has individual groups of fibres formed into an are at the input end 48 of the light guide. The width of the line scan 44 exaetly covers the arc at 48, so that the modulated beam 31' is seanned along -the arc at 48 for eaeh line of the image.
At the outpu-t end 56 of the fibre optie light guide 52, the individual groups of fibres are similarly formed into an are the fibre groups oecurring in the same sequenee at P.2097 .

the two ends 48 and 56, so tha-t the scanned image line at the input end 48 is exactly reproduced at -the output end 56.
The emergent rays from the output end 56 of the light guide 52 are focussed by the spherical lens 62 onto the face of the frame scanning mirror 60. As shown in Fig. 1, the mirror 60 is mounted on the pilot's helmet 12 in bearings provided by reciprocating motors 74 and 76.
With the mirror 60 stationary, the emergent rays are reflected from the mirror 60, as shown instantaneously at 66, to form a single line of the image. As the mirror 60 is moved, successive lines of the image are projected to form -the entire scanned image.
Fig. 5 shows, in side view, the output end S6 of the :light guide 52, the spherical lens 62, the mirror 60 and the reflected beam 66 as described above with reference to Fig. 4.
A second line scanner, comprising a second mirror drum, produces a second line scan over -the input end 50 of the second fibre optic light guide 54, as is shown in Fig. 1. The output end 58 of ~his second light guide 54 provides emergen-t rays which are focussed by a second spherical lens 64 onto the same reciprocating mirror 60. The -two helmet mounted optical systems, with the common frame scan mirror 60, together provide the right eye image and left eye image of the pilot's displayed view. As already explained, the identical right eye and left eye images provide -the pseudo collimated display for the pilot.

P.2097 Fig. 6 shows line scanning apparatus alternative to that of Fig. 4 and including a supplementary mirror 202.
The rnirror 202 is pivotable on an axis 203 which is parallel to the spin axis 204 of the polygon mirror line scanner 144.
To effect image derotation for head movement in the direction of line scan by the method (ii) described earlier, the mirror 202 is rotationally positioned abou-t its axis 203 by a motor 205 in a controlled manner so that the swept arc 44 is positioned at the required part of the arc 48 at -the inpu-t end of -the fibre optic light guide 52. The motor 205 is controlled from the throughput delay error control unit 100 by a signal on line 102.

P.2097 ~3~;~35 Throughput Delay Error Compensation Unit~ Line Scan Control aind Frame Scan Control As has been explained earlier in the description~ the C.G.I. image generator 20 takes an appreciable time to compute a new view for display when the pilot's line of view is changed. The delay is of the order of lO0 m secs.
However, when any viewer changes his line of view, by extensive head movement, there is a delay before the viewer appreciates the new view before him. This delay also is of the same order of time as the image generator delay.
In a simplified form of the apparatus according to the invention means are provided merely to ensure that the old display is not projected in the new line of view of the changed head position.
In this simplified form of the apparatus, a large change of head orientation signal on line ll9 is effective to blank out the projected view for a period of some lO0 m secs. until the new view has been computed.
The apparatus of Figure l provides means for the derotation of the projected image upon rotation of the pilot's hoad. Derotation is considered to be of especial importance when head movemen1: is such that the new field of view is not separate ~rom the old field of view but is within it or overlaps it.
The displayed view is some 100 in azimuth and some ~ . i.,.~....

~3~

70 in elevation, with respect to the pilot's line of view.
Although a viewer's ~ield of view may exceed -these angles, the marginal areas are low-in-terest and the central area of prime-interest may be a cone of perhaps only 5 about the line of vision. It is therefore readily possible for the pilot to change his line of view so as to move -this area of central interest within the initial displayed view area.
In the apparatus of Fig. 1, line scan is generally across the screen 14 and frame scan is orthogonal thereto. The head orientation sensor 22 provides signals resolved into head azimu-th movemen-t and head pitch movement.
The synchronising pulse genera-tor 106 provides a line synchronising and frame synchronising pulse output of equally spaced apart pulses. Upon change o:f head azimu-th, the output signal on line 119 causes unit lO0 -to provide a relative change of phase of the line synchronising pulses supplied by control unit 92 to the line scanner 42, and the video synchronising pulses supplied by the throughput error con-trol 100 to the frame buffer 20' on line 110 in the sense to displace the displayed image e~ually and oppositely to every change of head azimuth.
Similarly, the output signal on line 119 causes ~mit 100 with frame scan control unit 96 to provide a relative change of phase of the frame synchronising pulses supplied by control unit 96 to the frame scanning motors 74 and 76 and P.20~7 ~3~

