How can I read huge amount of image files and take the corresponding histogram data to a matrix in order?

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Hi all, Suppose I have a database of 100000 image files (the names of files are numbers 1.jpg to 100000.jpg) . I want to compute the histogram of individual grayscaled versions of the image and store it to a matrix in the order of file names numerically. ie Finally I will be having a matrix of size 256 * 100000, where the first column will be histogram of 1.jpg, 2nd column will be histogram of 2.jpg and so on.
As a first step, I tried the code below. But since the output was cell, I could not sort it the way I wanted. Also it took lot time.
file = dir('*.jpg');
n = length(file);
images = cell(n,1);
for k = 1 : n
images{k} = imread(fullfile( file(k).name));
end
Here file(2).name = 10.jpg , but what is required for me is 2.jpg. What is the efficient way to code this? Can I do this without for loop?

Accepted Answer

Image Analyst
Image Analyst on 2 Apr 2014
Code for creating those filenames is in the FAQ :<http://matlab.wikia.com/wiki/FAQ#How_can_I_process_a_sequence_of_files.3F>. Though be sure to add an "if exist(fullFileName, 'file')" before you attempt to use the file. Like Dishant said, no need to store all the images, and just append a row onto your accumulator array of histograms.
  5 Comments
Image Analyst
Image Analyst on 2 Apr 2014
Well, do you even have the parallel computing toolbox? I don't so I can't really help you. Most of my images just take 2-3 seconds to analyze, and I have usually less than a hundred or so, so speed is not really a big concern for me. If you have the toolbox, give it a shot. You could also try to perform some computations on the GPU.

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More Answers (2)

Anand
Anand on 2 Apr 2014
If you have the parallel computing toolbox, you could try something like this:
allHists = zeros(256,numel(file));
parpool;
parfor n = 1 : numel(file)
allHists(:,n) = imhist(imread(file(n).name));
end
Ideally you send this out to a cluster and not your local machine. If you have an older version of MATLAB, you might have to use matlabpool instead of parpool.
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Dishant Arora
Dishant Arora on 2 Apr 2014
file = dir('*.jpg');
fileNames = {file.name};
fileNames = sort_nat(fileNames); % sorts string efficiently
sort_nat is not a built-in function. You can get it from file exchange, here: sort_nat
And you need not to stack images one over another in cell, it will consume more memory. Just read an image compute histogram and append it to your result one by one in loop.
  1 Comment
DEVANAND
DEVANAND on 2 Apr 2014
Edited: DEVANAND on 2 Apr 2014
Thanks for the answer. sort_nat is a good function. And yes I am trying for appending histograms, but for loop takes a lot time for 100000 images. Any suggestions will be helpful.

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