Thicken a one pixel line
I'd take a look at morphological operations.  Dilation sounds closest to what you want.  You might need to work on a subregion with your line if you don't want to dilate the rest of the image.
I'm using OpenCV to do some image processing on Python. I'm trying to overlay an outline on an image where the outline was made from a mask. I'm using cv2.Canny() to get the outline of the mask, then changing that to a color using cv2.cvtColor() then finally converting that edge to cyan using outline[np.where((outline == [255,255,255]).all(axis=2))] = [180,105,255]. My issue now is that this is a one pixel thick line and can barely be seen on large images. This outline is all [0,0,0] except on the points I apply as a mask onto my color image using cv2.bitwise_or(img, outline. 
I'm currently thickening this outline by brute forcing and checking every single pixel in the bitmap to check if any of its neighbors are [180,105,255] and if so, that pixel will also change. This is very slow. Is there any way using numpy or openCV to do this automatically? I was hoping for some conditional indexing with numpy, but can't find anything.