Method and apparatus for augmented reality using a see-through head-mounted display

An image augmentation apparatus for superimposing an overlay image on a portion of a field of view of a user looking through the apparatus at a scene external to the apparatus. The apparatus includes a display screen for generating the overlay image. An optical system creates a virtual image of the overlay image which appears to the user as covering a portion of the field of view. The apparatus utilizes a transmission screen having a two-dimensional array of pixels to block portions of the image of the external scene. The pixels of the transmission screen are either set to a first state in which the pixels are transparent or a second state in which the pixels are opaque. The transmission screen is located such that a pixel set to the opaque state blocks the user from viewing a portion of the field of view of the external scene allowing the user to view only the virtual image in that portion of the field of view. This results in the virtual image appearing to occlude the external scene in that portion of the field of view.

FIELD OF THE INVENTION 
The present invention relates to display devices, and more particularly, to 
a head mounted display for combining computer generated information with 
the scene being viewed through the display. 
BACKGROUND OF THE INVENTION 
There are a number of situations in which it would be advantageous to 
superimpose computer generated information on a scene being viewed by a 
human viewer. For example, a mechanic working on a complex piece of 
equipment would benefit by having the relevant portion of the maintenance 
manual displayed within her field of view while she is looking at the 
equipment. Display systems that provide this feature are often referred to 
as "Augmented Reality" systems. Typically, these systems utilize a 
head-mounted display that allows the user's view of the real world to be 
enhanced or added to by "projecting" into it computer generated 
annotations or objects. 
In a typical optical see-through head-mounted display, the user's view is 
an optical combination (via the half-silvered combining mirror) of the 
external world view and the image presented at the display. The optics are 
arranged such that the displayed image appears enlarged and at a 
comfortable viewing distance. 
A major disadvantage to a simple optical combination between the 
transmitted view of real objects and the projected view of the "virtual 
objects" is the lack of the ability for the latter to occlude the former. 
The virtual objects appear as translucent objects which overlay the 
external scene. The "bleed-through" of the external scene makes the 
viewing of the virtual objects difficult. For example, if the virtual 
object is a page of text, the text is difficult to read since it is 
printed on "paper" that contains the external scene. 
It has been determined that the occlusion of farther objects by nearer ones 
is a very strong visual cue in augmented reality. The inability of 
conventional head-mounted displays to provide this leads to problems in 
perceiving the depth of the displayed objects. This is particularly 
problematic in medical applications. 
Prior art solutions to the occlusion problem for head-mounted displays have 
relied on the use of "video see-through" technology. In these systems the 
external view is captured electronically via video cameras and then 
combined with the image to be presented. Since the combination is done 
electronically rather than optically, occlusion can be handled easily in 
the compositing of the images. 
There are several drawbacks to this approach. The resolution, color 
fidelity etc. of the external view is now limited to that available via 
the camera/display components. The resolution of the display must now be 
that needed to accurately represent the scene. In contrast, the resolution 
of the display in the simple optical mixing systems is determined by the 
accuracy needed to display the computer generated information. The 
resolution needed to display a page of text is usually much less than that 
needed to display an external scene presented to the viewer. 
There are also deleterious effects due to the processing delay in the 
viewing electronics between the user's perception of their own head motion 
and the corresponding motion of the viewed scene. These processing delays 
increase as the resolution is increased to provide more realistic 
representations of the scene from the outside world. 
In addition, the computational workload is substantially higher when the 
computer must process both the scene from the outside world and the 
material to be added to that scene. This results in increased computer 
costs which restrict the market for such displays. Lastly the cameras etc. 
add significantly to the weight, cost, and complexity of the head-mounted 
display. 
Broadly, it is the object of the present invention to provide an improved 
head mounted display for use in augmented reality systems. 
It is a further object of the present invention to provide a display in 
which the computer generated information does not appear translucent. 
It is a still further object of the present invention to provide a display 
that does not require cameras and compositing software and hardware. 
These and other objects of the present invention will become apparent to 
those skilled in the art from the following detailed description of the 
invention and the accompanying drawings. 
SUMMARY OF THE INVENTION 
The present invention is an image augmentation apparatus for superimposing 
an overlay image on a portion of a field of view of a user looking through 
the apparatus at a scene external to the apparatus. The apparatus includes 
a display screen for generating the overlay image. An optical system 
creates a virtual image of the overlay image which appears to the user as 
covering a portion of the field of view. The apparatus utilizes a 
transmission screen having a two-dimensional array of pixels to block 
portions of the image of the external scene. The pixels of the 
transmission screen are either set to a first state in which the pixels 
are transparent or a second state in which the pixels are opaque. The 
transmission screen is located such that a pixel set to the opaque state 
blocks the user from viewing a portion of the field of view of the 
external scene allowing the user to view only the virtual image in that 
portion of the field of view. This results in the virtual image appearing 
to occlude the external scene in that portion of the field of view.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention may be more easily understood with reference to FIG. 
