MATLAB: Getting Rid of Nested For Loops

Question

So I have a bunch of nested for loops in the following code but I feel like there has to be a faster and better way to eliminate these things so it will run faster. If anyone had some insight into how I might do this I would be greatly appreciative.

    function [mask] = bbox_analysis(bboxes, mask, blackwhiteframe, th, fractional_th)
    % Cycle through every bbox in the frame
    bbox_dim = size(bboxes);
    for b = 1:bbox_dim(1)
        x_1 = bboxes(b, 1);
        y_1 = bboxes(b, 2);
        x_2 = x_1 + bboxes(b, 3);
        y_2 = y_1 + bboxes(b, 4);
        cropped_image = imcrop(blackwhiteframe, [x_1, y_1, bboxes(b, 3), bboxes(b, 4)]);      % Section of frame captured in bbox
        max_image = max(cropped_image);
        threshold = max(th/255, max_image*fractional_th);
        new_image = (cropped_image > threshold);
        for i = x_1:x_2-1
            for j = y_1:y_2-1
                mask(j, i) = new_image(j+1-y_1, i+1-x_1);
            end
        end
    end 
end

Answer 1

Replace

        for i = x_1:x_2-1
            for j = y_1:y_2-1
                mask(j, i) = new_image(j+1-y_1, i+1-x_1);
            end
        end

by

   mask(y_1:y_2-1, x_1:x_2-1) = new_image((y_1+1-y_1):(y_2-1+1-y_1), (x_1+1-x_1):(x_2-1+1-x_1));