How to create a mask for the image

I want to generate a mask for my image.How can I generate it?

The mask image should be like the below picture.

Answer 1

[Matt J]

 Mask = (YourImage>threshold);

[Ibrahim Usman]

How do you get the matrix dimensions for the image agree with the dimensions of the threshold?

[Image Analyst]

You don't. Here, threshold is just a scalar. No dimensions at all really, just a 1-by-1 scalar -- it's not an image with any dimensions. So the "matrix dimensions for the image" do not need to agree with anything, since threshold is a scalar.