1 which is a schematic drawing of a prior art image augmentation system. A 
human viewer 18 views a scene 15 through a set of "eye glasses" 10. 
Information that is to be added to the user's field of view is displayed 
on a back lighted display screen 12. The output of display screen 12 is 
combined with the view of the scene 15 with the aid of a half silvered 
mirror 11 which applies the image to a focusing reflector 13 which is also 
half silvered. Reflector 13 creates a virtual image of display screen 12 
in a plane 14 which is between the glasses and the objects of scene 15. 
The partially silvered mirror and reflector allow light from scene 15 to 
reach the viewer. 
The combined image generated by the glasses is shown in FIG. 2, The 
material displayed on screen 12 becomes a translucent image 17 in the 
field of view of viewer 18. Since mirror 11 and reflector 13 must be 
partially transmitting, the image is the "OR" of the image generated on 
display screen 12 and that passing through the optics from scene 15. As 
noted above, the translucent nature of image 17 makes it difficult to view 
when the scene has significant details in the same region as the image 
generated by display screen 12. 
The present invention provides an overlay image that is opaque by utilizing 
a transmission mask that blocks the portion of the scene 15 that would 
ordinarily be ORed with the material generated by display 12. Refer now to 
FIG. 3 which is a schematic drawing of an image augmentation 30 according 
to the present invention. Once again, a viewer 38 views a scene 15 through 
the image augmentation system. The material to be added to the viewer's 
field of view is displayed on a display screen 32 which generates an 
overlay image in a virtual plane 34 with the aid of partially reflecting 
mirror 31 and a partially reflecting focusing reflector 33. 
A transmission screen 40 is located between reflector 33 and scene 15. 
Transmission screen 40 is preferably a liquid crystal transmission screen 
whose pixels are controlled by controller 41. As will be explained in more 
detail below, controller 41 determines the portion of the image of scene 
15 that is to be overwritten by the material displayed on display screen 
32. The pixels of transmission screen 40 corresponding to the overwritten 
area are then rendered opaque by controller 41. The remainder of 
transmission screen 40 is left in a transparent state. 
The image seen by viewer 38 is shown in FIG. 4. The overlay image inserted 
via display screen 32 is shown at 45. Since transmission screen 40 has 
blocked the portion of the image from scene 15 that would have normally 
been ORed with the overlay image, the translucent quality of the overlay 
image has been eliminated. It should be noted that the embodiment of the 
present invention shown in FIG. 3 places transmission screen 40 at a 
location that is out of focus with respect to viewer 38, and hence, the 
overlay image appears to have an out of focus silhouette, or "mask", 
behind it. As a result, the edges of the mask are blurred as shown at 46. 
Methods for controlling, and even eliminating, this artifact will be 
discussed in more detail below. 
As noted above, controller 41 must determine which parts of transmission 
screen 40 are to be transparent and which parts are to be opaque. One 
method for providing this information is to provide separate video signals 
for the transmission screen and display 32. However, such a scheme 
requires greater complexity than schemes in which this information is 
encoded in the video signal used to generate the image on display 32. In 
the preferred embodiment of the present invention, a unique color is 
defined which is not used in the image to be projected into the viewer's 
field of view. This color defines the pixels of the displayed image that 
are to remain translucent. Controller 41 then sets the corresponding 
pixels in transmission screen 40 to the clear state. The remaining pixels 
are set to be opaque. Controller 41 also changes the pixels in the overlay 
image that were originally set to the unique color to black. In this way 
the overlay image display will contribute no light in the "transparent" 
parts of the image. 
The above described embodiment of the present invention assumes that the 
transmission screen has the same resolution as display 32. As noted above, 
the transmission screen is out of focus in these embodiments. Thus, the 
apparent resolution of the transmission screen, as seen by the viewer, 
will be less than that of display 32. Accordingly, a transmission screen 
having a resolution that is less than that of display 32 may be utilized 
without substantially reducing the quality of the image seen by the 
viewer. Since the cost of the transmission screen is related to the number 
of pixels in the screen, a significant cost savings can be realized by 
using a screen that has fewer pixels. 
