MediBang Pro’s Line Extraction tool is an excellent function that will allow you to extract the lines from an image to use as line-art.

This is best to use when getting traditional art ready for digital coloring.

For example, prepare the traditional art image (a scan or a clear photograph).

Go to Filter>Line Extraction and open the Line Extraction panel.

Drag the saturation to zero, and increase the brightness until you are satisfied with the result.

When extracted, you will be able to paint on layers underneath the lines the same way you would with an originally digitally drawn piece.