data import from figure

Dear all,

actually i'm tring to import data from an image .png in order to create a matrix to store them. I've seen the usage of imread function but what i obtained is a matrix 420x560x3 uint 8. I can't understand how to use it because what i would need is to enter in that matrix with an index i and j (for x axes and y axes) and read the value stored in it. Any opininon on how to proceed?

(If it would need of help i also downloaded the image toolbox).

Thanks,

NIccolò.

Answer 1

[Walter Roberson]

?? imread() already creates a matrix to store the information. The matrix returned by imread() is that matrix.

If you want to look at a particular x, y location in the matrix, then the sequence would look like

img = imread('YourFileNameGoesHere.png');

x_of_interest = 17;
y_of_interest = 83;
pixel_value_at_x_y = img(y_of_interest, x_of_interest, :);

Note that y is the row index, and that x is the column index.

Note that if you wanted to examine a number of individual pixels, then you would need a different solution:

x_of_interest = [17 17 184];
y_of_interest = [83 84 5];
[rows, columns, panes] = size(img);
ind = sub2ind([rows, columns], y_of_interest, x_of_interest);
pixel_value_at_x_y(:,1) = img(ind + 0*rows*columns);
pixel_value_at_x_y(:,2) = img(ind + 1*rows*columns);
pixel_value_at_x_y(:,3) = img(ind + 2*rows*columns);

then the first row would be the R then G then B components for x = 17, y = 83, and the second row would be RGB for x = 17, y = 84, and the third row would be RGB for x = 184, y = 5.

Or you could simply loop:

x_of_interest = [17 17 184];
y_of_interest = [83 84 5];
for K = 1 : length(x_of_interest)
    pixel_value_at_x_y(K, :) = img(y_of_interest(K), x_of_interest(K), :);
end

The way I used with sub2ind() takes more understanding of how images are stored and how subscripts are calculated. Because looping is a lot clearer for most people, I would recommend using the loop unless you are extracting values at a lot of different locations.