Kernel Fisher discriminant analysis
In statistics, kernel Fisher discrimant analysis (KFD), also known as generalized discriminant analysis and kernel discriminant analysis, is a kernelized version of linear discriminant analysis. It is named after Ronald Fisher. Using the kernel trick, LDA is implicitly performed in a new feature space, which allows non-linear mappings to be learned.
Linear discriminant analysis
Intuitively, the idea of LDA is to find a projection where class separation is maximized. Given two sets of labeled data, and , define the class means and to be
where is the number of examples of class . The goal of linear discriminant analysis is to give a large separation of the class means while also keeping the in-class variance small. This is formulated as maximizing
where is the between-class covariance matrix and is the total within-class covariance matrix:
Differentiating with respect to , setting equal to zero, and rearranging gives
Since we only care about the direction of and has the same direction as , can be replaced by and we can drop the scalars and to give
Kernel trick with LDA
To extend LDA to non-linear mappings, the data can be mapped to a new feature space, , via some function . In this new feature space, the function that needs to be maximized is
Further, note that . Explicitly computing the mappings and then performing LDA can be computationally expensive, and in many cases intractable. For example, may be infinitely dimensional. Thus, rather than explicitly mapping the data to , the data can be implicitly embedded by rewriting the algorithm in terms of dot products and using the kernel trick in which the dot product in the new feature space is replaced by a kernel function, .
LDA can be reformulated in terms of dot products by first noting that will have an expansion of the form
Then note that
The numerator of can then be written as:
where . Similarly, the denominator can be written as
with the component of defined as , is the identity matrix, and the matrix with all entries equal to . This identity can be derived by starting out with the expression for and using the expansion of and the definitions of and
With these equations for the numerator and denominator of , the equation for can be rewritten as
Then, differentiating and setting equal to zero gives
Since only the direction of , and hence the direction of , matters, the above can be solved for as
Note that in practice, is usually singular and so a multiple of the identity is added to it 
Given the solution for , the projection of a new data point is given by
The extension to cases where there are more than two classes is relatively straightforward. Let be the number of classes. Then multi-class KFD involves projecting the data into a -dimensional space using discriminant functions
This can be written in matrix notation
where the are the columns of . Further, the between-class covariance matrix is now
where is the mean of all the data in the new feature space. The within-class covariance matrix is
The solution is now obtained by maximizing
The kernel trick can again be used and the goal of multi-class KFD becomes
The are defined as in the above section and is defined as
where the component of is given by .
Classification using KFD
In both two-class and multi-class KFD, the class label of a new input can be assigned as
where is the projected mean for class and is a distance function.
Kernel discriminant analysis has been used in a variety of applications. These include:
- Face recognition and detection
- Hand-written digit recognition
- Palmprint recognition
- Classification of malignant and benign cluster microcalcifications
- Seed classification
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- Kernel Discriminant Analysis - This site gives a high level explanation of KFD.
- Kernel Discriminant Analysis in C# - C# code to perform KFD.
- Matlab Toolbox for Dimensionality Reduction - Includes a method for performing KFD.
- Handwriting Recognition using Kernel Discriminant Analysis - C# code that demonstrates handwritten digit recognition using KFD.