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Preprocessing: How Real Movie Magic is Made

May 5, 2008

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Real World Video Compression

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Noise Reduction

Noisy video is a big headache for video compressionists. Random noise causes pixels to change rapidly and constantly, making it difficult for the codec to encode the clip correctly.

Noise reduction encompasses a variety of techniques to remove that noise from an image, making it easier to compress. Very simple noise reduction algorithms are just blurs that hide grain but make the image softer as well, such as when a median filter is applied (Figure 4.5). More advanced algorithms try to blur only the parts of the image that have grain and may take advantage of differences among frames.

Figure 4.5 Here are two versions of a DV-NTSC clip. On the right it has had a median filter applied. Note the increased softness in the image on the left but the smoother quality of the image also.

Noise Reduction Before Compression

If the damage from video noise is significant enough, such as dust or scratches in film, no amount of blurring will help remove them completely from the image during compression. In this case, you may want to use a stand-alone application, such as Adobe After Effects or Photoshop CS3, to digitally remove the scratches either manually (with a clone tool) or with an automatic scratch removal filter. Once you've treated the source footage to remove the damage, you can then compress the footage as you normally would.

Noise reduction is not always required. If you have a clean video source to work with, simply skip the noise reduction step when preprocessing your work. If you do have damaged footage, it may take several experiments to find the right balance of cleaning up the source without degrading the finished video image.

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