Recently I’ve been experimenting with a technique for converting photos to line art, which can then be colorized. (I don’t claim this technique is original to me, but I’ve been refining it for my own purposes.)
The essence of the technique is the conversion to lines, using the color dodge and multiply blend modes. In the artwork above, I started from this photo:
To convert the photo to line art, I converted to black & white (Cmd/Ctrl+ Shift+U). I duplicated that layer and inverted it to a negative of the b/w image (Cmd/Ctrl+I ). This is where I change the blend mode to color dodge, which lightens the underlying image. Since the top layer is the inverse of the bottom one the result is total white. This seems nuts, but when a Gaussian blur (experiment with the amount) is applied to the top layer, what we are left with is a line version of the image where the white and black elements meet. These lines are too faint, however, so the result is merged (Cmd/Ctrl+Shift+Alt+E). This layer is duplicated, changing the blend mode to multiply. I just duplicate (Cmd/Ctrl+J) this multiply layer several times until the lines are the darkness desired. Here is the result:
You could clean this b/w image up with an image mask if you liked. This would be useful in making a cartoon version of a portrait, for example.
Rather than stopping there, I sometimes colorize the image. I do this by adding a white layer underneath the final line layer. With the line layer still set to multiply, I can then paint on the white layer. Here is the result compared to the original photo.