Automatic inbetweening is one of the main focuses in computer assisted cel animation as inbetweening process is traditionally time-consuming and labor-intensive in the production of cartoon animation. Most previous work demand either lots of modeling or working with complicated programs and their interfaces, which do not take much animator's expertise into consideration, as animators are usually more capable of drawing pictures on the paper than programming with computer. An approach proposed in this paper aims to provide an easy-to-use system which automatically models key frames "on the fly" and interpolates them in several layers, including object size and position, line curvatures and curve texture in order to preserve artistic style of the hand-drawn key frames and make the motion smooth and natural. Without much user interaction and complicated program interface the system may be tuned, uploaded into the Web so that not only professional artists but those who are interested in cartoon but don't have much time or skill on drawing frames or inexperienced at using computer, like students, free-lance animators, etc. may benefit from using the system, for example, creating animation clips for the Web. The results show that the algorithm works well although it leaves areas for further research to extend and improve the algorithm to work with as many different types of key frames as possible.