The first picture is a skull I make by 4 layers of different values I make on sketchbook.However, the density of the lines is not clear in different layers, and the skull is not presented clearly.
The next two have 5 layers of different values, made by Chinese and English handwriting respectively. They have the same problem as the previous photo. I didn’t save the documents in Photoshop PDF file, so the values cannot be differentiated.
After adjusting the density of lines by duplicating images, the image works better. The image looks even better after I inverse it.