Realistic Computer Graphics for Natural Scenes

本项目旨在提高室外自然场景的计算机图形图像质量。到目前为止，室外地形的效果图具有卡通般的质量，大大分散了现实感。这部分是由于计算和源数据的限制，限制了可以渲染的地形的几何复杂性。这里所描述的研究的论点是，照明和材料属性在从这些场景中创造真实感方面发挥着同样重要的作用。此外，在室外地形模型中，几何、照明和材料属性之间存在重要的相互作用，当实时约束也必须得到满足时，应该理解这些相互作用。在过去十年中，在理解如何生成室内场景的逼真渲染方面取得了重大进展。一般的方法是分析光在这种环境中传输的物理特性，然后在计算算法中体现对物理特性的近似。正确建模照明和材料属性是至关重要的。现在我们知道，真实感主要取决于对阴影、二次照明和非均匀反射函数的考虑。精确地近似这些性质的影响需要大量的计算费用。因此，绘制逼真图像的方法几乎总是利用可能遇到的几何结构的性质、照明和材料属性的假设。这些假设大多来自对室内环境的假设。室外场景呈现出非常不同的计算特性。虽然物理是相同的，但几何，照明和反射率属性都明显不同。许多为支持室内场景的逼真渲染而开发的技术将需要对自然的室外环境进行大量修改。要克服的最困难的计算问题是，需要能够将微观结构的效果聚合成足够大的单元，以便有效地渲染，同时保留视觉外观的关键方面。这个问题存在于广泛的尺度上，从叶子，单个叶子的集合产生的整体外观与组成成员的外观完全不同，到遥远的地标，细节必须被抑制，而不能去除那些使地标与众不同和有用的属性。

This project aims to improve the quality of computer graphic images of outdoor, natural scenes. To date, renderings of outdoor terrain have had a cartoon-like quality that significantly distracts from a sense of realism. Partially, this is due to computational and source data constraints that limit the geometric complexity of terrain that can be rendered. The thesis of the research described here is that illumination and material properties play an equally important role in creating a sense of realism from these scenes. Moreover, there are important interactions between geometry, illumination, and material properties in a model of outdoor terrain that should be understood when real-time constraints must also be satisfied. Significant progress has been made in the last decade in understanding how to generate realistic renderings of indoor scenes. The general approach is to analyze the physics of light transport in such environments and then to embody approximations to the physics in computational algorithms. Correct modeling of illumination and material properties is vital. It is now known that a sense of realism depends critically on accounting for shadows, secondary illumination, and non-uniform reflectance functions. Accurately approximating the effect of these properties involves great computational expense. As a result, methods for rendering realistic imagery almost always exploit assumptions about the nature of the geometric structure and illumination and materials properties likely to be encountered. Most of these assumptions derive from a presumption of indoor environments. Outdoor scenes present very different computational characteristics. While the physics is the same, geometry, illumination, and reflectance properties are all distinctly different. Many of the techniques developed to support realistic rendering of indoor scenes will require substantial modifications for natural, outdoor environments. The most difficult computational prob lem to overcome is the need to be able to aggregate the effects of micro-structures into large enough units that they can be rendered effectively, while at the same time preserving key aspects of visual appearance. This problem exists across a wide range of scales, ranging from foliage, in which a collection of individual leaves generates a collective appearance that is quite different than that of the constituent members, to distant landmarks, where detail must be suppressed without removing those properties that make landmarks distinctive and thus useful.