Displaying Stereo Images
If we have a stereo pair, an image that corresponds to the left eye and an image that corresponds to the right eye, if we can come up with a mechanism for directing those two slightly different images to the respective eyes, then we have a very, very vivid sense of the three-dimensionality of the scene and this is the underlying principle of 3D movies, 3D television and so on.
It's something human beings have been fascinated with for the longest time. I really love this movie poster in this scene from a cinema back in the 1950's. It shows us that there’s this long term human fascination with 3D movies. It's not a recent phenomenon. The technology keeps changing, but the underlying principle is exactly the same. The fundamental problem we need to address is how do we get different image into our left eye and our right eye? Let’s have a look at some of the technologies that have been developed over the years.
In order to reproduce the really vivid 3D perception that we get in everyday life, we need to use some rather elaborate mechanisms to recreate that effect artificially.
So the first one is back from probably early last century and what this device does is takes an image which corresponds to an image from the left eye and another image taken from a position that corresponds to the right eye.
Then this lens mechanism here sends the left image into my left eye and the right image into my right eye. So if I hold this up to my face, I see a really vivid three-dimensional representation of the scene that was captured more than 100 years ago.
This is the first kind of stereoscope and these things were really, really popular in the early 1900s. So a slightly more modern version of that technology is something from the 1960s and this one has a number of stereo image pairs on a little piece of cardboard and a little clicker so I can move from one picture to the other much more conveniently. If I hold this up to my head, again, I get a very vivid 3D perception of the scene that I’m looking at.
Another approach to providing different images into the left and right hand eyes, what’s called the Anaglyph Technique and you look at an anaglyph image through red and blue glasses. What happens is that the left image is printed in red and the right-handed image is printed in blue and by putting on these filtered glasses, the left image comes through the red filter into my left eye. It does not pass though the blue filter. So my right eye doesn't see that. For the right-hand image, the one that’s printed in blue, comes through the blue filter into my right eye but not into my left eye. So if I put this on and look at a printed anaglyph image or an anaglyph image on a webpage, for instance, NASA has lots of anaglyph images of Mars, we get a very, very vivid 3D sensation.
This is just one image from NASA’s collection of 3D anaglyph images. The most recent Mars Rovers all carry quite sophisticated stereovision systems and the images are transmitted back to Earth, processed by NASA and turned into anaglyph form and made available through their website.
What we don’t get though is a very full representation of the color because we’re looking at it through red and blue glasses. So it looks a bit monochromatic but the 3D perception is really quite vivid. If you’ve got access to a pair of anaglyph glasses, these are glasses with the red and blue lenses, you might be interested to grab them, put them on and have a look at these next few images. They contain some very vivid three-dimensional content.
The toolbox function that draws 3D coordinate frames has an anaglyph mode. If we call the trplot function which we've seen before, it will pass in a rotation of 90 degrees around the X axis so that’s the coordinate frame that we want to display. We pass the 3D plot option and if I view this through my anaglyph glasses, I get a very realistic 3D impression. I actually see that Z axis coming out of the screen somewhat.
Another technique and one you see in 3D movies, you can see Gravity or Avatar, Hobbit, something like that, and you put on glasses like this, what happens in the theater is the left-hand image is projected with light but it’s polarized perhaps in the vertical direction and that light goes through this filter which lets through polarized light into my right eye but not into my left eye because this filter is polarized in the horizontal direction. The right-hand image is projected with a vertical polarization comes through this filter but not into this filter. So when I put this on my head in the cinema, I see a really wonderful 3D colorful representation of the scene that was filmed.
For home use, with three-dimensional TV’s, the technology is a little bit different. What we use are these liquid crystal shutter glasses. What happens now is that the television presents the left and right images sequentially about 100 times per second. We have left image, right image, left image, right image and the lenses in these glasses are synchronized with the TV so when the left image is projected, this lens becomes clear and the right, this one, is opaque. So I only see the left image in my left eye. Then 1/100th of a second later, the right image is projected on the TV; this lens goes black and this one goes clear. So with these on my head, I get a very vivid, again colorful, and three-dimensional representation of the scene through the TV.
The final approach is to do away with trying to use colored glasses, filtered glasses, and shuttered glasses on my head and to project the left and right image directly into my eyes.
So in some ways it’s a high-tech analog of the old stereoscopic viewers like this one and this one. So here is the state-of-the-art head-mounted display. If I put this on my head, the left image is projected through this lens directly into my left eye and the right-hand image is projected through this lens directly into my right eye.
All of the techniques that we’ve looked at require us to wear something on our head and that’s not always very convenient. Maybe we don’t have the particular glasses to hand or maybe we find that it gets in the way of our everyday life.
So there’s been quite a bit of work in trying to create the stereo effect without needing to have anything on your head, so one fairly recent innovation is the technology shown diagrammatically here. It requires that your head is approximately the right distance away from the screen and there is a system of lenses which refract the light from the left-hand image in the general direction of your left eye and the right-hand image is refracted in the general direction of your right eye.
Humans have long been fascinated with seeing images and movies in ‘3D’. Let’s look at how human stereo vision works and some of the technologies used to present images to our eyes in ‘3D’.
This content assumes only general knowledge.