Histogram from masked area of grayscale picture

Hello!

I want to make a histogram from masked area of a grayscale picture. I've tried this:

maskedImage = bsxfun(@times, grayImage, cast(mask, 'like', grayImage));
histogram(maskedImage(mask), 256, 'EdgeColor', 'none')

But it keeps saying: "Array indices must be positive integers or logical values". There are no negative integers and the mask only contains 0 and 1.

Hope you can help! :-)

Answer 1

[Steven Lord]

When performing indexing, there is a difference between a logical array and a double array containing only 0 and 1. The former works and performs logical indexing, the latter does not work and throws an error because there's no such thing as element 0, row 0, column 0, page 0, etc. of an array in MATLAB.

x = 1:10
L = mod(x, 2) == 0
D = double(L)
onlyEvens1 = x(L) % Works
onlyEvens2 = x(D) % Errors

If mask is a double array containing just 0's and 1's you'll need to convert it into a logical array to use it for indexing. The easiest way to do this is to call the logical function on it.

L2 = logical(D)
isequal(L, L2) % True
onlyEvens3 = x(L2)

Answer 2

[Image Analyst]

Attach a .mat file containing your grayImage and mask.

Alternatively, just do

histogram(grayImage(mask), 256, 'EdgeColor', 'none')

Answer 3

[Anders Jensen]

Thanks for your answer! It still says "Array indices must be positive integers or logical values".

I have attached the .mat file and the grayscale image :-)