【Real Time Rendering 2】
1、The light vector l is usually defined pointing in a direction opposite to the direction the light is traveling.
2、The surface irradiance（辐照度） is equal to the irradiance measured perpendicularly to l, times the cosine of the angle θi between l and n.
Irradiance, is equivalent to the sum of energies of the photons passing through the surface in one second.
Light can be colored, so we will represent irradiance as an RGB vector containing three numbers: one for each of red, green and blue.
E is used in equations for irradiance, most commonly for irradiance perpendicular to n. We will use EL for irradiance perpendicular to l.
3、Scattering does not change the amount of light—it just causes it to change direction.
Shading equations typically incorporate a smoothness parameter that controls the distribution of the specular term.
Surfaces scatter reflection);
散射（scatter）分为两部分：折射（refraction or transmission）和反射（reflection）。
4、irradiance is equivalent to the sum of energies of the photons passing through the surface in one second.
Incoming illumination is measured as surface irradiance. We measure outgoing light as exitance, which similarly to irradiance is energy per second
per unit area. The symbol for exitance is M.
5、Radiance is the density of light flow per area and per incoming direction. Radiance (symbolized as L in equations) can be thought of as the measure of the brightness and color of a single ray of light.
The directional distribution of the specular term depends on the surface smoothness. We can see this dependence in Figure 5.8, which shows diagrams and photographs for two surfaces of different smoothness. The beam of reflected light is tighter for the smoother surface, and more spread out for the rougher surface.
对于 specular，光滑度（smoothness）越高，则反射（reflected）的光束越紧凑。
6、Physical sensors measure an average value of radiance over their area, over incoming directions focused by the lens, and over a time interval; the evaluation of a shader equation computes radiance in a single ray at a single instant.
7、Shading equation for the diffuse term:
8、Specular。
θh is the angle between h and n。Lspec increases when the angle θh between h and n decreases。 m, which represents the surface smoothness. Increasing m causes the specular highlights to be smaller and brighter。
9、Diffuse + Specular:
而经典的Blinn-Phong方程如下：
10、Equation 5.10 is for a single light source. However, scenes often contain multiple light sources. Lighting is additive in nature, so we can sum the contributions of the individual light sources to get the overall shading equation:
11、When designing a shading implementation, the computations need to be divided according to their frequency of evaluation. The lowest frequency of evaluation is per-model: Expressions that are constant over the entire model can be evaluated by the application once.
where Kd = cdiff/π and Ks = ((m + 8)/8π)cspec are computed by the application. Since we only use directional light sources, lk and ELk are constant and can be set once. The remaining variables θhk and θik vary over the mesh. As we have seen, cos θik is computed by performing a clamped dot product between lk and n. Similarly, cos θhk is computed with a dot product between hk and n.
12、法线。
13、The saturate intrinsic function returns its argument clamped between 0 and 1. In this case we just need it clamped to 0, but the argument is known not to exceed 1, and saturate is faster than the more general max function on most hardware.
14、the surface normal is scaled to length 1 in the vertex shader, interpolation can change its length, so it may be necessary to do so again in the pixel shader.
15、The goal of this sampling process is to represent information digitally. In doing so, the amount of information is reduced. However, the sampled signal needs to be reconstructed in order to recover the original signal. This is done by filtering the sampled signal.
16、The effect occurs because the images of the wheel are taken in a series of time steps, and is called temporal aliasing.
17、For a signal to be sampled properly (i.e., so that it is possible to reconstruct the original signal from the samples), the sampling frequency has to be more than twice the maximum frequency of the signal to be sampled.
This is often called the sampling theorem, and the sampling frequency is called the Nyquist rate or Nyquist limit.
采样频率必须大于 Nyquist Limit 才能不走样。上图中第三行，采样频率等于 Nyquist Limit，发现还是走样了。
it is impossible to entirely avoid aliasing problems when using point samples to render a scene.
18、三种 Filter：
lowpass filter:
19、The general strategy of screen-based antialiasing schemes is to use a sampling pattern for the screen and then weight and sum the samples to produce a pixel color, p:
20、Antialiasing algorithms that compute more than one full sample per pixel are called supersampling (or oversampling) methods. Conceptually simplest, full-scene antialiasing (FSAA) renders the scene at a higher resolution and then averages neighboring samples to create an image.
A related method is the accumulation bufferblurry, and depth of field.
are most disturbed by aliasing on near-horizontal and near-vertical edges.
21、among the samples.
fragment.
MSAA每个fragment只计算一次，然后将计算结果赋值给覆盖的 sub-sample，所以sub-sample保存了fragment shader的计算结果。当绘制到屏幕时，每个pixel中的sub-sample，将被求各平均，从而达到抗锯齿。
CSAA creates a coverage mask for each screen grid cell it fully or partially covers.
22、随机采样。
Awith noise.
stratified sampling.
column
23、per pixel, and up to 16 different user-defined sampling patterns that can be intermingled in a repeating pattern.
24、边缘重叠采样。
FSAA methods.
Quincunx method itself appears to have died out.
This results
in a pattern, called FLIPQUAD.
下图左边的左边是RGSS，左边的右边是FLIPQUAD。
25、透明。
Crendered, thereby leaving the object behind it partially visible. The problems with this technique include:
effect.
the three colors can appear on the checkerboard pattern.
26、标准透明算法是 back-to-front。front-to-back也是可以的，需要按如下这样做。
Blending front-to-back when rendering used in volume rendering techniques.
additive blending:
effect of saturating the colors of the phenomenon.
27、预乘 alpha.
are multiplied by the alpha value before being stored.
28、Gamma Correction
and encoded pixel values are slightly modified power curves.
The encoding gamma describes the encoding transfer function, which describes the end-to-end transfer function.
to produce the appropriate end-to-end gamma.
未Gamma-Correction的处理，易产生过曝。
未Gamma-Correction的处理易产生扭曲（Roping）
29、nonlinear space（Gamma Space）