sources include: James Helman's Siggraph '93 Applied VR course notes, Dennis Proffitts '94 Developing Advanced VR Applications course notes, Lou Harrison's Siggraph '97 Stereo Computer Graphics for Virtual Reality notes, and the IRIS Performer guide.
We are going to spend some time
talking about the human visual system.
The eye has a bandwidth of approx 1
gigabyte per second - much greater than the other senses. Any AR or VR
systems we create should try to match or exceed what the human body can
The real world
doesn't flicker. Classic theatrical films flickered and CRT displays
flickered because the image is constantly being refreshed. Most modern
displays also need to be refreshed regularly to keep energy flowing to
the various elements. If the image isn't refreshed fast enough, we
perceive flickering. Some people can perceive flickering even at 60Hz
(the image being refreshed 60 times per second) for a bright display
with a large field of view but most people stop perceiving the flicker
between 15Hz (for dark images) and 50Hz (for bright images). LCD panels
tend to have 60hz or 120hz refresh rates these days (though
manufacturers often claim to have higher rates. Direct view LED tiles
have similar rates.
Note that this is the
rate that the hardware refreshes, which is (mostly) independent from the
rate the graphics are being refreshed. We are at the point now that
HMDs, like TVs, and phones generally have sufficient hardware refresh
rate, though more exotic technologies for AR are still catching up.
Making sure the graphics refresh fast enough will always remain an
The eye can detect an
amazing range of 10^14 (10-6 to 108 cd/m2)
in luminance, but we can only see a range of 100:1 at the same time. The
size of our pupil automatically adjusts to the amount of light
available, allowing us to shift our window of much narrower range of
luminance detection across a much larger range. If we spend 30 minutes
dark adapting we can dramatically increase our vision at the low end.
The human eye is
sensitive to ratios of intensities not absolute magnitude. Brightness =
Luminance^0.33. To make something appear n times brighter the luminance
must be increased by n^3.
Most perceptual processes are driven by intensity not colour. The human motion system is colour blind, depth perception is colour blind, object recognition is colour blind.
but uniquely coloured objects are easy to find
Today's displays still can not
reproduce the full colour spectrum that human beings can see, but the
range increases every year, and with the new focus on OLED displays,
high contrast displays, and direct view LED displays the coverage is
getting much better.
Ditto the notes above on regrading
for the display device and its location.
Field of View
Each eye has
approximately 145 degrees horizontal (45 degrees towards the nose and
100 degrees to the side) and 100 degrees vertically (30 degrees up and
70 degrees down)
Current VR headsets like the VIVE and the Rift have 110 degree FoV, the new Quest has 115 degree FoV which are getting close to matching human field of view.
AR headsets currently
have much smaller fields of view which limits their ability to combine
real and augmented content as you can only see the augmented content
within a window within your field of view.
Google Glass display
had FoV of 15 degrees monoscopic in the corner of your vision
Microsoft HoloLens has a 30 degree horizontal and 18 degree vertical FoV
Magic Leap One has a 40 degree horizontal and 30 degree vertical FoV
When holding your
phone or a tablet the FoV depends on how close you are holding the
screen to your face - the closer to your face the more the screen fills
With human scale
systems like the CAVE and CAVE2, or even a planetarium dome, the field
of view depends on the size and location of the physical displays and
the type of 3D glasses used - 6 sided CAVEs surrounded the users with
screens, but the glasses typically don't cover the complete field of
view (roughly 120 horizontal and 90 vertical).
We often encounter
visual acuity as measured with a Snellan Chart at the doctor's office
where correct vision is described as being 20/20 (20/X where this viewer
sees at 20 feet detail that the average person can see at x feet, 20/200
is legally blind). If you are metricly minded then the fraction is 6/6.
Back in the 90s our
monitors at normal distance were 20/40, HMDs were 20/425, BOOMs were
20/85, and the original CAVE was 20/110.
Our current consumer
HMDs are around 20/60 with a wide variety of lens distortion effects to
give us a wider field of view, while most of our laptops and phones have
so many pixels that they give us better than 20/20 across a small range
of our vision.
Overview of How the Eye Works
Bill Sherman's diagram of the human eye
eye has 2 types of photosensitive receptors: cones and rods
cones are highly concentrated at the fovea and quickly taper
off around the retina. For colour vision we have the greatest
acuity at the fovea, or approximately at the center of out
field of vision. Visual acuity drops off as we move away from
the center of the field of view.
