how to change compression level in PCA?

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azizullah khan
azizullah khan 2014-2-21
评论： azizullah khan 2014-2-24
how to change compression in pca and how to find the compression ratio in pca...

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Walter Roberson
Walter Roberson 2014-2-21
pca is not in itself a compression scheme. pca is sometimes used for compression by finding the vectors that are most responsible for variation in the image and discarding some of the vectors that are least responsible (which are closer to 0.) To change the compression level, discard more or fewer components.
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Walter Roberson
Walter Roberson 2014-2-22
"intensity reduction" might be called "dynamic range compression" in some cases.
In general, "compression" is "to make smaller" and that is the sense used for dynamic range compression (amongst other uses.)
When compression of data is being discussed, the topic is compressing (making smaller) the memory required to reproduce the data afterwards ("decompression"). There are two important sub-varieties of data compression: lossless (in which decompression must be able to exactly recreate the data), and lossy (in which decompression just has to recreate the data "close enough to be usable" for whatever purpose the compression was devised.
In the more general case, "resizing" an image does not necessarily involve reducing the amount of data the image requires, and might only refer to the amount of display space used for the image.
In the case of a data array such as might be used to store an image, "resizing" it does change the amount of storage used. Resizing a data array to be smaller is one form of lossy compression.
Using PCA for image compression is a form of lossy compression: you keep the information that has the most effect on how the image looks, and you let some of the fine details be erased by not storing the information needed to recreate them.

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