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Thursday, September 15, 2011

Temporal Redundancy in Video Compression. For Laymen.

INTRODUCTION
Video Compression is something I have been involved with since a year and a half now and in this first official post, I am discussing analogies for fundamentals of compressing a video sequence.

I want to share two basic strategies behind compressing any video sequence so that people can appreciate and acknowledge how an entire movie sequence can be stored onto a hand held device or how can it be wirelessly  transmitted to far off receivers or how live videos can be streamed anywhere and anytime on demand . 

Video compression is an area of study important but is also critical given the advent of streaming videos, storage and transmission of HD quality video sequences and the overly populated frequency spectrum and the ever increasing demand for news, blogs and entertainment, all in the form of multimedia..!!

There are two important factors governing the compression of a video sequence namely,
  1. Temporal Redundancy and
  2. Spatial Redundancy.
The Concept
Temporal Redundancy is the redundancy of information which exists between a set of frames i.e. it is an inter frame redundancy. Given the nature of any video sequence, the numerical values of the luminance/brightness and chrominance/color of all the pixels in a given frame is either exactly similar or at least near similar to those values in the previous frames. This redundancy or in simpler words the 'repetition of information between frames' is exploited by all the video compression algorithms. 
The basic idea is not to encode the similar or the near similar pixel values which have already been encoded and transmitted.


Example
  • In video telephony there are fine movements of the face and hands yet the background information from frame to frame remains the same throughout the sequence.

  • In a live steaming of a tennis match, the motion is represented by the players and the ball while the remaining information that of the stadium and the audience remains similar all through the video.

The Analogy
People travel everyday to office, school or research labs and the weight/size of the bag we carry each day remains important irrespective of our mode of transport. Hence, we make all efforts to minimize/compress the size of the same and remain intelligent in selection of the items which go into the bag. We should be clever enough to reduce the redundancy in the items which exists between days by not carrying rather not repeating the stuff which we would need the immediate day. 
Hence a bit of knowledge about what we carried the previous day, what we have got today and the prediction for the next day combined with some intelligent and wise choices saves us a lot of weight..!!


Example
  • There are items like laptop and phone chargers, water bottles and clothes for gym which need not to be carried back everyday from office and
  • In case of students going to schools, we need not repeat carrying all our books, pens and other stationary back to home (by using a locker at school), reduce the redundancy and hence the keep the size of the bag smaller.
All of us reduce redundancy of items between days to wisely carry our bags everyday and this is similar to how a video gets compressed by exploiting redundancy between frames..!!. 


"Do not carry already encoded information in case of video compression and do not carry items back and forth if you need them the immediate day and you end up compressing  your bag..!!!."


So, this is how analogies work for me, they might not be entirely convincing or not every time do they provide insight into a technical concept. But yes, if they are related to real world examples from our daily lives, things/concepts seem simpler, creates trigger/interest for further reading and understanding, serves as a quick tool for revision too and sometimes just helps me appreciate a concept, I might never get an opportunity to learn or read about in detail.


I would be grateful for all the honest feedback and suggestions from all my well wishers so that I have a fair idea about the acceptance of this IDEA and the scope for its improvement and of course I ll come back with more analogies..Spatial Redundancy is next..:)    


Keep Simplifying,
Sunny



PS - Belated "Engineers Day" to all the engineers around the world and it is celebrated in India on Sep 15 on the august occasion of the birthday of Sir M Visvesvaraya, the greatest engineer India has seen and a greater human being..!!  
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