The rods are highly concentrated 10-20 degrees around the fovea, but almost none are at the fovea itself - which is why if you are stargazing and want to see something dim you can not look directly at it.
happens when we walk from a bright area into a dark area, say into a
movie theatre? When we are outside the rods are saturated from the
brightness. The cones which operate better at high illumination levels
provide all the stimulus. When we walk into the darkened theatre the
cones don't have enough illumination to do much good, and the rods
take time to de-saturate before they can be useful in the new lower
illumination environment so we cant see very well.
It takes about 20 minutes for the rods to become very sensitive, so
dark adjust for about 20 minutes before going stargazing.
Since the cones do not operate well at low light intensities we can not see colour in dim light as only the rods are capable of giving us information. The rods are also more sensitive to the blue end of the spectrum so it is especially hard to see red in the dark (it appears black).
VR where the programmer has total control of the visuals this means
taking into account how long it will take a person to be able to see
something in a dark area after being in a bright one, and how bright a
scene can be after being in a dark one without blinding the user.
AR this means we need to be careful how bright the computer graphics
are, and we need to take into account the current conditions (is it a
bright sunny day, is it overcast, is it dark) so the generated
graphics are visible and comfortable.
is also the optic nerve which is 10-20 degrees away from the fovea
which connects your eye to your brain. This is the blind spot where
there are no cones and no rods. We can not see anything at this point
and our brain compensates by filling in that part of our vision with
surrounding colours and simple patterns.
This page has a nice simple visual to help you see the location of both of your blind spots and gives a hint at the amount of processing the brain is doing to the information coming from your eyes: https://medicine.yale.edu/lab/mccormick/seminars/vision-fun-1/slide2/
Here is a nice page on colour blindness:https://www.color-blindness.com/ishiharas-test-for-colour-deficiency-38-plates-edition/
8 percent of men
1 percent of women
Vision Simulator is a nice phone based tool for simulating
what colour blind people see. Its free and available at http://asada.tukusi.ne.jp/cvsimulator/e/
Just as in the real world where we make sure that traffic lights work for colour blind people, we need to make sure that our VR and AR worlds work as well. You should make sure that the worlds you create work for color blind people.
In general it is a good idea to give
the user control over colors, text size, contrast and brightness etc. in
VR and AR just like it is a good idea to do this on a smart phone or a
laptop to enable more people to use your hardware and software.
the major takeaways from this is that as our display technology gets
better and better we will increasingly be running into the limitations
of the human eye rather than the limitations of the display technology -
just as we are on our smartphone and laptop screens.
major takeaway is that as we are creating worlds, in the case of VR,
where we can take total control of various senses , this allows us to
get a better understanding of how those senses operate, alone and
together, and the more we understand, the less likely we are to make
people sick, and the more likely we are to give them richer experiences
that take advantage of all the relevant senses.
One of the most important things in VR, and increasingly in AR, is creating an artificial world with a sense of depth.
Real World Depth Cues
Photography, Computer graphics give 1,2,3,4
Interactive computer graphics, and monoscopic AR (hand held smartphone or tablet) adds 5
VR gives and stereoscopic
AR adds on 5,6,7
some specialized multi-plane displays and light field displays can give 8.
In VR, as in stereo movies or stereo photographs, the brain is getting two different cues about the virtual world. Some of these cues indicate this world is 3D (convergence and stereopsis). Some of these cues indicate that the world is flat (accommodation). The eyes are converging depending on the position of the virtual objects floating in front of or behind the screen, but the eyes are focusing on the flat screen. This can lead to headaches, or more serious forms of simulator sickness, but in general its not a serious problem, especially if you are doing it for only a couple hours at a time.
For most people VR
'works' and virtual objects appear to be the correct size because they
'trust' convergence more
than they trust accomodation
(focus on the screen), but some people trust
accomodation more, and for them the virtual objects are always on the
projection plane and generally the wrong size.
If we start thinking
about wearing AR glasses (which could potentially become VR glasses
whenever we wanted) all the time then we may run into more issues.
there are some potential issues of having young children seeing this
kind of mediated 3D and research is ongoing there in terms of whether it
can alter the normal development of their vision. Most of the current
headsets are for 12 or 13 year olds or older to be safe.
How to Generate and View Stereo Images
We have stereo vision because each eye sees a slightly different image. Well, almost all of us (90-95%) do.
imagery through technology dates back to the mid 1800ds. Here is a
stereoscope from the early 1900ds.
As a kid you may have had a View-Master(tm) and saw the cool 3D pictures. Those worked exactly the same way as the stereoscope. Your left eye was shown one image on the disc, while your right eye was shown a different image. These images were typically taken with a pair of identical cameras with identical settings sitting horizontally next to each other, or two images taken by the same camera that is quickly moved horizontally. Often cameras, especially film cameras, were too large requiring a series of mirrors to get their lenses close enough to mirror human vision.