the video synchronising pulses supplied by the throughput error con-trol 100 to the frame buffer 20' on line 110.
Thereby, upon head rotation in azirnuth or pitch or bo-th, the displayed view is displaced oppositely. The dero-tation is maintained for a period of some 100 m secs., until the new view is computed. The original relative timing of the synchronising pulses is then restored, so that the new view is displayed in the direction of the new line of view.

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Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a head-coupled area-of-interest visual display system, for pro-jecting upon a screen at least partly surrounding a viewer a scene generated by an image generator of the computer-generated image type and projected as a raster-scanned light-spot image by optical means mounted for movement with the head of the viewer, said raster-scanned light-spot image being built up by a scanning raster involving line-scanning from a projected line start defined by any stationary head position of the viewer and frame-scanning orthogonal to said line scanning, the characteristics of which image computer are such that, upon change of the viewer's head orientation from a first head orientation to a new head orientation, the changed scene corresponding to the new head orientation is delayed for an interval noticeable to the viewwer, throughput delay error control means responsive to a viewer's head orientation sensor for projecting the said image substantially at the screen position corresponding to said first head orientation for a predetermined time interval not less than the said computer delay interval and at the screen position corresponding to said new head orient-ation after said time interval, the said control means controlling both the line-scanning and frame-scanning synchronisation of the scanning raster according to the following rules:
i) Advancing the video signal with respect to the projected line start when the new position is the result of an angular rotation in the direction of the line scan;
ii) Delaying the video signal with respect to the projected line start when the new position is the result of an angular rotation in the direction opposite to that of the line scan;
iii) Advancing the video signal with respect to the projected frame start when the new position is the result of an angular rotation in the direction of the frame scan;
iv) Delaying the video signal with respect to the projected frame start when the new position is the result of an angular rotation in the direction opposite to that of the frame scan; and v) Restoring the original relative time sequence of both line- and frame-scanning synchronising pulses at the end of said time interval.

2. In a head-coupled area-of-interest visual display system for a ground-based craft-flight simulator, throughput delay error control means as claimed in Claim 1, responsive to a combined signal derived both from the viewer's head orientation sensor, defining the viewer's instantaneous head orientation, and from a craft flight computer, defining instantaneous craft attitude.

3. In a head-coupled area-of-interest visual display system having rotating mirror line scanning means, throughput delay error control means as claimed in Claim 1 or Claim 2, connected to control the line-scanning mirror rotation, upon change of viewer's head-orientation in the direction of line scan, in the manner to change the phase of rotation while maintaining constant the speed of rotation.

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4. In a head-coupled area-of-interest visual display system having rotating mirror line scanning means together with a supplementary oscillating mirror, throughput delay error control means as claimed in Claim 1 or Claim 2, connected to control the orientation of the supplementary oscillating mirror, upon change of viewer's head-orientation in the direction of line scan, in the manner to delay movement of the projected scene upon the screen.

5. In a head-coupled area-of-interest visual display system having an image generator of the computer-generated image type with an integral frame buffer store, throughput delay error control means as claimed in Claim 1 or Claim 2, connected to selectively interrogate the frame buffer store, upon change of viewer's head-orientation in the direction of line scan, in the dire-ction of frame scan or in both directions, in the manner to delay movement of the projected scene upon the screen.

6. In a head-coupled area-of-interest visual display system having oscil-lating mirror frame scanning means, throughput delay error control means as claimed in Claim 1 or Claim 2, connected to control the phase of frame scan-ning mirror oscillation, in the manner to delay movement of the projected scene upon the screen.