Key points
- A video is simply a succession of still images/sequence of still images.
- Frame rate: the number of frames displayed per second, usually expressed in FPS.
- The resolution of a video is the number of lines per frame and the number of pixels per line, as the resolution of an image.
- The eye will notice the flicker unless the screen is refreshed at least 50 times per second
- The eye cannot see that any motion on the screen is not actually continuous provided that the refresh rate is 25 times per second
Two methods to display video frames
Interlaced encoding
It is used in television broadcasting and adapted for video recording.
- The image of each frame is spitted into halves.
- One contains the odd numbered lines and the other contains the even numbered lines
- The first half is display completely and then the second half
Advantage: low transmission bandwidth
Progressive encoding
A full image is displayed each time. As the improved bandwidth become more generally available, this method is becoming more norm.
Compression
Spatial redundancy
The image compression techniques might be appliable to a video since a video is a sequence of images. However, this is unlikely to be efficient enough. Because a frame of a video is usually very similar to its preceding one.
Temporal redundancy
Temporal redundancy means that there is a large amount of similarity between video frames. So, it is not always necessary to encode each full frame. Because only a small percentage of pixels are different between the previous one and the next one, it possible that the next frame can be get from the previous one and the differences. This method provides substantial compression. Sometimes it is called interframe encoding. However, it depends entirely on the content of the video. For example: a room full of people dancing has less temporal redundancy then a room with just 1 person sitting and talking.