As a kid you might
have read some 3D comics which came with red/ blue (anaglyphic) glasses,
or you might have even seen a movie with the red/blue glasses (maybe
down at U of Chicago's Documentary Film Group with films like 'House of
Wax' or 'Creature from the Black Lagoon' or 'It Came from Outer Space').
You can either use 2 synchronized projectors each with a black and white
print of the film for one eye, one projector with a red filter and one
with a blue filter, or, less expensively, you can have one projector
with the red and blue on a single color print of the film , which is why
this would become popular at second run theatres.
If you have a pair of red/cyan glasses and a correctly calibrated display then this image will become 3D.) The coloured lenses make one image more visible to one of your eyes and less visible to your other eye, even though both eyes are viewing both images.
What people saw in
the theatre back in the 1950s, including that photo above from Life
magazine, was closer to the image below, using
2 synchronized projectors each with a black and white print of the
film for one eye, each projector with a different polarizing filter,
and each person in the audience wearing polarized glasses. This is
basically the same technology as when 3D movies came back in the 1980s
and when they came back again in the early 2000s. Now we have digital
projectors so the image is much more stable and we don't have film
breaks so the final stereo effect is much cleaner but the core
technology is identical.
we can also put polarizing material on the front of our flat
panel displays in alternating horizontal lines of the left and right eye
images, so a single video (in color or black and white) can contain
imagery for both eyes, though in half resolution each.
here is another
red/blue image from an Ocean Going Core Drilling ship. The red/blue or
red/cyan trick works ok for greyscale images (like old movies or black
and white comics) but has a hard time with color. Other color variants
have been used over the years.
and here is a nice movie on youtube https://www.youtube.com/watch_popup?v=MQEkFppWaRI
Another inexpensive way to show stereo imagery is to draw two slightly different images onto the screen (or onto a piece of paper), place them next to each other and tell the person to fuse the stereo pair into a single image without any additional hardware. This is easy for some people, very hard for other people, and impossible for a few people.
Some of these images
require your left eye to look at the left image, others require your
left eye to look at the right image (cross-eye stereo).
To see the pictures below as a single stereo image look at the left image with your right eye and the right image with your left eye. If you aren't used to doing this then try this: Hold a finger up in front of your eyes between the two images on the screen and look at the finger. You should notice that the two images on the screen are now 4. As you move the finger towards or away from your head the two innermost images will move towards or away from each other. When you get them to merge together (that is you only see 3 images on the screen) then all you have to do is re-focus your eyes on the screen rather than on your finger. Remove your finger, and you should be seeing a 3D image in the middle.
Passively polarized stereo - https://en.wikipedia.org/wiki/Polarized_3D_system
As mentioned above,
in the 1950s when stereo films like 'Creature from the Black Lagoon'
were shown theatrically, the theatres did not use red/cyan. They used a
system pretty much identical to what 3D movies used in the 80s and again
today - passive polarization. In this case one projector shows the left
eye image with a polarizing filter. A second synced projector shows the
right eye image with a different polarizing filter. The screen preserves
polarization and the viewers wear lightweight polarized glasses that
allow the correct image through and not the wrong one.
This same technique
can be used on flat panel displays where a layer is added on top of the
display with lines of alternating polarization matching up with the rows
of pixels below. The same glasses are used but now one eye only sees the
even lines of the display while the other eye only sees the odd lines.
This has the disadvantage of cutting your vertical resolution in half.
An alternative approach for both projectors and flat panels is to use Active Stereo - https://en.wikipedia.org/wiki/Active_shutter_3D_system
In active stereo the
projector or the panel shows the image for the left eye, then black,
then the image for the right eye, then black, and then repeats. The
viewer wears glasses with LCD shutters the turn either clear or opaque
and the shutter for the appropriate eye is open when the display shows
the image for that eye. This requires the display to also talk to the
glasses (usually via IR) and for the glasses to be powered.
sure your visuals look good, its very important to keep in mind what
kind of display you will be using, the space that it will be used in,
who will be using it, and what kinds of things they will be doing - are
these designers interacting with a new car model, or museum visitors
exploring an ancient site?
systems, even modern laser based 4K ones, can give much larger
borderless fields of view, but typically need to be in dimly lit or dark
rooms. If the system is front-projection then how close will the user be
able to get to the display before they begin to cast shadows on the
screen? If its rear-projection from behind the screen then how much
space are you prepared to waste behind the screen.
