PCA

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(Reducing the Data Dimension)
 
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Now, <math>\textstyle \tilde{x} \in \Re^k</math> is a lower-dimensional, "compressed" representation
Now, <math>\textstyle \tilde{x} \in \Re^k</math> is a lower-dimensional, "compressed" representation
of the original <math>\textstyle x \in \Re^n</math>.  Given <math>\textstyle \tilde{x}</math>, how can we recover an approximation <math>\textstyle \hat{x}</math> to  
of the original <math>\textstyle x \in \Re^n</math>.  Given <math>\textstyle \tilde{x}</math>, how can we recover an approximation <math>\textstyle \hat{x}</math> to  
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the original value of <math>\textstyle x</math>?  From the [[#Rotating the Data|previous section]], we know that <math>\textstyle x = U x_{\rm rot}</math>.  Further,  
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the original value of <math>\textstyle x</math>?  From an [[#Rotating the Data|earlier section]], we know that <math>\textstyle x = U x_{\rm rot}</math>.  Further,  
we can think of <math>\textstyle \tilde{x}</math> as an approximation to <math>\textstyle x_{\rm rot}</math>, where we have
we can think of <math>\textstyle \tilde{x}</math> as an approximation to <math>\textstyle x_{\rm rot}</math>, where we have
set the last <math>\textstyle n-k</math> components to zeros.  Thus, given <math>\textstyle \tilde{x} \in \Re^k</math>, we can  
set the last <math>\textstyle n-k</math> components to zeros.  Thus, given <math>\textstyle \tilde{x} \in \Re^k</math>, we can  
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approximation to the data.  
approximation to the data.  
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To decide how to set <math>\textstyle k</math>, we will usually look at the '''percentage of variance
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To decide how to set <math>\textstyle k</math>, we will usually look at the '''percentage of variance retained'''  
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retained''' for different values of <math>\textstyle k</math>.  Concretely, if <math>\textstyle k=n</math>, then we have
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for different values of <math>\textstyle k</math>.  Concretely, if <math>\textstyle k=n</math>, then we have
an exact approximation to the data, and we say that 100% of the variance is
an exact approximation to the data, and we say that 100% of the variance is
retained.  I.e., all of the variation of the original data is retained.   
retained.  I.e., all of the variation of the original data is retained.   
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Note: Usually we use images of outdoor scenes with grass, trees, etc., and cut out small (say 16x16) image patches randomly from these to train the algorithm.  But in practice most feature learning algorithms are extremely robust to the exact type of image  it is trained on, so most images taken with a normal camera, so long as they aren't excessively blurry or have strange artifacts, should work.   
Note: Usually we use images of outdoor scenes with grass, trees, etc., and cut out small (say 16x16) image patches randomly from these to train the algorithm.  But in practice most feature learning algorithms are extremely robust to the exact type of image  it is trained on, so most images taken with a normal camera, so long as they aren't excessively blurry or have strange artifacts, should work.   
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In this case, it makes little sense to estimate a separate mean and
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When training on natural images, it makes little sense to estimate a separate mean and
variance for each pixel, because the statistics in one part
variance for each pixel, because the statistics in one part
of the image should (theoretically) be the same as any other.   
of the image should (theoretically) be the same as any other.   
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This property of images is called '''stationarity'''.
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This property of images is called '''stationarity.'''  
In detail, in order for PCA to work well, informally we require that (i) The
In detail, in order for PCA to work well, informally we require that (i) The
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this is not the same thing as estimating a mean value separately for each pixel <math>\textstyle x_j</math>.
this is not the same thing as estimating a mean value separately for each pixel <math>\textstyle x_j</math>.
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If you are training your algorithm on images other than natural images (for example, images of handwritten characters, or images of single isolated objects centered against a white background), other types of normalization might be worth considering, and the best choice may be application dependent. But when training on natural images, using the per-image mean normalization as the normalization equations above would be a reasonable default.
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If you are training your algorithm on images other than natural images (for example, images of handwritten characters, or images of single isolated objects centered against a white background), other types of normalization might be worth considering, and the best choice may be application dependent. But when training on natural images, using the per-image mean normalization method as given in the equations above would be a reasonable default.
== References ==
== References ==
http://cs229.stanford.edu
http://cs229.stanford.edu
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{{PCA}}
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{{Languages|主成分分析|中文}}

Latest revision as of 13:18, 7 April 2013

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