Research On Digital Bas-relief Generation

Over the past few years, computer aided art design has been one of the most challenging topics in computer graphics, art design, computer aided design and advanced mechanical manufacturing. The research hotspots include visual psychology analysis, model simulation and art stylization on different kinds of artistic products. In this paper, we take bas-relief as the research object, and investigate the key techniques of digital bas-relief generation, such as viewpoint selection, geometric modeling and relief editing.So far, most existing methods for bas-relief generation take advantage of2D image processing that tend to convert the useful information of a well-designed image to the height field of the relief. Since the depth information is often limited in a single image, the quality of the generated bas-relief depends largely on the experience of the designer. In recent research on automatic bas-relief generation, a great deal of attention has been devoted to view-dependent3D object compression, which begins with the input of original3D scenes. Compared with traditional2D image algorithms,3D object-based methods have competitive advantages in viewpoint selection, depth information extraction, and geometric processing. Although the technology is now in its developing stage, it will play a more important role in future bas-relief creation with the development of the research.To generate the desired bas-relief from a3D object, several problems need to be solved. First, for a given3D object, how to select a most representative viewpoint for subsequent geometric modeling? Second, when a large compression is conducted on the input object, the original feature details will be lost in the bas-relief, so how to preserve the fine details and ensure the generation quality? Third, the computation efficiency is critically important to bas-relief generation, so how to present a real-time algorithm and ensure the availability of the production? Finally, when local shape of the bas-relief needs to be modified, how to provide an efficient solution for further relief editing?Currently, there have been abundant researches to deal with the above problems. Although some satisfactory results have been obtained, there is still some difference from the practical application. Among them, most existing methods operate in image space that take the rendering of3D object as the input, and represent the depth information in z-direction displacements based on regular pixels sampling. Thus, the topology of the generated relief becomes completely different from that of the input mesh. Furthermore, the pixel-based representation of the output makes shape editing of the relief much more difficult, largely limiting the extensibility of the bas-relief. Our approach differs from existing methods in that it operates directly on triangular mesh, maintaining the topology of the mesh unchanged during geometric processing. On this basis, more achievements from3D mesh processing techniques can be explored for further bas-relief producing, merging and reusing. In this paper, the main research contents are as follows:A new saliency-driven approach for best viewpoint selection is proposed based on information theory. To discriminate some features are more salient than the others, we define mesh saliency in two ways:a view-independent method using third-order surface derivatives, and a viewpoint sampling approach based on the line drawing technique. After that, the mesh saliency is incorporated in the computation of the most influential descriptors:viewpoint information, viewpoint stability and viewpoint visibility. Finally, we combine the three descriptors together to evaluate the goodness of a viewpoint and to select the most representative views for the bas-relief generation.Aiming at preserving fine details and achieving high computational efficiency, a real-time bas-relief generation algorithm is proposed based on geometric compression. The feature details are first extracted from the original objects using a spatial bilateral filtering technique. Then, a view-dependent coordinate mapping method is applied to build the height domain for the current view. After fitting the compression datum plane, we use an adaptive compression function to scale and combine the Z values of the base mesh and the fine details. The proposed approach operates in object space that offers flexible control over the extracted details, making it possible for real-time bas-relief generation and details adjustment.A novel approach for bas-relief generation and editing is proposed based on gradient-based mesh deformation. By deforming the input mesh implicitly through gradient manipulation and Poisson reconstruction, our approach works well both for plane surface bas-relief and curved surface bas-relief generation. We put forward a series of gradient-based algorithms such as height field deformation, surface continuity enhancement, fine details preserving, curved surface flattening and so on to ensure high-quality of the generation. Besides that, we present several editing tools, including boundary height editing, sketching-based boundary shape editing and skeleton-driven shape editing that allow the user to interactively modify the bas-relief to a desired shape. On this basis, a basic framework for bas-relief generation and editing is developed.The problem of direct generation of line-based sunken relief from a3D mesh is studied. We present a suitable way for the combination of feature lines and relief height field. After extracting view-dependent feature lines from a3D mesh, we concentrate on the post-processing of the lines including clipping, chaining, and smoothing. Based on this, the extracted lines are organized in good forms and appearance, appropriate for line engraving. We construct the base height field of the sunken relief using Lambertian shading. The input3D mesh is finally transformed into three basic layers:a contour line layer to strengthen the sense of outlines, a decorative line layer to represent the relief details, and a smooth base layer to convey2.5D illusion of the depth. Using a continuous pitting corrosion method, we combine the three layers together and generate the sunken relief with desired line depth and stylization.