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

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DEVANAND 2014 年 4 月 2 日
コメント済み: Anand 2014 年 4 月 3 日
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
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?

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### 採用された回答

Image Analyst 2014 年 4 月 2 日
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.
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DEVANAND 2014 年 4 月 3 日
Yes I do have Parallel computing toolbox, but I am on a isolated machine.

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### その他の回答 (2 件)

Anand 2014 年 4 月 2 日
If you have the parallel computing toolbox, you could try something like this:
allHists = zeros(256,numel(file));
parpool;
parfor n = 1 : numel(file)
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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Anand 2014 年 4 月 3 日

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Dishant Arora 2014 年 4 月 2 日
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.
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DEVANAND 2014 年 4 月 2 日
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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