Especially structure sketch, also known as "body sketch". This kind of sketch is characterized by taking lines as the main means of expression, without changes in light and shade, but emphasizing the structural characteristics of objects. In order to understand and express the structural essence of an object itself, structure sketch's observation is often combined with measurement and reasoning. The application of perspective principle runs through the observation process, not just focusing on the intuitive way. This method of expression is rational, and external factors such as light and shadow, texture, volume, light and shade of objects can be ignored.