October 14, 2012 at 21:09About 3D movie technology
3D movie theaters have recently bred in large numbers. Not much lately behind them in prevalence and 3d-TVs. However, what exactly is behind the marketing “3D” in each case is not always clear and obvious.
It is worth noting that it would be more correct to call this “stereo” movie, but the term “stereo” has long been firmly and firmly (simply, apparently, by the right of primacy) fixed to the sound (in this regard, for example, the name of the magazine “Stereo and video ”). Therefore, marketers had to use the term “3D”, which is associated with a three-dimensional image in one sense or another. In this case, the brain perceives the volume due to the supply to each of the eyes of an image slightly different from the image for the other eye, similar to how the images obtained by the eyes in life differ.
Theory
So, to create a sense of volume, you need to convey to each eye a picture.
This can be done in the following ways:
- 1. Shutter technology
Each eye has its own frame and these frames are interleaved. In order to separate frames from one another, glasses are needed that will skip one frame and show another, simultaneously with the display of these frames. Such glasses always contain some kind of electronic stuffing, require batteries (which means they need to be replaced regularly), and, worst of all, flicker. This technology is already quite old, back in the days of CRT, NVidia released video cards that doubled the frame rate and had special glasses connected to the video card that closed (using LCS - Liquid Crystal Shutter) one of the eyes in synchronization with the image. On the LCD that replaced it, this was no longer real, because the refresh rate of the first LCDs was below the required 120 Hz. - 2. The second method is to combine the picture for both eyes simultaneously on one screen and share it using filters in glasses. In this case, the filters on the glasses are passive, do not contain electronics, but divide the luminous flux based on some physical properties of this flux. You can share in different ways:
- a) by color:
these are long-known blue-red (or some other colors with a disjoint spectrum) glasses. The easiest and most affordable way. The disadvantages of this method are that colors are lost, in addition, after sitting for a long time in such multi-colored glasses, for some time after removing them, the eyes see in different colors, because they managed to adapt and correct the "white balance" as they could. - b) spectrum:
This is a somewhat complicated first method: each eye is given all three colors, but in slightly different disjoint frequency ranges corresponding to each of the primary colors. - c) by polarization [5]. In this case, two sub-options can be considered:
- Linear polarization:
Linearly polarized light is an electromagnetic wave in which the oscillations of the field vector lie in the same plane. In this case, each lens of the glasses is a linear polarizing filter that transmits light with polarization in one plane and blocks light with polarization in a plane perpendicular to the first. In the intermediate planes, some part of the light is transmitted, depending on which of the main planes is closer to polarization. Accordingly, you can depict a picture where, for the left eye, there will be, for example, vertical polarization, and for the right eye, horizontal (or vice versa). Then glasses with the corresponding poles, instead of glasses, will filter the image for one eye from the image for the other. There is a nuance: if the glasses are rotated 90 degrees, then the skipped pictures will change places. And at 45 degrees there will be no separation at all: both equally darkened pictures (with double “three-dimensional” objects) will pass through the glass. Thus, linear polarized glasses are very sensitive to tilting the head. - Circular polarization:
For light with such polarization, the field strength vector runs in a circle. It is very convenient here that the eye has only two things, as well as the directions in which this vector can run (clockwise and counterclockwise). The filters for the respective glasses are circular poles. Rotate them no matter how they filter, they will be the same light. Of course, you will not look at 3D lying, but it is already quite possible to tilt your head by 30 degrees.
- Linear polarization:
- a) by color:
Practice
Now let's move on to practice, that is, to which of these technologies are now being used.
Cinema looks in cinemas, and cinemas are public and home. For them, the appropriateness of using various technologies for obvious reasons is different.
Public Cinema Technology
At the moment, two are more common: IMAX 3d [3] and RealD 3d [2]. Both use passive polarized glasses. Besides them, Xpan 3D [6] and Dolby 3D [4] technologies are also known, but it seems they are less common.
IMAX 3D
The aimax uses linear polarization in glasses, and the image is projected by two projectors onto one screen. The resulting image in my experience turns out to be very bright, saturated, the glasses almost do not darken the image, there is only one thing: sometimes the so-called crosstalk is visible, that is, the eye can see a translucent image that is intended for the other eye. For my taste, a very unpleasant effect.
RealD 3D
For RealD 3D, the polarization is circular, but the glasses are darker, and a movie is shown using one projector, which 144 frames per second shows frames for the left or right eye, and there is a synchronized filter in front of the projector’s lens, which gives appropriate polarization to light. In this sense, here is some kind of mix from the first and second types of technologies, the separation of pictures by time is transferred from glasses (which are passive and, accordingly, cheap, which is critical for public cinemas) to an additional filter in front of the projector. This filter, by the way, reduces brightness even more, so RealD-technology is very "dark". From my own experience I can still say that there are some problems with the flowers, in theory they should not be, but they are. Not only are they dim so also for some reason the number of perceived shades of color decreases. In addition, for some reason I still distinguish much less details in RealD glasses than without them.
