Matlab storing values in array

Question

I would like to read an image pixel by pixel and store how many of each pixel value there are (grayscale 0-255):

img = imread('jetplaneCor.jpg');
imgGray = rgb2gray(img);
sizex = size(imgGray,1);
sizey = size(imgGray,2);
grayArray = [0:0:255]; %Not working

for i=0:1:sizex
   for j=0:1:sizey
       pixelValue = imgGray(i,j);
       grayArray(pixelValue)=grayArray(pixelValue)+1;
   end
end

How can i allocate an array with 256 places?

You should use initialize grayArray as zeros(1,256). But see my solution, which is much faster than using two loops — Luis Mendo
– Luis Mendo, Commented Sep 29, 2013 at 23:30

Luis Mendo · Accepted Answer · 2013-09-29 23:49:36Z

2

You can do that easily with hist. No need to use loops:

img = imread('jetplaneCor.jpg');
imgGray = rgb2gray(img);
grayArray = hist(imgGray(:),0:255);

edited Sep 29, 2013 at 23:49

answered Sep 29, 2013 at 23:28

Luis Mendo

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2 Comments

KevinCameron1337 Over a year ago

Hey, thanks. The point of the program is to write what imhist() does and compare it to our code.

Luis Mendo Over a year ago

I've just edited my answer, there was a (:) missing. As for your code, as I said in my first commnent, you only had to initialize grayArray as zeros(1,256).

Bucket · Accepted Answer · 2013-09-29 23:29:03Z

0

This will create a 1 x 256 array where every entry is a 0:

grayArray = zeroes(1, 256);

You can reference each element with:

grayArray(1, index);

answered Sep 29, 2013 at 23:29

Bucket

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2 Comments

KevinCameron1337 Over a year ago

How can i hist grayArray once its filled in. When i use hist(grayArray) i dont get the right data.

Bucket Over a year ago

Have you tried hist(transpose(grayArray))? Or you can reverse what I told you and use grayArray = zeroes(256, 1) and reference an element with grayArray(index, 1).

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