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Streaming Video: An Overview

Temporal Compression

Temporal compression, sometimes known as “inter-frame compression”,  sets aside just enough memory for one frame. When the video moves from the first frame to the second frame, all that happens is that the compressor looks at frame one and simply paints over the things that changed.

By recording the changes between frames, rather than the frames themselves, you don't need a lot of extra data to be processed. Unfortunately, when there is a big change- a pan or a zoom- the process starts all over.

Temporal compression

Image 3: Temporal compression is traditionally used when you see a "quality" slider in you compression selection.

Spatial Compression

Spatial compression or “intra-frame compression” is totally different. It looks at the frames in the video as being nothing more than a bunch of pixels with pretty colors. Sometimes referred to as "run length encoding", this method records consecutive runs of color and can achieve significant data stream reductions in areas where nothing changes. This system really falls apart when things change.

Spatial compression

Image 4: Spatial compression introduces a color dimension to your choices.

These two concepts are where your video headaches really start because they are at the core of the most common compression schemes used. The issues you will confront around working with video involve not only whether the compression is temporal or spatial, but whether the compression is “lossy” or “lossless”.

Lossless compression is good, and should be used if you are going to be subsequently compressing the movie for web playback. The file will reduce- usually between 10 and 50 per cent- but the files are still huge. If you are targeting a video for compression in the Flash Video Encoder then “Lossless” compression is the way to go. When you compress an already compressed movie - a video that uses Cinepac or Sorenson compression is compressed in the Flash Video Encoder as well- there will be a definite loss of image quality.

As you may have guessed, "lossy compression" loses information... forever. Even so, the file size reductions can be significant. In certain instances these reductions can be in the order of 100:1 as is the case with Sorenson Squeeze.

The amount of the compression is under your control but understand, lossy compression introduces artifacts to your image. As the file size starts decreasing, the severity of the artifacts increases. They usually first appear as "blotches" in areas of fine shading. At high compression, prepare to lose fine details and encounter a nasty sharpening effect called "ringing".

Ringing

Image 5: As a video gets over compressed artifacts, called "ringing", are introduced to the image.

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photo Tom Green
Tom Green

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Teacher, author, raconteur. Here's a run down of what I have been up to over the past few years. My Bio.

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