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Item Faster Phong Shading via Angular Interpolation(Blackwell Publishing Ltd and the Eurographics Association, 1989) Kuijk, AAM.; Blake, E.H.One of the most successful algorithms that brought realism to the world of 3D image generation is Phong shading. It is an algorithm for smooth shading meshes of planar polygons used to represent curved surfaces. The level of realism and depth perception that can be obtained by Phong shading is attractive for 3D CAD applications and related areas. However, per pixel computation costs which were too high and/or artifacts, introduced by some of the more efficient evaluation methods and apparent only when displaying moving objects, are major factors mat blocked the common usage of Phong shading in highly interactive applications.In this paper we present angular interpolation for Phong shading planar polygons. Angular interpolation was a method especially designed to meet requirements as imposed by special purpose hardware we developed1, but turned out to be generally applicable. The angular interpolation method appears to be very efficient and reduces artifacts when displaying moving objects. Ideally a shading algorithm imposes no need for subdivision of patches as presented by the solid modelling system. Shading calculation via angular interpolation yields such an ideal algorithm. We will describe two alternative evaluation methods that trade off evaluation cost against level of accuracy. They both can handle light source and view point at arbitrary distances, but differ in level of accuracy. As a consequence these alternative evaluation methods do impose restrictions on the topology of patches and light sources. However, generally, the limitations imposed by these alternative shading methods are much more liberal than the limitations on patch size imposed by the geometry.The most economic evaluation method we present can incrementally compute the colour intensity along a scanline by two additions per pixel. The methods presented are generally applicable and can easily be implemented in hardware.Item Algorithm for Clipping Arbitrary Polygons(Blackwell Publishing Ltd and the Eurographics Association, 1989) Andreev, Rumen D.Item Blending Rational B-Spline Surfaces(Eurographics Association, 1989) Bardis, L.; Patrikalakis, N.M.A method for blendin non uniform rational B-spline surface patches, either open or periodic, is developed. he blending surface is expressed in terms of an integral, bicubic B-spline patch. The blend ensures position and normal vector continuity along linkage curves to within a specified accuracy. The linkage curves are either user-defined or are obtained by offsetting the intersection of the two patches using geodesics on each patch. An example illustrates the applicability of our method.Item A Virtual Memory System Organization for Bit-Mapped Graphics Displays(The Eurographics Association, 1989) Barkans, Anthony C.; Richard Grimsdale and Wolfgang StrasserDescribed is a display sub-system, designed for support of a very high speed rendering engine. It provides high-performance graphics to an enVironment that consists of a hierarchy of resizable windows. The concept of virtual memory has been applied with the organization of the virtual to physical address spaces having a unique mapping that fits the organization of a bit-mapped graphics memory display.Item Variations on a Dither Algorithm(Eurographics Association, 1989) Pins, Markus; Hild, HermannMapping continuous-tone pictures into digital halftone pictures, i.e. 0/1-pictures, for printing purposes is a well explored technique. In this paper, one of these algorithms, the two-dimensional error-diffusion algorithm is extended to color pictures and animated pictures. The color picture algorithm is superior to existing algorithms by considering extreme color values as well as adjacent color values. The animation algorithm eliminates the noise created by the correct but varying pixel patterns generated by applying a single picture dithering algorithm on every frame. The power of the algorithms is demonstrated by experiments carried out on synthetic images generated by ray tracing.Item A Hardware Algorithm for Fast Realistic Image Synthesis(The Eurographics Association, 1989) Yilmaz, A. C.; Hagestein, S.; Deprettere, E.; Dewilde, P.; Richard Grimsdale and Wolfgang StrasserA VLSI oriented algorithm, for the implementation of a generalized two-pass radiosity method is presented. The method allows any reflection behavior, varying from purely diffuse to perfect mirroring. Moreover, objects may be defined in terms of curved (Bezier) surfaces. All computations in the pre- and postprocess are similar and ray-tracing based, consequently a single architecture can be devised for both passes. This architecture, when built on ray-rotating and ray-tracing pipelined processors such as Cordics, results in a very high throughput VLSI implementation o f the proposed generalized two-pass procedure.Item On The Projective Invariant Representation of Conies in Computer Graphics(Blackwell Publishing Ltd and the Eurographics Association, 1989) Herman, IvanA general formulation for conies and conic arcs for the purpose of computer graphics is given, based on principles and theorems of projective geometry. This approach allows the approximation of these curves by line segments to be postponed in the graphics output pipeline- it results in a more compact storage, faster approximation algorithms and smoother outlook of the curves.Item Hidden Octree Node Removal as a Pipeline Process(Blackwell Publishing Ltd and the Eurographics Association, 1989) Heal, B.W.A processing pipeline solution to the generation and display of octree encoded 3-D objects is described. Particular emphasis is given to an algorithm which removes that part a of linear octree code which describes parts of the object which are invisible from a prescribed viewing position. The scope of the algorithm is restricted to viewing directions which are parallel to the Cartesian axes and to orthogonal projections. The algorithm is specifically designed to operate as a single stage within a pipeline of processes.Item Two Object-Oriented Models to Design Graphical User Interfaces(Eurographics Association, 1989) Hübner, Wolfgang; Gomes, Mario RuiObject-oriented concepts are well-suited to deal with the characteristics of user interfaces. Up to now several attempts to integrate the object-oriented paradigm in user interface models were evolved and led to distinctive resulting models due to different requirements of the target application area. Within this paper two independently developed object-oriented interaction models are presented which emphasize the graphical requirements to user interfaces. These are among others its hierarchical nature, dynamical topology of the user interface, strong connection between input, output and the semantics of the application and the diversity of the graphics input devices and interaction techniques. Both approaches converge in the following aspects: Instead of having separated user interface layers the components of an interactive graphics application's user interface are embedded locally within interaction objects. Therefore dialogue control, input, output and the dynamical behavior are organized as a micro-cosmos within each object. Compound interaction objects can be designed. Temporal logical operators are used to specify the dialogue. Tools to support the implementation of each model are described. By describing both approaches this paper could be a contribution toward establishing a uniform object-oriented framework for the design of graphical user interfaces.Item The Incremental Display of a Single-Valued Curve(Blackwell Publishing Ltd and the Eurographics Association, 1989) Smith, F.J.; Leitch, S.A new algorithm is described for the drawing of a single-valued smooth function on a raster screen. It first approximates the function by a spline, normally cubic, and then displays the spline using a combination of finite differences and a special Bresenham type algorithm in integer arithmetic. Finite difference have not been successful previously, partly because of the build up of rounding errors. We show that for a modern workstation with 32 bit-integers these errors can be estimated and the algorithm modified to minimise their effect.The result of combining all of these factors, spline, finite differences, error control, geometry and integer arithmetic is a powerful algorithm which we believe is generally more accurate and faster than previous algorithms for smooth curves.It is worth noting that when the curve is a straight line, our algorithm becomes an extension (and minor improvement) on Bresenham s algorithm.