How to do data dimensionailty reduction? - matlab

Question

I have a set of 25 images of label 'Infected' and 25 images of label 'Normal'.
I am trying to extract the dual-tree complex wavelet transform based coefficients as features for each of the images.
My code to obtain coefficients using DT-CWT ia as follows:
I = imread('infected_img1.jpg'); %read image
I = rgb2gray(I); %rgb ro gray-scale
L = 6; %no. of levels for wavelet decomposition
I = reshape(I',1,size(I,1)*size(I,2)); %change into a vector
I = [I,zeros(1,2^L - rem(length(I),2^L))]; %pad zeros to make dim(I) a multiple of 2^L
I = double(I);
dt = dddtree('cplxdt',I,L,'dtf3'); %perform DT-CWT
dt_Coeffs = (dt.cfs{L}(:,:,1) + 1i*dt.cfs{L}(:,:,2)); %extract coefficents at Level 6
Now, since I have 24 more images to extract coefficients from, I do this block for each of the images. My ultimate aim is to append all coefficient vectors generated in each iteration to form a matrix. But each image tends to give a different sized coefficient vector.
I want to know about some dimension reduction method that can reduce each vector to a uniform size and at the same time preserve its information.
Can anyone suggest methods with a good amount of clarity?

Answer 1

As I mentioned in my comment,
You can't shrink something (i.e. remove information) and still preserve all of the information.
Instead you can pad all of the vectors to the length of the largest vector and then concatenate them to create a matrix. You could program your own method but in the spirit of not reinventing the wheel I've previously used padcat(). It is very easy to use and pads with NaN but you can easily change this to 0.
Here's an example usage which also contains a handy conversion from NaN to 0.
>> a = [1 2 3 4];
>> b = [1 2 3];
>> c = padcat(a, b);
c =
1 2 3 4
1 2 3 NaN
>> c(isnan(c)) = 0
c =
1 2 3 4
1 2 3 0