Large flat panel
displays have a different problem - they are designed for on-axis TV
viewing and the further you are off-axis (horizontal or vertical)
increases the chance that you will see degraded colour / contrast /
stereo vision. They also tend to have borders which can be distracting
when they are tiled. They are also thin so you cant stand on them
without breaking them.
Head mounted VR
displays avoid issues of dark rooms and off-axis viewing but you still
need enough space for the user to move around in and you need to make
sure the user doesn't hurt himself/herself while moving around without
seeing the real world.
Head mounted AR
displays allow the user to see the real world, making them safer than VR
displays, but as these displays may be used in a dim room or in bright
sunlight the graphics need to be visible across that range of use cases,
similar to how you want to be able to use your phone or smart watch in a
variety of settings, so the contrast and brightness ranges need to be
greater than for displays that are only used indoors.
Distance - the
distance between the viewer's left and right eyes, usually about 2.5
inches - in order for virtual objects to appear to have the exact
correct size the AR or VR system needs to know your specific
interocular distance. https://en.wikipedia.org/wiki/Interpupillary_distance
that there can be good reasons to change the interocular distance if you
are looking at very small things or very big things, i.e. back in 2006
we helped out with the NASA STEREO project using two satellites ahead
and behind the earth in its orbit to take stereo movies of the sun, and
on the other end of scale we work with biologists that take stereo
images of cells.
Stereo Window (Plane) - the
point at which there is no difference in parallax between the two eye
views - usually at the same depth as the monitor screen or the
projection surface. HMD optics create their own projection plane,
commonly about 1-2 meters away from the user. Looking at things on the
plane is most comfortable since convergence and accommodation are both
correct, but there is no stereo effect there. Often the subject of a
stereo photograph or movie is placed on the plane with other elements in
front or behind to heighten the 3D effect.
Homologous Points - points
which correspond to each other in the separate eye views (e.g. the tip
of a person's finger, or a person's left pupil)
Parallax - when
the retinal images of an object fall on disparate points on the two
retinas, these points differ only in their horizontal position (since
our eyes are at the same vertical position). The value given by R - L.
Note that this is obviously true for stereo movies, but in full VR you
may be tilting your head (and your two eyes along with your head) so
the 'horizontal' part refers to the line between your eyes defining
horizontal and vertical.
Parallax - the point lies behind the stereo window -
in the Creature from the Black Lagoon image, the background is far from
the camera and has blue to the right of red for homologous points
Zero Parallax - the point is
at the same depth as the stereo window -
in the Creature from the Black Lagoon image, the base of the
fossilized hand is at zero parallax and there is no red / blue
Negative Parallax - the point
lies in front of the stereo window - in the Creature from the Black
Lagoon image, the fingers of the fossilized hand are near to the camera
and have red to the right of blue for
homologous points like the fingertips
Vertical Displacement -
vertical parallax between homologous points relative to the line that
the two eyes form - this should be zero in a correctly calibrated setup.
Our eyes and brain can actual handle a certain amount of vertical
parallax but it gets uncomfortable.
Interocular Crosstalk (Ghosting)
- when one eye can see part of the other eye's view as well, reducing
the effect of the stereo imagery - Crosstalk should also be zero in a
correctly calibrated setup, but this can be hard to achieve, especially
in scenes with very high contrast.
In an HMD or the boom, the screens in front of each user's eye move with the user - that is the screens are always perpendicular to the eye's line of site (assuming the eyes are looking straight ahead). This allows the traditional computer graphics 'camera paradigm' to be extended so that there are 2 cameras in use - one of each eye.
In the CAVE, Fish
Tank VR, or any large screen VR, this is not the case. The projection
planes stay at a fixed position as the user moves around. The user may
not be looking perpendicular to the screen and certainly cant be looking
perpendicular to all of the screens in the CAVE simultaneously - in this
case 'off-axis projection' is used, and the math/geometry is a bit more
complex to be more general, though still something that someone with a
knowledge of computer graphics from CS 425 could implement. One of our
former students, Robert Kooima, has a nice page on the geometry and math
involved : http://csc.lsu.edu/~kooima/articles/genperspective/
Visuals for Multiple Viewers
With head mounted
displays for AR or VR there is a single user looking at the display.
With room scale or spatially immersive displays like CAVEs there are
typically multiple users, but usually only one user being head tracked.