Xpand 3D
This is the only representative of the technology of the first type - active glasses synchronized with the signal from the projector. I haven’t seen it in cinemas, but it’s possible that somewhere we use it, if anyone knows where, tell me, it’s interesting to try.
Dolby 3D
Representative of type 2b technology according to classification from the first part of the article. They say glasses for this technology are expensive, so they are made heavy enough to reduce the likelihood of theft. Again, I have not met, but I would like to try, even more than Xpan 3D.
Home theaters
Although, in principle, a home theater can also be based on a projector, this is relatively rare, so we will only talk about televisions. Moreover, about their most common type at the moment - about LCD TVs. A computer with a monitor can also act as a home theater, but almost all modern monitors are also LCDs and all the same technologies can be used there.
Basically, there are two technologies that are prominent representatives of the first and second types.
Shutter technology
Most manufacturers (for example, Samsung, Sony) equip TVs with a 3D shutter technology that requires active glasses. Due to the limitations of the LCD (well, liquid crystals do not know how to switch states quickly enough), each frame of the film shown has four frames shown: a frame for one eye, a dark frame, a frame for the second eye and another dark frame. A dark frame is necessary, because driving the LCD pixel into black is faster than driving it to another intermediate state. Accordingly, in fact, 25% of the 2d brightness of the TV reaches the eye. Plus, they filter the glasses. So the brightness of the picture is the disadvantage of this technology the number of times.
Deficiency number 2 I already mentioned: glasses flicker. And flicker with a frequency not the highest, for example, 60 Hz. Anyone sitting on old CRT monitors will understand and shudder. Moreover, if this flicker on the film itself is not very noticeable (we watched TVs at 50Hz), then the flicker of an external light source filtered by glasses already looks completely disgusting. Plus, an additional worsening factor may still be present, consisting in the fact that the flicker frequencies of the glasses may be close to the flicker frequencies of the source itself, but not coincide in phase.
Other disadvantages of active glasses: heavy, expensive, incompatible - each manufacturer has its own protocol for synchronizing with a TV.
In fairness, I must say that most likely this technology will develop and may have already developed. For example, you can raise the frequency and then the problem with flicker will not be so pronounced.
Polarization technology
Everything is completely different with TVs using passive polarization technology (such TVs are produced, for example, by LG).
The essence of the technology is as follows: each line of the TV has a filter different from the neighboring ones, due to which all even lines have circular polarization in one direction, and odd lines in the other. If you watch 3d on such a TV without glasses, you will see a "comb", that is, the mismatch of even and odd lines. Glasses simply filter the appropriate polarization for each eye. They are lightweight, cheap and without batteries. Do not flicker . In addition, they are interchangeable with RealD glasses (and similar glasses from other manufacturers), so you can remove the glasses from the movie and watch TV at home, or, better yet, take your glasses from the TV to the cinema.
These were all pluses. Theoretically, the disadvantages of the technology are as follows: 1080p is shown for each eye through 540 lines. True, the frame rate doubles and on the same line for one eye it shows an even, then an odd line of content. In addition, for technological reasons, the shadow mask between the lines on such a TV is slightly wider than usual (because you have to go somewhere from one filter to another).
In practice [1], the following is clarified: since the content in the vertical coordinate for adjacent lines is almost identical, after the formation of a three-dimensional picture in the brain, the absence of half of the lines is leveled and the perceived sharpness of the picture is only slightly lower than in the 2d version.
The shadow mask is larger than usual from a practical point of view by such a small amount that it is not worth mentioning it.
Other technology
Firstly, there is evidence that there are televisions that do not require glasses for viewing surround content. Apparently, a technology is used here, similar to that which allows you to create postcards with a sense of volume, that is, the image is divided vertically into strips, in front of which there is a prism that directs light from one strip into one eye, and from the next into the other. Obviously, in this case, the range of places from which the volume will be observed is rather limited. However, this is not a significant limitation for small screens and this technology is used in one of LG phones and in Nintendo’s handheld game console.
Secondly, you can make two small screens and hang them directly in front of your eyes, you will get a helmet (or glasses) of virtual reality. Together, this way you won’t watch a movie either.
Thirdly, I had the idea that it’s possible to adapt a technology similar to Dolby 3D for a TV, that is, to make 6 subpixels for a pixel with different, disjoint spectra. Most likely it will be expensive in terms of production of both TV and glasses, but suddenly someone has already done or will do it?