Understanding Compression


If you wonder why compression matters, it's easy to explain. Let's say your video is short and clocks in at a mere 60 seconds or so. In its native DV format, that one-minute video is a huge 220MB file that takes 30 minutes or more to download. But compress the video into a QuickTime or Windows Media movie, and suddenly your monstrous masterpiece becomes a skinny 4MB gem that downloads in less than a minute.

The compression software you use to transform a video file lets you fiddle with a variety of factors, including frame size, frame rate, image quality, and audio quality so you can maintain optimal quality overall. For vlogs, you'll reduce the original DV video frame size from 720 by 480 pixels to 320 by 240 pixels, a whopping 78 percent reduction in the number of pixels that make up the image. Other compression techniques are more complex and rely on sophisticated compression/ decompression algorithms, called codecs, to perform the necessary audio-video wizardry.

Codecs come in two basic flavors. "Lossless" codecs squeeze files into a smaller space without removing any data. It's a little like stuffing three suits into a small suitcase and then sitting on top of it so you can zip the bag shut. "Lossy" codecs, like the ones you'll use for vlogging, actually reduce the amount of information in a file permanently. That's kind of like fitting your suits into a small bag by snipping off bits of fabric. It may sound drastic, but imagine a skilled tailor trimming fabric from the inside seams; few people would see the difference. Lossy codecs do something similar by removing video and audio information that our eyes and ears don't usually notice and won't miss.

For example, it's hard for the human eye to distinguish between very small differences in color, so video frames can be compressed by averaging out the color in some areas. That means making areas that are similar in color, the same color. Another compression trick saves space by repeating information that's identical from one frame to the next, rather than storing duplicate information for each complete frame.

Compression Tradeoffs

Compression techniques are useful, but they're not foolproof. Some video footage is difficult to compress without a noticeable loss in quality. Flip back to Figures 4.18 and 4.19 at the end of Chapter 4 to see one example. There are so many areas of small detail that it's not possible to color-average over large areas. Much of the action changes from frame to frame as well, so a codec can't economize by simply repeating redundant information from one frame to the next.

With challenging footage like this, a compression program can usually produce a gorgeous fileif you let it use as much data as it wants. But then you have a file that is much smaller than the original but still too large to post on the Web. To avoid that problem, you can limit the amount of data that a program uses to compress any one frame. But if your limit is too strict and the program doesn't have enough data to do its job, compression artifacts will show up. Viewers will notice blocky chunks of pixels instead of smooth, seamless video. So compression, like so many other aspects of vlogging involves juggling various factors so you can balance the file size you need with the video quality you want.

In Chapter 3 we explained how the various video formats are like containers that hold video and audio information. Any of these formats.mov, .mpg, .wmv, and .avican be compressed with a number of different codecs, each with its own advantages and disadvantages. For now, we recommend that vloggers use the MPEG-4 codec for compression. We find it does the best job of compressing files efficiently while maintaining video quality.

There are several different versions of MPEG-4 codecs out there, including plain-vanilla MPEG-4, H.264, 3ivx, and the version Microsoft created for Windows Media files. The versions you'll use for vlogging don't produce the ultimate in either video quality or file size, but they are compatible with older computers and Web browser plug-ins, and best of all, exist within the software already on your computer. So keep reading!




Secrets of Videoblogging
Secrets of Videoblogging
ISBN: 0321429176
EAN: 2147483647
Year: 2006
Pages: 81

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