Multi-Stroke Combination Algorithm In The Automotive Styling From 2D To 3D

It is crucial for manufacturers to make a decision quickly from many concept sketches in the early stage of automotive styling, which has substantial effects on the downstream engineering, the time-to-market, the competitiveness of product and the investment. Therefore, the developing model should be finalized as early as possible. The number of 3-D models developed from sketches is, however, limited because of the cost and time involved in the 3-D modelling process, and the 2-D sketches give little real 3-D information at early stages, limit the variety at late stages and opportunity for management intervention. In the past decades, people paid more attention to 3-D modelling quickly from 2-D sketches. Many measures have been tried to reconstruct 3-D models by applying the existing CAD systems. Some researchers focused on the issues of "2-D to 3-D" and "Shape from X" (X stands for the different sources of 2-D). The regular geometries, such as cube, cylinder and sphere, have been now successfully reconstructed into 3-D models. However, the free-form surface reconstruction is still difficult especially from the automotive sketches - the non-realistic images. The 2-D automotive sketches are actually the quick reflections of stylists' creative thinking. The styling, in the most cases, is a process to use vague or uncertain strokes to express the design intent, which usually needs a lot of strokes to express one line or contour. For such non-realistic images, free-form surface reconstruction exists theoretically endless solutions. The algorithm of multi-stroke combination in the automotive styling from 2-D to 3-D is an effective way to combine the vague or uncertain strokes used by stylists in the concept design to a single and determinate stroke, which can be used to generate a NURBS curve, resulting in the formation of NURBS surfaces. This novel algorithm will change the traditional way of styling, i.e. from using sketches as background references to create 3-D models to a continuous design process that combines 2-D styling with 3-D modeling seamlessly. Therefore, the multi-stroke combination is one of the key techniques to merge continuous creative thinking into 3-D modeling.The data acquisition and processing of single stroke are the base of multi-stroke combination algorithms. By using the digital tablet, stylists draw arbitrarily some free-form strokes, and then the corresponding ones containing the separate points can be seen from computer screen. Therefore, dealing with these points is a start point. A data structure has been established to store the strokes, including the total number of strokes and the total number of data points in each stoke, and the coordinates of each point. In order to express the actual trend of a stroke, the least squares polynomial approximation and B-spline interpolation are applied to fit the points. It is found by comparison that the least squares polynomial approximation curve is smooth and beautiful, but lack of fixed ends nature, and the B-spline interpolation curve is good to lock the ends, but lack of smoothness. After acquiring an intuitional single stroke, the next problem to solve is how to merge two or more strokes into a single one on-line.The combination algorithm of two consecutive strokes provides a feasible method for solving the multi-stroke combination. It is known that stylists draw strokes freely only following the design intent. So it cannot simply infer that the first input is the first stroke, the second input is the second stroke, and so on. The algorithm should meet with the design intent, determine the sequence of strokes, and then combine the points from different strokes. The keys of the algorithm are the distance comparison and the two-stroke analysis. The distance comparison doesn't consider the sequence and the direction of strokes in advance, but compare the distance between the points belonged to two strokes to determine the sequence of strokes. The initial purpose of this algorithm is to imitate stylists' "free strokes" and give completely unrestricted forms, such as those drawn by using pen and paper, and above all be easy to realize. After prototyping and analyzing, it is found that the algorithm has higher requirements for strokes, such as intensive points, simple stroke form and so on. Otherwise, it will not determine accurately and effectively the sequence of strokes, get what the stylists want and reflect the initial design intent finally. Thus, a new way free of restraints - the two-stroke analysis - has been explored. The algorithm has given a detailed analysis and classification for common styles of stroke. It divides the two consecutive strokes into several sections, and combines the points in each section separately. The algorithm gives a good result.According to the two consecutive strokes processing, the multi-stroke combination has been investigated. In order to express a precise stroke, automotive stylists usually use many intensive strokes in sketching. Those intensive strokes play different roles in stvling. Some are given to ensure the length of final profile and some to modify the degree of bending after length fixed. The algorithm of multi-stroke combination has taken it into account to seize the beginning and ending points of all strokes to ensure the length of the final curve and allow a local modifiability to the final curve at the same time. The algorithm mainly includes the consecutive linkage and sequential combination. The consecutive linkage uses the minimum distance method to determine the sequence of two consecutive strokes and then combine them. It can grasp the length of the strokes much better. But when meeting the modified strokes, it also adds them to the tail of the fitting curve. it is what we are not expected. The sequential combination is a algorithm that combines two strokes into a new stroke, and then combine the new stroke with the third stroke. Following the same procedure, a single combined stroke can be obtained finally. The sequential combination algorithm is actually an extension of the combination of two consecutive strokes. It also contains the distance comparison and stroke analysis, which will not be discussed here in details.Finally, the combined strokes are processed further for surface reconstruction, including the specific work of curve fitting and stroke dividing.Above all, the algorithm of multi-stroke combination has investigated a new way for automotive styling with computer interactively in order to set up a bridge between the 2-D conceptual design and 3-D modeling. The algorithm includes some innovative and unique ideas which would be discussed in further research.