This means everyone is seeing through this one person's eyes, meaning
the farther you are standing form this person the less correct the
visuals are. The visuals are also moving based on that tracked person's
movements, so if that person is moving around a lot the visuals are
moving around a lot.
|For a single large screen the users are all typically looking at the (single) screen so head tracked stereo visuals are reasonably correct most of the time.|
|In a more immersive multi-screen setup like a classic CAVE with a front, left, and right walls, and a tracked user looking at the front wall, the imagery on the left and right walls will be monoscopic as the tracked user's eyes are in line towards those walls. In full 6 wall CAVES with a tracked user looking at the front wall this means that stereo on the back wall is reversed, which is uncomfortable for anyone looking in that direction. While correct for the one tracked user, this 'correct' head tracking can cause issues for groups of users.|
One solution is to use the tracked user's head position to generate (rotated) tracked user eye positions independently for each wall to maintain correct stereo on the assumption that the tracked user's eyes are 'looking at' each wall simultaneously. This is slightly less correct for the one tracked user, but much more comfortable for everyone else who don't have stereo disappearing or reversing as the tracked user moves and turns.
Updating Visuals based on Head Tracking
Naively we would like to update the graphics every frame in order to use the most recent head (eye) positions.
Since there will be jitter in the tracker values and latencies in getting information from the tracking hardware to deal with, this may result in the computer generated image jittering.
One way to avoid this
is to only update the image when the head has moved (or rotated) a
certain amount so the software knows that the motion was intentional.
Another common option is to interpolate between the previous and current position and rotation values in order to smooth out this motion. This results in smoother transitions, and minimizes tracker error, but will also increase the lag slightly.
Another option is to extrapolate into the future by predicting how the user is going to move in the next couple seconds and proactively render for the position you believe the user will be in since people typically do not move their heads too abruptly.
How to generate Graphics Quickly
In the 1990s Silicon Graphics (the
folks who created GL, OpenGL, and would go on to found nVidia) optimized
graphics through a pipelined approach (image from SGI Performer manual)
and overlapping different parts of the pipeline.
and then it helps to have tools to
help visualize how long each part of the process takes and where
improvements can be made.
The following image show
a-application, c-cull, d-draw in the same color for the same frame, e.g.
when its drawing frame 5 its culling frame 4 and doing the application
phase of frame 3. In this case the application is spending almost all
its time drawing, so it has time to do more application work or culling
which might improve the overall experience.
Unity has a similar tool with its
Profiler - https://docs.unity3d.com/Manual/ProfilerWindow.html
While in videogames it is good to
maintain a particular frame rate, in VR and AR it is much more important
to do that.
Current game engines are pretty good
at optimizing what is drawn based on where the user is in the scene. If
you create your own software from scratch you will need to take care of
Models can be replaced by models with less detail
3D models of far away objects can be replaced by texture mapped billboards
The horizon can be moved in - moving in Z-far and perhaps covering this with fog
A less complex lighting model can be
2 things are needed: a functioning vestibular system (canals in the inner ear) and a sense of motion
Symptoms: Nausea, eyestrain, blurred vision, difficulty concentrating, headache, drowsiness, fatigue
These symptoms can persist after the VR experience is finished.
Causes: still unknown but one common hypothesis is a mismatch between visual motion (what your eyes tell you) and the vestibular system (what your ears tell you)
Why would this cause us to become sick? Possibly an inherited trait - a mismatch between the eyes and ears might be caused by ingesting a poisonous substance so vomiting would be helpful in that case.
bright images are more likely to cause it than dark ones
wide field of view is more likely to cause it than narrow field of view
HMDs are more likely to cause it than projection systems
low resolution, low frame rate and high latency are also likely causes
Another hypothesis deals with the lack of a rest frame. When a user views images on a screen with an obvious border that border locates the user in the real world. Without that border the user loses his/her link to the real world and the affects of motion in the virtual world are more pronounced.
Current HMD environments default to
allowing the user to walk around a physical space but then use
teleporting rather than controller based 'flying' common in first person
shooters to move larger distances. This is primarily to reduce the
chances of people getting sick.
Fighter pilots have 20 to 40 percent sickness rates in flight simulators - but experienced pilots get sick more often than novice pilots.
In a rotating field when walking
forward, people tilt their heads and feel like they are rotating in the
opposite direction. Here is a physical version but you can do the same
thing with just computer graphics
If a person is walking on a treadmill holding onto a stationary bar and you change the rate the the visuals are passing by, it will feel to the person like the bar is pushing or pulling on their hands.
Open fields are less likely to cause problems than walking through tight tunnels; tunnels are very aggressive in terms of peripheral motion. This doesn't mean that you should not have any tunnels, but you should be careful how much time the users spend there.This all affects the kinds of worlds you create and how long a person can safely stay in that world.
Its easy (and fun) to induce vertigo. Most people really seem to enjoy jumping off of high places and falling in VR.
With VR, you very likely have visuals covering the user's field of view, like a child up close to the TV, and you have a full array of special effects to chose from. Choose carefully.
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