06Rasterization 2 (Antialiasing and Z-Buffering
- 一、Rasterization 1 (Antialiasing and Z-Buffering)
一、Rasterization 1 (Antialiasing and Z-Buffering)
Recap: Testing in/out△at pixels’ centers
如何解决锯齿（学名走样aliasing）是本节课的主要内容
照片就是采样的过程，采样可以发生在不同的时间。
Sampling Artifacts/瑕疵
(Errors / Mistakes / Inaccuracies) in Computer Graphics
Moire Patterns in lmaging摩尔纹
拿手机拍显示器的屏幕也会出现同类情况
Wagon heel illusion (False Motion)
有时看到高速行驶的汽车轮胎倒转
Artifacts due to sampling - "Aliasing"
Jaggies - sampling in space
Moire - undersampling images
Wagon wheel effect - sampling in time
Antialiasing ldea: Blurring (Pre-Filtering) Before
Sampling（在采样之前做一个模糊，即滤波）
若先采样再模糊叫Blurred Aliasing，实际还是走样了
？？采样速度跟不上信号变化速度就会走样
频率知识：
Frequency Domain 频域
Fourier Transform 傅里叶变换
傅里叶级数展开可以将函数图像转换成正弦余弦变换。
相同采样方法采样不同函数，采样结果一样。
Filtering 滤波= Getting rid of certain frequency contents去掉特定频率
高通滤波：只让高频信息留下，只留下图片边界
边界（颜色的突变）——高频
低通滤波：只让低频信息留下，模糊边界
Filtering = Convolution卷积(= Averaging平均)
卷积定理
空间域中的卷积等于频域中的乘法，反之亦然
实域的卷积等于频域的乘积。
Box Filter
九分之一是为了保持图像颜色亮度不改变（归一化）
盒子变大了，反映在频率上反而变小了。
Sampiing = Repeating Frequency Contents
Aliasing = Mixed Frequency Contents
解决方法：
Option 1: lncrease sampling rate
- Essentially increasing the distance between replicas in the Fourier domain
- Higher resolution displays, sensors, frame buffers…
- But: costly & may need very high resolution
Option 2: Antialiasing - Making Fourier contents “narrower” before
- repeating i.e.Filtering out high frequencies before sampling
Solution:
- Convolve f(,y) by a 1-pixel box-blur
- Then sample at every pixel’s center
MSAA多重采样反走样
将一个像素分解为多个小格，然后平均采样
并未实际改变分辨率，只是近似覆盖
What’s the cost of MSAA? 增大计算量
其他抗锯齿方法：
- FXAA(Fast Approximate AA)
- TAA (Temporal AA)
Super resolution / super sampling
- From low resolution to high resolution
- Essentially still “not enough samples” problem
- DLSS (Deep Learning Super Sampling)