Hi
Finiding the 4 corners of the rectangle - is very easy (looking for minimum/maximum non zero index on each dimention)
Here is a small example:
W = 20; H = 120;
Image_BW_filled = zeros(W*3,H*1.2);
Image_BW_filled(W*1:W*2-1,20:H+20-1) = rand(W,H)> 0.1;
hFig = figure;
ax = axes('Parent',hFig);
imshow(Image_BW_filled,'Parent',ax)
x1 = find(sum(Image_BW_filled)>0,1, 'first');
x2 = find(sum(Image_BW_filled)>0,1, 'last');
y1 = find(sum(Image_BW_filled,2)>0,1, 'first');
y2 = find(sum(Image_BW_filled,2)>0,1, 'last');
corners = [...
x1,y1; ...
x1,y2; ...
x2,y1; ...
x2,y2];
hold(ax,'on');
hLine = line('XData',corners(:,1),'YData',corners(:,2),'Parent',ax,'Marker','o','MarkerFaceColor','r','LineStyle','none');
