the rightmost one isn't straight. that's troublesome.
if they were perfectly straight, you could do a radon transform, i.e. rotate by given angle, then sum along one axis that hopefully coincides with the line.
edge filters might be able to pick these up. try a gabor kernel. you'll want to vary the angle of the gabor kernel too. that is commonly called a "filter bank". the larger the kernel, the finer the angle step needs to be too.
