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3D Perspective - And The Reason Behind “3D Headaches”
3D Perspective - The Less Technical Version

“3D Perspective” is “what an individual sees.”  It means that two people looking at the same thing can see it very differently.  For these two people, there could be a lot of real differences or they might just be interpreting the same things differently.  A person can certainly see a huge ball in the distance and say, “It’s a small ball up close” without really being wrong about their own experience.  It is this individual experience that matters to us most.  Even so, we still want to know the actual size of the ball.  That’s where the display technology used in 3D TVs comes into play.  Dual-image 3D displays send a different image to each of our eyes and enhance our 3D perspective, an important step in the quest to ‘find the real size of the ball,’ or to “see what’s really there.”

Widespread techniques for creating drawings and paintings with 3D perspective took a very long time to develop in the world of art.  This is the complete opposite of photography and cinema, where cameras could never help but record the true appearance of a scene with all its 3D perspective intact.  Indeed, the number of 3D traits automatically present in a flat image is quite surprising.

Human eyes are slightly separated by a distance.  Each eye sees a slightly different scene in the same way that two people standing next to each other each see something different.  People normally consider “seeing 3D” as each of their eyes seeing slightly different images, but this doesn’t account for all the 3D perspective traits already present in a flat picture.  Understanding these traits will help explain why dual-image 3D is actually just a ‘3D upgrade’, not a transition from 2D to 3D like most people think.  The traits of 3D perspective are: 3D projection (shape skewing), parallax (relative motion differences), occlusion (objects blocking other objects), and focal distance (the ‘unblurry’ range in an image).

The first trait of 3D perspective is 3D projection.  It means that when something 3D is shown on something flat, it gains a unique shape that depends on its position.  In particular, near objects appear larger and more skewed than distant ones.  3D projection can sometimes cause confusion and optical illusions if there are objects are unusual shaped or neighbor each other artificially (like ‘holding up’ the Hollywood sign).  Dual-image 3D displays can help our eyes identify these kinds of objects with a little more scrutiny because two different images gives the brain additional information about the shapes.  This means that optical illusions are harder to pull off on dual image displays, which is a good step towards “seeing what’s really there.”

The next trait of 3D perspective is parallax.  This concept is mostly used to describe how further objects appear to move slower than closer ones.  The geometric reasoning for this is not as important as the benefits of the effect.  Everyone is accustomed to judging distance by how much something changes during motion.  For example, a mountain in the distance appears to stay in place no matter how fast you’re driving, whereas a speed limit sign moves slow at a distance and then really fast when you get right next to it.  This sort of motion parallax is an important part of 3D, but it can be somewhat deceptive.  When something is moving on it’s own or it’s size is other than expected, our minds can be tricked into thinking something else is happening.  Dual-image 3D displays give us a second way to judge parallax, so objects that move confusingly can be interpreted more quickly and accurately.

Occlusion is another trait of 3D perspective.  It is “when something closer gets in the way of something further.”  This result is obvious for most objects, but what about two objects that appear to be the same distance away?  If you were looking at two parallel pipes running forward on the ceiling and you knew one was higher, you can use occlusion to find out which one it is.  If you move one way or the other, one of the pipes will eventually get in the way of the other and that’s the lower one.  In the case of a camera, its motion can reveal new information in the same manner.  But what if the camera’s not moving?  Dual-image 3D gives us an advantage in this case because it allows us see two slightly different positions at the same time.  The second image gives you a whole new set of clues you can use to dissect an environment.

The final trait of 3D perspective is focal distance.  Every camera and eye needs to be focused at a specific distance to correctly capture light.  This means that if there are many objects at many distances in a picture, some of them will always have to look blurry.  Although “blurry” isn’t usually equated with “good”, it does have its advantages.  Seeing unfocused parts of an image against focused ones tells a viewer which objects are at similar distances.  That, however, is the only advantage.  The prominent downside is that unfocused parts of an image can never be refocused by our eyes (the light necessary to do so wasn’t recorded).  Dual-image 3D cannot help with this particular problem.  In fact, it actually makes it worse.  The increased 3D effect entices our eyes to look around more and focus in different places, but they often only end up trying to focus on unfocusable blur. 

Ultimately, there are many advantages to dual-image 3D and at least one disadvantage.  Sending a different image to each of our eyes means that 3D projection, parallax, and occlusion each gain an extra bit of detail.  This makes it harder for imagery to trick us, and the result is that our minds become more confident in what we see.  It gives us a greater sense of connection to the recreated environment.

