Note: Descriptions are shown in the official language in which they were submitted.
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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.
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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
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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
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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.
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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
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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
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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
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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.
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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),
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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
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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
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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
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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
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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
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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.
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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.
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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
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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.
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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.
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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
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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
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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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