计算机图形学视觉外观浙江大学.ppt
88页
Chapter 3. Visual AppearanceChapter 3. Visual Appearance 视觉外观视觉外观金小刚金小刚 Email: Email: jin@jin@ 浙江大学浙江大学CAD当a Require “n” passes for all geometry> # of passes is practically unbounded• T-buffer> Require 1 pass for all static geometry > # of samples depends on implementation limits> Multiple rendering for moving objects# Could be 2-4 for slow moving objects# More for fast moving objectsT-buffer T-buffer 和和 Accumulation buffer Accumulation buffer比较比较Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.• T-buffer 和 Accumulation buffer与FSAA（全屏 幕反走样）相比的共同优点：在一个象素单元内 ，采样模板不一定是均匀的。
• 每个Pass与其它Pass无关，因而可采用均匀模板 外其它的模板，如(0,0.25), (0.5,0.0), (0.75, 0.5), (0.25,0.75)• 上述模板称为旋转网格超采样(Rotated Grid Super-Sampling, 简称 RGSS)，这种模板对于接 近水平或垂直的直线能提供更好的反走样效果， 而这也是应用中最需要的T-buffer T-buffer 和和 Accumulation buffer Accumulation buffer的优点的优点Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.各各 种种 象象 素素 采采 样样 模模 板板RGSSEvaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.A-bufferA-buffer• Multisampling：在单个pass中对每个象素采样多 次的方法。
• A-Buffer是一种Multisampling方法，由 Carpenter提出通常用于在软件中生成高质量 的绘制结果，但不能实时生成它的主要思想是 计算覆盖象素网格单元的多边形逼近• 是一种边反走样方法，可以绘制透明物体Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.• 在硬件A-buffer中，为了节省计算量，象素网格 单元对应的多边形光亮度值只计算一次，所有象 素网格单元内的采样点共享该光亮度值因此， 基本的A-buffer算法不能处理纹理和阴影的反走 样• 在A-buffer中，对于每个屏幕网格单元，每个绘 制的多边形生成一个Coverage Mask(完全或部分 覆盖象素单元)Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.多边形的角 部分覆盖象素网格单元，网格单元细分为 4X4的子网格。
覆盖的子网格单元为1，其余为016 位的mask为：0000 0111 1111 01110 0 0 00 1 1 10 1 1 11 1 1 1Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.NVIDIANVIDIA的的QuincunxQuincunx方法方法• 一般的反走样方法一个采样点只影响一个象素 ， NVIDIA的Quincunx方法单个采样点影响的 象素多于一个。
1/21/81/81/81/8由于共享，每个象素 平均只需2个采样点 ，但是效果却比2个 采样点的全屏幕反走 样好不少Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.随机采样（随机采样（stochastic samplingstochastic sampling））• 由于场景中可能包含在屏幕上投影非常小的物 体，因此规整的采样模板总会存在某种形式的 走样• 解决方案：把采样点在象素内随机分布，每个 象素有不同的采样模板这种方法称为随机采 样• 随机采样原理：用噪声取代重复性的走样现象 ，人的视觉系统更能接收Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.JitteringJittering（抖动）（抖动）• 最常用的随机采样方法为Jittering，这是一种 分层采样(stratified sampling).• 原理: 假设每个象素需要n个采样点，我们把象 素区域分成n个面积相同的区域，把每个采样 点随机地置于其中的一个区域。
象素的最终颜 色取为采样点的加权平均Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.一个典型的一个典型的JitteringJittering模板模板每个象素划分成3X3的子单元，每个采样点在子 单元内随机出现Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.交替采样交替采样(Interleaved Sampling)(Interleaved Sampling)• 采用交替随机采样可以把由于每个象素采用相 同模板引起的走样缺陷最小化。
采用累积缓存反走样， 每个象素采样四个点每个象素的采样模板不相 同，模板交替效果好！可见重复模板走样现象Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.透明、透明、AlphaAlpha和合成和合成v在实时绘制中，透明效果的处理通常是简单化和 有限的v如下效果通常没有：折射、透明物体厚度引起的 光的衰减等v有一些透明效果总比一点透明效果都没有好！v通过把表面颜色与其后面物体的颜色相混合，实 时绘制系统确实提供了绘制半透明表面的能力Evaluation only.Evaluation only. Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0.Created with Aspose.Slides for .NET 3.5 Client Profile 5.2.0.0. Copyright 2004-2011 Aspose Pty Ltd.Copyright 2004-2011 Aspose Pty Ltd.Alph。