Unfortunately, having two images with focal blur will confuse us when we try to look around, because our eyes think that they should be able to focus wherever they want.  This drawback is noticeable in all 3D technology and is the main cause of “3D headaches.”  The best way to reduce the effect is to focus on the same objects as the camera and try not let your eyes wander.  Focal blur is the reason why CG (Computer Graphic) movies often look better in dual-image 3D than live action ones.  The CG environments aren’t real, so focal blur can simply be ‘left out’, allowing the viewers to look wherever they want without strain.  Though it is possible to eliminate focal blur in live action movies, it requires multiple cameras to focus on different distances simultaneously and then a computer to stitch those images together.  The only exception to this is when everything on screen is far away and a single camera can use a focal distance of “infinity” to get a good approximation of everything.

3D perspective has one more effect on displays, which appears ‘outside’ the images on a screen.  It is the 3D projection of the flat display onto your eye.  If you move to the side of a TV or movie screen really far, you’ll notice that the pictures looks skewed (when watching “The Lord of the Rings” from an angle, you might wonder if the “One Ring” is really the “One Oval”*).  This problem is not corrected by dual-image 3D and can create an additional level of discontinuity when 3D objects viewed at a high angle ‘pop out askew’.

A more advanced type of 3D than dual-image is “single parallax” (e.g. Holografika’s Holovizio).  This kind of device can show a different image for every position of a viewer who moves from one side to another (but not up and down).  The additional imagery corrects some of the focal problems present in dual-image 3D and it makes it so you can look around stuff and sit in different positions without seeing skewed objects.  It still has the same problems as dual-image 3D along the other direction.

The next step beyond single parallax is “double parallax” or “volumetric.”  Such a system would have continuous imagery for every position and would resolve all of the problems present in 3D displays and create a completely natural appearance.  Crafting such a system is no small task, and that is the reason for the book “How to Make a Holodeck.”  The more people that read about the volumetric display techniques it presents, the sooner such displays will become commercially available.

Dual-image 3D gives a boost to the single-image 3D that has been forever present in televisions and monitors.  The 3D perspective traits we are accustomed to each gain something new through the addition of a second perspective.  The transition also puts us one step closer to fully recreating reality, the end results of which will have effects on every field of science and technology.

*This was the experience my dad had when the first movie was in theaters.  All the seats were full so he got stuck with the far end seat in the first row.


Change is Silver

Author of “How to Make a Holodeck” (5Deck.com)
~A silly and informative book that describes how to create fully volumetric 3D and 4D displays.
Creator of Unili arT (UniliarT.com)
~Original designs placed on multiple stickers to be readable through rear view mirrors.

3D Perspective - The More Technical Version

The term “Perspective” has several meanings: it can be “how an individual sees the world”; it can be, “the literal geometric relationship between objects and light”; or it could just be “how two or more people looking at the same thing see it differently.”  These definitions are all related, but they are not exactly the same.  For example, if I said, “that large building is really far away”, you might reply, “no, that building is close and small.”  One or the other may be true, but that doesn’t define our individual experiences.  It is our own impressions that matter to us the most.

Still… we all want to be able to see the ‘right’ things around us, at least for posterity.  That’s where 3D perspective comes in.  It is “the attempt to make relative experiences more uniform” or “to see what is really there.”  Artists may take exception this embarkment (i.e. art is practically defined as an individual’s unique perspective), yet it remains an important step in humanity’s attempts to recreate the world; that is after all, the ultimate purpose behind every home entertainment system.

The article “What is 3D? (More Technical)” describes two types of 3D.  “Old 3D,” which is when a recreation of the world appeals to a given sense as a whole (like a flat TV screen or a single speaker); and “New 3D”, which is a recreation of the world that appeals to each input of a given sense (an unique image or speaker for each eye or ear).  This difference is what defines our modern attempts to improve the 3D perspective of our technology, to finally “see what is really there.”

In terms of modern technology, Old 3D and New 3D only refer to the recreation of sight and sound.  Additionally, since we are discussing “perspective”, we’ll limit that discussion even further to just sight.  “Perspective” does have meaning for sound, but it is very different than with sight.  Our ears face opposite directions and each one does not have the same capacity for directional differentiation as do each of our eyes (if you are interested in the concept of directional sound, you can read an advanced treatment of the subject in the “How to Make a Holodeck+++” article “3D Sound” at 5Deck.com).  So now we’ve narrowed down our discussion of 3D perspective to its visual meaning in Old 3D and in New 3D; let’s start with its meaning in Old 3D.

