Hi again, Lee.
1) We can use the Hough transform for extracting lines, assuming that the pixels that we want are shown in the image (if we are not pleased with the obtained lines, we need to extract edges again from our image).
[H,T,R] = hough(cdata);%cdata is our black and white image
P = houghpeaks(H,10,'threshold',ceil(0.3*max(H(:)))); % 10 strongest peaks
lines = houghlines(cdata,T,R,P,'FillGap',5,'MinLength',7); % extract lines
% draw them in the image
img=cdata*255;
for k = 1:length(lines)
xy = [lines(k).point1; lines(k).point2];
img = insertShape(img,'Line',[xy(1,1) xy(1,2) xy(2,1) xy(2,2)],'LineWidth',2,'Color','blue');
end
imshow(img);
2) For the distance between lines, in this case, we can simply take the absolute difference between the y-coordinates (assuming that we know the m and n indexes for them):
dist = abs(lines(m).point1(2)-lines(n).point1(2));
If we want this distance to be in cm, then we need to measure the size of our tube (or desired object). This will provide us with the pixel density of our image: the ratio between pixels and its real size.
