I was messing around with some image processing and tried out a fun (albeit trivial) use for PCA. For a given image, I took each pixel's RGB value as a 3 dimensional vector. Then projected each pixel onto the first PC and normalized for the appropriate value.
The results are here: http://imgur.com/a/a4kwY
I know this isn't anything crazy, I just thought it was kind of a fun use of a data processing algorithm. Also, if you think about it more deeply, this method of creating a black and white photo could possible preserve more of the original qualities of the color version than just taking the average of the color pixels.
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