Widespread use of 3D perspective is a relatively recent development in the art world (recent, as in hundreds of years compared to its much longer history).  However, it was an instantaneous part of the cinema industry.  A video camera cannot avoid recording the world from a single 3D perspective.  In this sense there is not much to say about Old 3D-no one had to put any effort into recreating the 3D perspective found in the real world because it was naturally recorded onto film.  Despite the ‘automatic’ nature of Old 3D, there are several 3D perspective concepts that can be picked out and analyzed in retrospect.  Each of these concepts also applies to New 3D, so they will lay a good foundation for our discussion.

The first 3D perspective concept present in Old 3D is 3D projection.  When light from the real world is collected on a flat surface, distant objects appear small and normal compared to nearer objects that look large and skewed.  You could manually trace each object’s defining points onto a flat surface to get the exact idea, but more generally, it is the same process our Retina’s already use when taking in the light from an environment.  We see exactly what the light coming from every point of every object naturally allows us to see.  Since our eyes use this technique already, it feels natural to view 3D on a 2D screen.  The major problem with this type of 3D projection is that it can be just as deceptive as it is informative.  A distorted object at a distance can appear identical to an undistorted one up close.  Fortunately, New 3D can help account for such situations.

Another Old 3D concept is parallax.  This is a descriptive term for how a viewer’s motion will cause distant objects to appear slower than near ones.  This term is relatively limited because it can cause incorrect assumptions under atypical circumstances.  For example, a distant object could be moving on its own, a near object could be a different size than a proportionally distant one and appear out of sync, and one or more objects could have a changing parallax because of subtle inward or outward motion.  Each of these circumstances will cause false assumptions about an object’s motion when compared to expected parallax.  You’ll understand how difficult it is to read something like inward and outward motion the next time you are driving behind a car and it slam on its breaks.  Trouble with judging parallax is a natural part of 3D perspective, but the degree to which false assumptions are present can be reduced by New 3D.

The next Old 3D concept is occlusion.  Further objects disappear when nearer objects are in front of them.  It may seem like an obvious statement, but it becomes less obvious when there are multiple viewers.  If people next to you can see one object behind another, but you cannot, there’s no way for you to know that the object exists even though it’s obvious to others.  Video games often use perspective occlusion tricks to hide treasure chests, even when doing so wouldn’t work in real life.  Moving around to discovering new objects is an inherent part of a 3D perspective, because that’s what real life is like.  New 3D can waylay some of the forced occlusion present in Old 3D, but only a little.

The final Old 3D concept to discuss is focal distance.  If an object is near, you can alter your eye’s focus closer to match the distance of the object.  This allows your brain to discover the distance to the object because it knows ‘what dilation’ of your pupil is associated with ‘what distance’.  Old 3D accounts for this, but only in a partially successful way.  Because Old 3D does not actually create objects at their correct positions, the light the camera gathers is only correct for the camera’s actual focal distance.  When a movie shows a blurry part of the screen, your eyes will get a sense of depth by distinguishing the blur and lack of blur, but your eyes cannot focus on the blurred section (the light they need to do so does not exist on the screen) so your pupils won’t be able to tell you how far away things really are.  To illustrate: let’s say someone puts two barrels on a grid drawn on the ground, takes a picture, and then uses software to forcibly blur one but not the other.  The grid lines in the picture will show your eyes that the barrels are the same distance away, but one will be blurry and the other won’t.  This will appear strange and irreconcilable to your eyes.  A similar confusion will occur when you can’t focus on a specific part of an apparently 3D image (Old 3D).  This confusion, unfortunately, only worsens with New 3D.

So we’ve discussed the 3D perspective present in Old 3D images by way of projection, parallax, occlusion, and focus (that’s a lot of 3D for just one flat picture!).  You’ve already gotten some hints of how these concepts are affected by the transition to New 3D, but let’s discuss it a little more thoroughly.  With two eyes, humans can each perceive two different images; so we each have “two perspectives on everything.”  Much like two different people can see the same things in two different ways.  Or at least that’s the way it should be.  In reality it’s a little different.

If two people can look at one object (from the same position) and say “it’s big and far” and “it’s small and close,” then so too can each of one person’s eyes.  However, this is never the case.  Our brains just don’t have the power to simultaneously process two different interpretations of the same thing, which means that we can never get two complete perspectives (even though we physically have the potential to do so).  Instead of our two eyes giving us two completely unique perspectives, they give us one consistent upgrade to our existing 3D perspective.  It’s probably better that way, because two conflicting perspectives would likely hinder our ability to make decisions.

