Extract pixel values inside a Binary Mask

I have an image and the ROI points. I did the following to extract the ROI by creating a binary mask with poly2mask. now I want to extact the values of masked image.

My problem is that I cannot retain zeros inside the masked image using the code below. Could you please help me?

S = size(img); % Size of the binary masked
bw = poly2mask(x,y,S(1),S(2)); % Create a binary mask
ROI = img;
ROI(bw == 0) = 0;
ROI = ROI(ROI>0); % Just keep the non-zero values

Answer 1

[Joseph Cheng]

since you already created a binary mask you can just index multiply " .* " the mask with the original image as outside is 0 and inside the mask is 1.

%dummy image  for example
    img = imread('pears.png');
    bwimg = single(rgb2gray(img));
    [r c]=size(bwimg);
   figure(1),subplot(131),imagesc(bwimg); colormap gray
   title('original image')
%dummy mask gen for example
    x = c*rand(1,4);
    y = r*rand(1,4);
    x(end+1) = x(1);
    y(end+1) = y(1);
    %create mask    
    bw = poly2mask(x,y,r,c);
    subplot(132),imagesc(bw);colormap gray
    title('mask')
%extractROI
    %index multiply image with mask as outside is 0 and inside is 1
    ROI=bwimg.*bw; 
    subplot(133),imagesc(ROI);colormap gray;title('Mask applied')