Video Compression Why is it Necessary?Video compression, is an aspect of the digital age that is massively taken for granted, most would associate the term in correlation with file sizes. It is true that it is mostly associated with it but there are further uses to compression than would be expected.

There are two types of compression they are lossless and lossy compression. Lossless is a reversible type of compression, they can be compressed and uncompressed without any loss of data. The only drawback to this is even in its compressed format, it still retains a very large file size.

For example in Understanding Digital Cinema a Proffessional Handbook it states that A movie with an aspect ratio of 1.85:1 is 39962160 pixels, for a total of 8,631,360 pixels per frame. Each pixel is represented in the Digital Cinema Distribution Master (DCDM) by three 12-bit values; this works out to 310,728,960 bits, or nearly 40 megabytes, per frame. A 2-hour movie is 172,800 frames (at 24 fps) for a total of nearly 7 terabytes. This is a phenomenal amount of data; the largest commonly available portable medium is the 2-layer DVD that can store 9.4 gigabytes, so the 2-hour movie would need over 700 DVDs in its uncompressed form. Transmission of a single movie over a satellite transponder, even if we assume a totally error-free circuit, would take more than 2 weeks.This is a staggering amount of data, now thinking in terms of uploading your own lossless video codecs such as DNxHD codec developed by Avid, it would take a staggering amount of time, for your audiences to load a video, not to mention there are only a minimal amount of devices that support this format or could even play it. This is why lossless videos compressed and exported into a different format for wider playback availability. Lossy compression tend to discard or distort some of the information in the original image dataThe intent of such systems is to provide the maximum amount of compression of the image data consistent with an acceptably low level of distortion of the images A good example of a lossy compression is the H.264 codec which tends to value speed over quality of data. This is codec is mostly used for online videos in sites such as YouTube because it slightly compromises in video quality for faster and lower bandwidth streaming. It is also a widely supported codec, with a large quantity of devices capable of playback, thus allowing a wider range of users to view the video.

Lossy compression takes advantage of the Human Visual System (HSV), which are the eyes, optic nerves, and parts of the brain that interpret what we see. HVS does not completely capable of processing all the information we see, and thats what lossy compression uses to its advantage, it removes the information that could not be seen by HSV. A good example of this is early stop motions that has lower frame rates, technically early stop motions are a form of lossy compression, they are choppy compared to higher frame rates such as 24fps, but are HSV comprehend whatever is being and shown and stiches it together to form a comprehensible motion. Reference:

Swartz, Charles, 2004. Understanding Digital Cinema: A Professional Handbook. 1st ed. USA: Focal Press.