The transition from Old 3D to New 3D alleviates some of the problems with 3D projection, parallax, and occlusion.  For 3D projection and parallax, two perspectives give our eyes the chance to see how a slight change in position affects the shape and motion of an object.  That slight change is usually all that’s needed to determine distance, position, and speed, even amongst artificially similar or confusingly contradictory objects.  Forced occlusion is also remedied by the slightly different object information we receive in our second eye.

But what about New 3D’s effect on focal mismatches?  Unfortunately, two flat images cannot remedy the problem of focal differentiation any more than one flat image, because focusing on a point is a relatively fine procedure for our eyes.  Each ray of light from each point in a 3D space must be magnified the correct amount to appear in focus.  If all of these rays are approximated on a 2D surface as being at one point, they will only combine correctly when they match the original focus of the camera.  This means that you can never properly focus on a 2D image unless you look at what the camera wants you to look at.  And even then, that distance may not appear correct to your eyes relative to the other 3D cues they are receiving (for exact correlation, the screen must be at the same distance from your eyes as the original object was from the camera).  In theory, software could measure your position, place of focus, and intended focal distance, then correct the image accordingly; but that would only work for extremely sophisticated dynamic interactive software and not for your average movie.

Focal discrepancies can become even more confusing when two images and eyes are involved.  This is the main reason behind some of the unpleasant effects of 3D TVs (e.g. 3D headaches).  The problem of unmatched focal distance in Old 3D is worsened by New 3D.  The second 3D perspective sends ‘happy signals’ to your brain telling it that “everything is more like it should be” (i.e. more like real life than before), which convinces your eyes that it’s okay to ‘search around the excitingly new 3D environment’.  The end result is that you look where you will become the most confused.  The problem suggests its own solution, which is that the best way to enjoy a modern 3D TV is to follow the camera’s focus and intentions and try not to look around like your easily excitable brain demands.

In the case of CG movies, the creators can make everything on screen be in focus.  That is why 3D CG movies can be much easier to watch.  Your eyes may still know that the focal distance isn’t quite right because they won’t have to adjust when looking around (despite apparent differences in depth), but at least they aren’t also looking at something unchangeably blurry.

A final perspective problem inherent to both Old 3D and New 3D is that the recorded images were taken from a specific position, and any flat screen that uniformly projects those images will seem skewed when viewed from anywhere other than original position (or ‘positions’ in the case of New 3D).  Aside from this unnatural perspective skewing (which our eyes are surprisingly good at ignoring for small angles), this also creates a greater problem with forced Occlusion.  Not only do new objects not become visible upon moving, but the image becomes distorted.  Not to mention that a quickly moving viewer will not see any parallax.  This ‘myriad’ of problems is often ignorable because most viewers are just sitting in one place.

Single parallax systems (e.g. Holografika’s Holovizio) improve the focal problem and remove an entire direction worth of forced Occlusion, directional skewing, and unchanging parallax; however, they ‘gain’ an asymmetric blur and do not resolve the any other problems along the perpendicular direction (if you are interested in how a single parallax system can be created, you may want to look at the “How to Make a Holodeck+++” article “Linear xV’s” on 5Deck.com).

The limitations of New 3D are one of reasons why I have been pushing interest in the concepts from “How to Make a Holodeck”.  Creating a full light field that matches a real life “wall” of light is the only way to correctly and completely recreate an environment.  Anything less has to make approximations of that light and will lead to one or more of the visual discontinuities described in this article.

New 3D accounts for many of the aspects of 3D perspective that Old 3D fails to address.  It has some drawbacks, but when viewed with an understanding of its machinations (most importantly, that you should try to match your focus to the camera), it can be an extremely enjoyable experience.

Note: The term “3D” used in this article means three “spatial dimensions.”  In “How to Make a Holodeck,” I use the term 3D with time as a possible dimension.  Because of this, I prefer the term 4D for most instances where people use 3D, but I still use 3D for ease of understanding.


Change is Silver

Author of “How to Make a Holodeck” (5Deck.com)
~A silly and informative book that describes how to create fully volumetric 3D and 4D displays.
Creator of Unili arT (UniliarT.com)
~Original designs placed on multiple stickers to be readable through rear view mirrors.

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