When display 32 has a higher resolution than transmission screen 40, each 
pixel of the transmission screen will overlap a number of pixels in the 
display. A number of different algorithms may be used to determine the 
state of a pixel in the transmission screen from the state of the 
overlapping pixels on the display. For example, a pixel in the 
transmission screen can be set to opaque if any pixel in the corresponding 
region of the display is to be displayed on an opaque background. In this 
case, the opaque mask will typically be larger than the opaque region 
specified by the pixels of the display. In the other extreme, the 
transmission screen pixel is set to opaque only if all of the pixels in 
the corresponding region of the display are marked to be displayed on an 
opaque background. In this case, the opaque mask will typically be smaller 
than the opaque region specified by the pixels of the display. 
Intermediate algorithms, which depend on the fraction of the pixels in the 
corresponding region of the display that are marked to be displayed on an 
opaque background, may also be utilized. 
The extent to which the mask overlaps the displayed region that is marked 
as opaque may be controlled independent of any resolution difference 
between display 32 and transmission screen 40. In general, the 
transmission screen can be viewed as a display having a binary image. Each 
pixel is either opaque or clear. As noted above, this image is out of 
focus in the embodiment of the present invention shown in FIG. 3. As a 
result, the viewer sees the mask as extending outside of the specified 
opaque region with a blurred boundary. 
This boundary can be caused to expand or contract by altering the binary 
mask image on the transmission screen in a manner which either "grows" or 
"shrinks" the image. In general, the image on the transmission screen will 
consist of one or more compact "objects". Each object forms one of the 
masks discussed above. Algorithms for expanding or contracting such 
objects are discussed in more detail below. By expanding the mask objects, 
the displayed image will have the sharpest boundaries, since it will be on 
a larger mask further from the blurred edges of the mask. However, the 
portion of scene 15 presented to the viewer will be reduced. Similarly, by 
shrinking the mask objects on the transmission screen, the edges of the 
mask will move into the images being displayed leaving the edges of the 
images translucent as shown at 49 in FIG. 5. However, the portion of scene 
15 seen by the viewer will be increased. Accordingly, the preferred 
embodiment of the present invention has an input on the glasses for 
expanding or contracting the masks displayed on transmission screen 40. 
Embodiments of the present invention which eliminate the out of focus mask 
may also be constructed. Refer now to FIG. 6 which is a schematic drawing 
of an image augmentation system 50 according to the present invention in 
which the mask is always in focus. Augmentation system 50 utilizes two 
sets of lenses shown at 56 and 57 to collimate the light originating in 
image plane 55. A transmission screen 54 is placed between these two sets 
of lenses between mirror 53 and the plane 55. The overlay image to be 
added to the scene is again generated by a display screen 52 which is 
controlled by controller 51 which receives the augmentation image. The 
image generated by display screen 52 is reflected by a half silvered 
mirror 53. The first set of lenses 56 has a focal length chosen to image 
display screen 52 at image plane 55. Controller 51 sets the pixels of 
transmission screen 54 in a manner analogous to that described for 
augmentation in system 30. 
While display system 50 removes the effects associated with the 
out-of-focus transmission screen, display system 50 has its own 
limitations. The focusing optics 57 only allow a limited depth of field. 
Hence, the viewer will not be able to focus on objects that are 
substantially out of plane 55. 
In the above described embodiments of the present invention, a controller 
controls the transmission screen and performs the computations needed in 
the case in which the transmission screen image is to have a different 
resolution or size from that specified in the augmentation image. Such 
control computations and functions may be accomplished using programmable 
controllers or discrete logic elements. Those skilled in the control arts 
will recognize that the computations needed to control the transmission 
screen are relatively simple. For example, to determine the pixels of the 
transmission screen that are to be left transparent, the controller need 
only compare the color assigned to each pixel in the augmentation image to 
a predetermined color which marks the transparent pixels. The 
corresponding pixels in the transmission screen are then set accordingly. 
Similarly, the computations needed to control a screen having a coarser 
resolution than the display screen requires only that the pixels in the 
display image corresponding to each pixel in the transmission screen be 
identified and a simple counting algorithm applied in the case that all of 
the corresponding pixels are not transparent. 
Algorithms for expanding the image on the transmission screen by a pixel 
are also known to the image processing arts. To expand the image, each 
pixel on the image is replaced by 3.times.3 block of pixels. This 
increases the boundary of the mask image by one pixel while leaving the 
internal pixels unchanged. Similarly, the image may be shrunk by one pixel 
by scanning the image with a 3.times.3 pixel window. Each time the window 
has all pixels set to be opaque, the current pixel at the center of the 
window is set to opaque, otherwise the pixel is set to transparent. These 
algorithms may be applied iteratively, or with larger masks (e.g. a 
5.times.5 mask for up to two pixel grow/shrink operations), to generate 
images that are expanded or contracted by more than one pixel. 
Various modifications to the present invention will become apparent to 
those skilled in the art from the foregoing description and accompanying 
drawings. Accordingly, the present invention is to be limited solely by 
the scope of the following claims.