You watch videos on an electronic device, tablet, smartphone or PC every day, but have you ever wondered how they work?
If we can stream high-definition films on our computers today, it is ideal for the technologies that have been developed over the past 30 years and that have revolutionized our view of digital film archiving. Technologies that are part of video compression systems.
How does a video work?
In general, a video is nothing more than a series of photos that appear in quick succession as quickly as possible.
By displaying the images in succession on the order of a few tens per second, it is possible to achieve the motion effect typical of films.
In particular, the video we see every day generally shows 24 to 25 frames per second. The number of frames per second displayed in a video is called the frame rate or even FPS (frames per second), which you have probably heard in the field of video games.
Video compression
If you do a quick calculation, it is clear that displaying 25 frames per second and saving exactly as you would save a photo on your computer would reach dimensions that are not realistic in terms of space.
A short example: A medium resolution photo in JPEG requires 300 KB. If 25 photos per second have to be displayed, we need 7.5 MB per second. So a 1 hour movie would be 27 GB. Considering that a DVD can generally hold 4.7 GB of data, this would mean that the average movie should be spread over a dozen DVDs, with the effort required to replace the disc every ten minutes of the movie .
Fortunately, we have managed to find a smarter way to play these films, and today we are generally used to recording video in kbps. H. eighth KB per second.
An hour of 300 kbps video only consumes 135 MB, much less than the previous 27 GB, and a value that allows you to realistically transfer these videos over the Internet, creating systems like YouTube, Netflix, and the competition.
Reducing the size of these videos is called “compression”. We generally speak of lossy compression, which is a type of compression that results in a slight loss of quality, most of which is aimed at being invisible to the eye. Human.
The higher the compression, the greater the loss of quality, of course. And if poorly compressed video can be of very high quality if you increase the compression, you will easily get into the mistake of moving around the screen.
How does video compression work?
The compression process of a video is quite long and varied. A lot depends on the type of format used. MPEG2, H264, WEBM … There are dozens of ways to compress a video, and they all give more or less different results depending on the video type.
In principle, however, all of these video compression formats have some strengths in common, which we will describe below.
First we said that the video consists of a series of pictures and that we have to show all of these pictures one after the other to show the video.
The problem is that storing all of these images takes up a lot of space. To save space, video formats generally don’t save all of the images. However, you save a single frame for every N images. Usually a complete picture is saved every 12 pictures. Of the remaining 11, only the parts of the image that have changed from the previous image are saved.
This way, instead of having to save 12 whole images, we can save the first in its entirety and then only the piece that changes between the first and the second, then only the piece that changes between the second and the third and so on.
The images that are completely saved are called I-FRAME and those whose only difference from the previous image is P-FRAME.
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