How do I make all pixels under an assisted line boundary = 1?

I have manually segmented an image into two halves using:

roi = drawassisted('Closed',false);

Im = createMask(roi);

The generated mask (Im) is attached as an image

I want to make all pixels below that line = 1

The problem I am having is that the boundary sometimes loops back on itself, so x values are not unique, meaning I can't write a simple if statement using the pixel co-ordinates.

I'm sure the solution is quite easy, but I am really struggling to find it.

Any help much appreciated,

Thank you

Answer 1

[Image Analyst]

Try this (untested) with your mask to create a mask where the lower portion is true and the upper portion is false.:

[rows, columns] = size(mask); % Get size of your binary image with the line in it.
lastRow = ones(1, columns); % Initialize to top of image.
% Initialize a lower mask
lowerMask = false(rows, columns);
% Scan columns finding the last
for col = 1 : columns
    thisColumn = mask(:, col);
    t = find(thisColumn, 1, 'last');
    if ~isempty(t)
        lastRow(col) = t;
    end
    lowerMask(lastRow(col) : end, col) = true;
end
imshow(lowerMask);

[Lydia Bradley]

This worked perfectly, thank you so much!