Actually, it's a tool in the program I used to make the image, called PaintBBS, which is an oekaki applet. It's called the tones feature- the higher the value, the more dots, and the more solid the brush. Lesser value means fewer dots so it's a little more "transparent". I just make a gradient palette and start with higher tones and work down to smaller values from there. The thing is, I had to shut down my personal oekaki due to financial reasons, and Java will eventually be releasing a security patch that will essentially shut down all oekakis. I don't know when it is though, so some places are still open and the applets usable. So I might be able to do some more tones work, hopefully!