【摘要】 植被、地形以及人工建构筑物是三维虚拟地理场景可视化表达的基本内容,树木是植被的主要组成部分.由于树木自然形态的复杂性,其真实感表达非常困难.基于几何模型的树木表达可以产生逼真的细节,但场景实时渲染具有巨大的计算负担,而基于纹理的简化模型在真实感表达上有所欠缺,但具有更好的渲染性能.如何兼顾场景渲染效率与视觉真实感受一直是树木三维可视化表达的研究热点.本文提出一种基于自适应布告板的三维树木表达方法,该方法在通常的平面布告板方法基础上,根据视点与布告板的相对位置关系,从预先获得的多张树木影像中选取最符合视点与树木相对位置关系的影像作为树木纹理渲染于该布告板上,实现了布告板纹理的动态调整,使得不同视点下的树木渲染效果尽量接近真实,同时也具有布告板的渲染性能优势.本文基于WebGL在浏览器中构建了一个三维场景,其中包含若干树木模型.实验结果表明本方法在表达树林时,在浏览器这样的低渲染计算能力环境下也能取得可接受的渲染性能与表达效果.更多还原

【Abstract】 Vegetation, terrain, and man-made buildings are the primary parts in a 3D geography scene. Trees, as the main component of vegetation, bring greater difficulties in presentation of themselves in a 3D scene than others due to complex natural forms. Geometry-based model, with huge computational burden to real-time scene rendering, could present 3D tree details much better. However, a simplified tree model based on texture has generally better rendering performance than the geometry-based model, but with rather rough 3D effect. Reasonable compromise on the rendering performance and visual 3D presentation is a big challenge, becoming a research focus in the field of virtual geographic environment.This study proposes a self-adapting billboard to present a tree in a 3D scene. Firstly, a set of tree pictures should be taken from different viewpoints or pre-constructed based on different perspectives of the 3D tree model. Then, a billboard is taken as a geometrical carrier of dynamical texture. While viewpoint moves, the direction from the viewpoint to the target tree will be regarded as a determinative parameter to pick out one best-match picture of the tree from the pre-constructed picture set to replace current texture on the billboard. It makes the rendering effect of trees much similar to the real observations from a relative viewing angle. At last, a 3D scene with many trees is rendered in a browser based on WebGL.The result proves that the rendering efficiency and effect are both acceptable.更多还原