How to get centerline of the mask images

Question

I am working on the centerline extraction, I have tried skeletonize, RivGraph and several other methods but could not get a good centerline. First the line on the ends are not following the center and it is also shorter than the mask itself.

Here is the code that I have tried:

# Thresholding the mask values to binary
mask[mask < 200] = 0
mask[mask > 200] = 255

# Convert the grayscale image to binary image
img = img_as_ubyte(mask)

# Perform thinning
thinned = skeletonize(img, method= 'lee')

# Get the x, y coordinates of non-zero (white) pixels
y, x = np.nonzero(thinned)

---

I = imageio.v2.imread("mask.jpg")

Imask = np.zeros((I.shape[0], I.shape[1]), dtype=bool)

Imask[I[:,:] != 255] = True   

coords, widths = ru.mask_to_centerline(Imask, 'ew')

plt.imshow(Imask)
plt.plot(coords[:,0], coords[:,1])

Could anybody give me some hint?

Answer 1

Since your first tag is grass-gis, I suggest you try the GRASS module v.centerline. You'll have to perform the following steps:

1. convert the raster to a polygon vector (r.to.vect)
2. extract the boundary of the polygon "mask" as a line (v.to.lines)
3. Then try v.centerline on that line feature

If you encounter any problems with the above, post again with your GRASS specific code, and we'll try to help.