This paper explores a concept of informal user interface for graphic computing, which can be characterized by two properties: stroke-based input and perceptual processing of strokes. A prototype system, named Magic-Sketch, is also outlined, which is well suited to natural and creative processes by making abstract problems more concrete informally. Two important issues to capture users' intention effectively for implementing sketch-based graphic input system, adaptive stroke segmentation and incremental sketch recognition, are discussed in details.