How to use an array as index in a 3D matrix?

Question

Hi I have loaded an image, thus creating a 3D matrix.

img1 = imread('bluebird.jpg')

I know that for a vector, i can create another vector based on a logic test and use this other vector as index like this:

speeds = [20 77 55 90 87 65 67 72 55]
invalid = speed > 70
speeds(invalid) = 0

This will set all invalid speed in speeds to 0.

But I have not figured out how to do it with a 3D matrix (an image).

What I want to do is swap color components 1 (red) and 3 (blue) for every pixel where the blue component is at least 20% higher than the average of the three components (the gray scale).

I have tried this:

img1 = imread('bluebird.jpg');
img2 = img1;
m = mean(img1, 3);
blues = img1(:,:,3) > 1.2*m;
img2(blues, [3,2,1]) = img1(blues, [1,2,3]);

But that did not work. The variable blues successfully get the pixels I want (the ones with a dominant blue component), but I get a illegal syntax on the last line.

Is it possible to do what I want? If so, how?

Thanks in advance.

Suever · Accepted Answer · 2016-05-23 15:52:51Z

The issue is because your logical array is 2D (takes care of the first two dimensions) and your linear index is only applied to the third dimension. You can combine logical indices however you must have one array of either per-dimension.

data = magic(3);
data([true false, true], [1 3])

%   8   6
%   4   2

An easy way around this for your case is to reshape your input to an [M*N x 3] array and then you can do exactly what you want because your logical array will now be a column vector of length M*N.

img1 = imread('bluebird.jpg');

% Remember the original size
shape = size(img1);

% Reshape to (M*N x 3)
img2 = reshape(img1, [], 3);

isBlue = img2(:,3) > 1.2 * mean(img2, 2);
img2(isBlue, [3 2 1]) = img2(isBlue, [1 2 3]);

% Reshape it back to the original size
img2 = reshape(img2, shape);

Or rather than using indexing, you could simply call fliplr.

img1 = imread('bluebird.jpg');

% Remember the original size
shape = size(img1);

% Reshape to (M*N x 3)
img2 = reshape(img1, [], 3);

isBlue = img2(:,3) > 1.2 * mean(img2, 2);
img2(isBlue, :) = fliplr(img2(isBlue, :));

% Reshape it back to the original size
img2 = reshape(img2, shape);

This is going to be more performant than creating a 3D logical matrix because reshape commands are very cheap as they don't actually alter the underlying data.

zeeMonkeez · Accepted Answer · 2016-05-23 15:53:29Z

1

Logical indexing (using matrices) and indexing by integers cannot be mixed. Instead, you can construct full logical index matrices:

img2 = rand(2, 4, 3);
m = mean(img2, 3);
blues = img2(:,:,3) > 1.2*m;
f_ind = false(size(blues));
ind = cat(3, blues, f_ind, blues);
img2(ind) = cat(3, img2(cat(3, f_ind, f_ind, blues)), img2(cat(3, blues, f_ind, f_ind)));

Or, instead of the last two lines:

r_ind = cat(3, blues, f_ind, f_ind);
b_ind = cat(3, f_ind, f_ind, blues);
img2(b_ind) = img1(r_ind);
img2(r_ind) = img1(b_ind);

edited May 23, 2016 at 15:53

answered May 23, 2016 at 15:39

zeeMonkeez

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2 Comments

Suever Over a year ago

That first statement isn't actually true. You just can't combine logical matrices and integer-based indexing.

zeeMonkeez Over a year ago

@Suever thanks for the clarification. And 'integer-based' is the correct term, too...

ibezito · Accepted Answer · 2016-05-23 16:51:35Z

1

Use:

[Y,X] = find(blues);
inds1 = sub2ind(size(img1),Y,X,ones(length(Y),1));
inds2 = sub2ind(size(img1),Y,X,3*ones(length(Y),1));
img2([inds1,inds2]) = img1([inds2,inds1]);

edited May 23, 2016 at 16:51

answered May 23, 2016 at 15:43

ibezito

5,8322 gold badges28 silver badges46 bronze badges

2 Comments

Jeff Over a year ago

Hi. I tried your code and the image displayed was identical to the original image.

ibezito Over a year ago

Thanks for your comment, I fixed it (had a small typo)

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