PRACTICAL DV FILMMAKING

Practical DV FilmMaking

• Overview
• Inside video
• Films and how they work
• Pre-production
• Production tools
• Production aesthetics
• Post-production tools
• Post-production aesthetics
• Careers and distribution
• Mapping the industry
• Glossary of film and video terms

Video compression

Now that there are so many different ways of watching video - phones, the web, DVD, CD - it was inevitable that at some point we would have to find a way of reducing these movies to a size that our mobile devices can cope with. Video data is huge and it is inconceivable that it could be sent to any new format just as it appears on your master tape. To reduce this data, we use a process called 'compression'.

Over the last decade there have been big advances in the way we squash movies. There are now very sophisticated methods which present to the viewer a version of the movie that they would find hard to detect next to an uncompressed version. Much of this work is down to a group called the Motion Picture Experts Group (MPEG).

In this section we will take a tour through the basics of compression, as it is used by filmmakers on a day-to-day level. Compression is one of those subjects that gets rather involved and more than a little technical, but we will focus instead on the ways it affects a movie and how you can lessen these effects, utilizing the new mobile and web formats to the full.

How it works

In essence, compression looks at individual frames and decides which parts of them we would not notice if they were discarded. For instance, a blue sky that is the same over five frames doesn't need a new bit of information for each of those frames, so the whole group is assigned a code.

At its most sophisticated, compression works by doing three jobs: first to analyse the video signal, then to sort out which parts of each frame the viewer wouldn't miss if they were discarded, and finally by encoding this process. The best way of doing this is called discrete cosine transform (DCT), which sam­ples a piece of video at regular intervals and figures out what an image can lose that the human eye would not detect. It is a complex algorithm that is behind the main methods of squashing down video that we use every day, including DVDs, Internet gaming, web broadcasts, CD-ROMs and cell phone movies.

The key to compression is that if you are going to squash something down, you have to be able to unsquash it again later. We could think of this as similar to sending a large poster to a friend via the postal system. The poster has to be rolled up so that the mail service can transport it efficiently. But at the other end, when it reaches your friend, it needs to be able to be unrolled again. We could