I came across a review for an iPhone/iPad app called Zen Brush. It looked like a lot of fun and got a good review so I thought I would try it. The frog picture is my first attempt with it. I think it must be easier to get in a lot of detail if you are using iPad. There is no zoom-in function so it is hard to do detailed work on the smaller screen. You can change brush sizes, but the small screen just isn't a lot of canvas. This is probably a plus for me since it means I just have to accept that the drawings need to be simple. Zen-like.
The maker PSoft has some excellent examples on their website. They are based in Sendai and have put together a book to raise money for the earthquake.
