This is an extremely important concept in the field of automobile design, and it is also a necessary skill for automobile designers and A-side designers.
CAS refers to high-quality surface, and the so-called visual expression of surface quality is unsmooth, wrinkle-free and unreasonable discontinuity.
Mathematically speaking, CAS refers to a surface with G2 continuity between facets and curvature radius jump less than a certain value. G2 continuity is necessary, but the curvature jump is different for different car factories. Among them, the requirements of GM are relatively low, 0.05mm, followed by Toyota, 0.03mm, and BMW and Volkswagen have the highest requirements, reaching 0.02 mm.
The surfaces of ordinary cars are extremely complicated. If you look at my space, you should be able to find the Alias wireframe of a car I made. You can see that some large surfaces are definitely not formed at one time, but pieced together into several small planes, so the connection between surfaces is extremely important, which is the requirement of continuity. However, after meeting the continuity requirements, there is still a radius jump. If the radius jump is too large, the results of zebra stripe detection and environmental reflection detection are S stripes and folds, which looks very uncomfortable. Therefore, surface A requires simplicity, directness and fluency. Generally, there are no more than 6 horizontal and vertical CVS, that is, both U and V are the highest D5 curves.
Making a face requires extremely high technology, and it is impossible without some knowledge of computer graphics and long-term experience in NURBS modeling. Its core is facet, and how to divide the surface is the easiest way to achieve the effect, because many cars are not quadrangular structures of standard NURBS surfaces, but are three-sided, five-sided or even six-sided, which requires high facet skills and is also the focus of AliasL3 designers' assessment. Many freshmen and sophomores are full of confidence and want to make children's shoes for cars, but few seniors really enter the automobile industry, and most of them are scared away by this A-level surface construction.