Abstract In order to compare the quality of different implementations of GKS, the ISO0 standard for computer graphics, an evaluation method for GKS implementations is presented. It is based upon several groups of criteria. One group of criteria is concerned with performance, by which we understand here the memory requirements and time requirements for programs using GKS functions. A program that measures the performance of GKS packages is presented. Results of this evaluation method with several commercially available GKS implementations are described in summary. A checklist for evaluation of standard graphics packages is added as an appendix.

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Deformations, such as bending, twisting and tapering, are important operations for modifying the shape of solid objects. In this paper, we present methods to deform objects defined by sweeping. A closed 2D polygon (the contour) moves along a 3D polyline (the trajectory), thus producing a polygonal approximation of a generalized cylinder. During sweeping, the contour can be incrementally scaled and rotated, resulting in tapered and twisted objects. Also, smoothly bent objects can be produced by performing local rounding operations on trajectories. The methods have been implemented in the solid modeller PLAMO, in which the deformed sweep objects can be used, as well as other types of bodies, as primitives in a CSG tree.

An algorithm is presented for ray tracing generalized cylinders, that is, objects defined by sweeping a two-dimensional contour along a three-dimensional trajectory. The contour can be any 'well-behaved' curve in the sense that it is continuous, and that the points where the tangent is horizontal or vertical can be determined, the trajectory can be any spline curve. First a definition is given of generalized cylinders in terms of the Frenet frame of the trajectory. Then the main problem in ray tracing these objects, the computation of the intersection points with a ray, is reduced to the problem of intersecting two two-dimensional curves. This problem is solved by a subdivision algorithm. The three-dimensional normal at the intersection point closest to the eye point, necessary to perform shading, is obtained by transforming the two-dimensional normal at the corresponding intersection point of the two two-dimensional curves. In this way it is possible to obtain highly realistic images for a very broad class of objects.