The goal for today is for you to learn some of the science of human vision and the history of film and video so that you understand the context for some of the various technical terms and ideas you will need to be able to identify and use video in your web pages.
Without context these are hard to grasp. They’re listed here and then we will cover them again in context.
Persistence of Vision
Frames Per Second (FPS)
Cathode Ray Tube (CRT)
Our eyes alone are not responsible for vision. They send the signals of what light is out there but it is up to our brains to interpret those signals. Film and video are basically based on understanding this relationship and using it to trick our brains into thinking a bunch of still images are actually moving.
We are going to visit this link. But to prepare, when you go to it there is a box with two images. Focus your eyes in the middle between the images and not on each image. Watch for a while and remember what the faces look like. Then cover one side with your hand and watch again. What is the difference?
This illustrates the Phi Phenomenon where our eyes, especially in our peripheral vision (the edges of vision not what is centered in focus), try to blend separate images into one to smooth movement.
This has to do with our brains thinking that when what looks like the same thing appears in different locations along a path we see it as movement along a path.
Persistence of Vision
Go to this site and pick a pattern that is big, like the red and white zig zag.
- Look at it for 10-15 seconds.
- Then close your eyes. ‘
- What do you “see?”
This is the phenomenon that our eyes tend to hold an image for a bit even after it is gone. Sometimes called after image because you can see part of the image after it is gone. This is often used to explain the entire phenomenon of seeing movement in film and video but it is more about why we don’t see a flicker in the transition between images.
To make that transition smooth we need to refresh the image before the persistence of vision wears off. Open this document
Technical History of Film and Video
Long before we had film we knew that if you displayed a series of images fast enough you would get the illusion of movement. You can create this by hand when you draw something like a flip-book animation. A mechanical device that did this before film was the Zoetrope.
Not long after we figured out how to create a continuous roll of film that could be used to expose a number of images, we created motion picture cameras.
When images are being recorded to film each image is know as a frame. The number of images/frames recorded in a second is known as the frame rate. In the full process we have a capture rate, the rate at which the images are recorded and the display rate, the rate at which the images are shown back to people.
Relationship between Capture Rate and Display Rate
If the two rates are the same then you see normal motion, basically like it was when the images were rcorded. If the capture rate is much faster than the display rate then you will see slow motion. If the capture rate is slower than the display rate then you will see speeded up motion.
When film cameras were first made both the cameras and projectors were operated by a hand crank. It’s really hard to keep those rates consistent and so a lot of early film was jerky looking compared to what we see today.
The new iPhone is an example of this where they allowed you to do slow motion by increasing the capture rate. When a camera can do this it is referred to as overcranking in a nod to the early days of hand cranked film: http://news.cnet.com/8301-1023_3-57605103-93/15-more-must-see-iphone-5s-slow-motion-videos/.
Here is an example of super slow motion at around 10,00 frames per second: http://www.youtube.com/watch?v=EUuIoBZJWMg.
To make that transition between frames smooth we need to change the image fast enough to kick the phi phenomenon and beta movements into gear and also do it before the persistence of vision wears off.
Watch this video to see how continuous images help us to “see” movement between images:
It turns out that the speed needed for persistance of vision is also adequate for the phi and beta movements.
Open this document and we will find out more about how we got the frame rates used today.
Find the two numbers as outlined in the document.
Technical limitations of the display device
In order for capture rate and display rate to match, the device you’re using to display the film or video must be capable of displaying at the capture rate. In film this was not a problem but in early television it was a big problem. Part of that was the technology in the TV and part of it was in the way the images were broadcast.
Old Television Tubes
Orginal televisions make an image by essentially shooting electrons at a screen of phosphor coated material. These phosphors glow when hit by electrons. Orginal TVs were black and white because they were just these phosphors glowing at different amounts of lightness. When you put red, green and blue filters over them the glow with those lights. Here is an image:
Cathode Ray Tube
That system of TV is known as cathode ray tube or CRT.
Some images: http://www.flickr.com/photos/audin/2324980526/, http://www.flickr.com/photos/renaissancechambara/442691040/
Persistence of Display
Television had to take into account the fact that the phosphors only stay lit about 15-20ms. This time is know as persistence of display. If they showed images at 24 frames per second on a television the image would only change every 42ms. After the first 20ms the phosphors would die out which meant that there would be a 22ms time where there was no image.
They could have solved the problem by simply increasing the frame rate to around 60 frames per second. However this would have required more than twice as many images as was captured on film.That would have meant more data and a higher amound of data. Early broadcast systems could not handle that data.
To fix this problem they came up with the idea of interlacing. This means that they break each image frame into two and only showed half of the image in each interlaced frame.
When computers came about, they originally had basically a TV display. As technology improved both TV and computer displays got better. One way was that they were able to use progressive scan where images were not interlaced. Because what you see on your computer is not generally from a broadcast TV signal then the data rate was not a problem and we began seeing more video that wasn’t interlaced.
LCD and LED
Modern displays use LCD or LED technology that allows lights to be continually on and can adapt to a number of different display rates.
Because there are so many interdependencies (capture rate, display rate, display resolution), the television and film industries have created a number of standards for how film and video are captured and displayed. This helps to make sure that what you record on your device can b seen by other people on their screens.
In television the resolution of a display is normally given in the number of horizontal scan lines in the screen. Original TVs were had a lot fewer lines (lower resolution) then moved up to about 520 lines and eventually what we now call standard TV is usually translated to 640 pixels on a computer (generally computer screens are been capable of higher resolution than T V screens).
Then HD tv came along with 720 lines of resolution and 1080 lines of resolution. With this change also came a change in the aspect ratio. Most TVs today are capable of 1080 HD resolution.
This refers to the ratio of the width to the height of the screen used to watch the film or video. Early film didn’t have many standards but generally shot at a 4:3 ratio. Starting around 1929 standards were developed for the film industry and we eventually got wider aspect ratios. 4:3 was also used for TV when it started.
Withe the move to HD the aspect ratio also changed to 16:9
The previous standard
Here is are some links on common resolutions and aspect ratios: