GLSL 中的光照计算

理论知识转载地址：http://blog.csdn.net/ym19860303/article/details/25545933

1.Lambert模型(漫反射)

环境光：

Iambdiff = Kd*Ia

其中Ia 表示环境光强度，Kd(0<K<1)为材质对环境光的反射系数，Iambdiff是漫反射体与环境光交互反射的光强。

方向光：

Ildiff = Kd * Il * Cos(θ)

其中Il是点光源强度，θ是入射光方向与顶点法线的夹角，称入射角(0<=A<=90°)，Ildiff是漫反射体与方向光交互反射的光强，若 N为顶点单位法向量，L表示从顶点指向光源的单位向量(注意顶点指向光源)，则Cos(θ)等价于dot(N,L)，故又有：

Ildiff = Kd * Il * dot(N,L)

最后综合环境光和方向光源，Lambert光照模型可以写成：

Idiff = Iambdiff + Ildiff = Kd * Ia + Kd * Il * dot(N,L)

2.Phong模型(镜面反射)

Phong模型认为镜面反射的光强与反射光线和视线的夹角相关：

Ispec = Ks * Il * ( dot(V,R) )^Ns

其中Ks 为镜面反射系数，Ns是高光指数，V表示从顶点到视点的观察方向，R代表反射光方向。由于反射光的方向R可以通过入射光方向L(从顶点指向光源)和物体的法向量求出，
R + L = 2 * dot(N, L) * N  即 R = 2 * dot(N,L) * N - L

所以最终的计算式为：

Ispec = Ks * Il * ( dot(V, (2 * dot(N,L) * N – L ) )^Ns

3.Blinn-Phong光照模型(修正镜面光)

Blinn-Phong是一个基于Phong模型修正的模型，其公式为：

Ispec = Ks * Il * ( dot(N,H) )^Ns

其中N是入射点的单位法向量，H是光入射方向L和视点方向V的中间向量，通常也称之为半角向量（半角向量被广泛用于各类光照模型，原因不但在于半角向量蕴含的信息价值，也在于半角向量是很简单的计算：H = (L + V) / |L + V|  ）。

4.Rendering Equation(全局光照模型）

Rendering Equation 是Kajia在1986年提出的，

Lo(X, Wo) = Le(X, Wo) + ∫fr(X, Wi, Wo) Li(X, Wi) dot(N, Wi) dWi

其中X表示入射点，Lo(X, Wo)即从物体表面X点，沿方向Wo反射的光强，Le(X, Wo)表示从物体表面X以方向Wo 发射出去的光强，该值仅对自发光体有效，fr(X, Wi, Wo)为，入射光线方向为Wi, 照射到点X上，然后从Wo方向发射出去的BRDF值，Li(X, Wi)为入射方向为Wi照射到点X上的入射光强，N表示点X处的法向量，然后对入射方向进行积分(因为光线入射的方向是四面八方的，积分的意义是对每个方向进行一遍计算后相加)，计算的结果就是全局光照的辐射率。

对于单个点光源照射到不会自发光的物体上，公式可以简化成：

Lo(X, Wo) = fr(X, Wi, Wo) Li(X, Wi) dot(N, Wi)

这个公式非常有用，通常会将该公式分解为漫反射表达式和镜面表达式之和。对于漫反射表面，BRDF可以忽略不计，因为它总是返回某个恒定值，所以可以写成如下形式：

Lo(X, Wo) = Idiff + frs(X, Wi, Wo) Li(X, Wi) dot(N, Wi)

其中Idiff表示漫反射分量，使用公式的计算方法，frs(X, Wi, Wo)表示镜面反射的BRDF函数，前面的Phong高光模型，其实是rendering equation在单一光源下针对理想镜面反射的特定推导，对于Phong高光而言：

frs(X, Wi, Wo) = Ks (dot(N, H)^Ns  / dot(N, Wi)

基于GLSL的实现（所有光照和材质参数在实际应用时，应该从常量改为uniform，由外部进行控制）：

// 在顶点着色器中处理光照计算（环境光+漫射光+镜面反射）
const char* ccPositionTextureColorForLight1_v = " \
uniform vec2 translate; \n\
\n\
attribute vec4 a_position; \n\
attribute vec4 a_color; \n\
attribute vec2 a_texCoord; \n\
attribute vec3 a_normal; \n\
\n\
#ifdef GL_ES \n\
varying mediump vec4 v_color; \n\
varying mediump vec2 v_texCoord; \n\
#else \n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
#endif \n\
\n\
const vec3 lightAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightSpecular = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialSpecular = vec3(1.0, 1.0, 1.0); \n\
const int shininess = 8; // 0-128 \n\
\n\
const vec3 eyePosition = vec3(512.0, 128.0, 442.0); \n\
const vec3 lightDirection = vec3(1.0, 1.0, 1.0); \n\
\n\
void main() { \n\
vec4 position = CC_MVMatrix * a_position; \n\
position.x += translate.x; \n\
position.y += translate.y; \n\
gl_Position = CC_PMatrix * position; \n\
\n\
vec3 normal = (CC_MVMatrix * vec4(a_normal, 1.0)).xyz; \n\
vec3 n_normal = normalize(normal); \n\
vec3 n_lightDirection = normalize(lightDirection); \n\
float cosNL = max(dot(n_normal, n_lightDirection), 0.0); \n\
vec4 diffuse = vec4(lightDiffuse * materialDiffuse, 1.0) * cosNL; \n\
\n\
vec4 ambient = vec4(lightAmbient * materialAmbient, 1.0); \n\
\n\
vec3 n_eyePosition = normalize(eyePosition - position.xyz); \n\
vec3 reflection = 2 * max(dot(n_normal, n_lightDirection), 0.0) * n_normal - n_lightDirection; \n\
vec4 specular = vec4(lightSpecular * materialSpecular * pow(max(dot(n_eyePosition, reflection), 0.0), shininess), 1.0); \n\
\n\
v_color = a_color * (diffuse + ambient + specular); \n\
v_texCoord = a_texCoord; \n\
} \n\
";
const char* ccPositionTextureColorForLight1_f = " \
#ifdef GL_ES \n\
precision lowp float; \n\
#endif \n\
\n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
\n\
void main() { \n\
vec4 color = v_color * texture2D(CC_Texture0, v_texCoord); \n\
color.a = 1.0; \n\
\n\
gl_FragColor = color; \n\
} \n\
";

效果：

// 在片段着色器中处理光照计算（环境光+漫反射+镜面反射）
const char* ccPositionTextureColorForLight2_v = " \
uniform vec2 translate; \n\
\n\
attribute vec4 a_position; \n\
attribute vec4 a_color; \n\
attribute vec2 a_texCoord; \n\
attribute vec3 a_normal; \n\
\n\
#ifdef GL_ES \n\
varying mediump vec4 v_position; \n\
varying mediump vec4 v_color; \n\
varying mediump vec2 v_texCoord; \n\
varying mediump vec3 v_normal; \n\
#else \n\
varying vec4 v_position; \n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
varying vec3 v_normal; \n\
#endif \n\
\n\
void main() { \n\
vec4 position = CC_MVMatrix * a_position; \n\
position.x += translate.x; \n\
position.y += translate.y; \n\
gl_Position = CC_PMatrix * position; \n\
\n\
v_position = position; \n\
v_color = a_color; \n\
v_texCoord = a_texCoord; \n\
v_normal = (CC_MVMatrix * vec4(a_normal, 1.0)).xyz; \n\
} \n\
";
const char* ccPositionTextureColorForLight2_f = " \
const vec3 lightAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightSpecular = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialSpecular = vec3(1.0, 1.0, 1.0); \n\
const int shininess = 8; // 0-128 \n\
\n\
const vec3 eyePosition = vec3(512.0, 128.0, 442.0); \n\
const vec3 lightDirection = vec3(1.0, 1.0, 1.0); \n\
\n\
#ifdef GL_ES \n\
precision lowp float; \n\
#endif \n\
\n\
varying vec4 v_position; \n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
varying vec3 v_normal; \n\
\n\
void main() { \n\
vec4 color = v_color * texture2D(CC_Texture0, v_texCoord); \n\
color.a = 1.0; \n\
\n\
vec3 normal = v_normal; \n\
vec3 n_normal = normalize(normal); \n\
vec3 n_lightDirection = normalize(lightDirection); \n\
float cosNL = max(dot(n_normal, n_lightDirection), 0.0); \n\
vec4 diffuse = vec4(lightDiffuse * materialDiffuse, 1.0) * cosNL; \n\
\n\
vec4 ambient = vec4(lightAmbient * materialAmbient, 1.0); \n\
\n\
vec3 n_eyeDirection = normalize(eyePosition - v_position.xyz); \n\
vec3 reflection = 2 * max(dot(n_normal, n_lightDirection), 0.0) * n_normal - n_lightDirection; \n\
vec4 specular = vec4(lightSpecular * materialSpecular * pow(max(dot(n_eyeDirection, reflection), 0.0), shininess), 1.0); \n\
//vec3 reflection = normalize(n_eyeDirection + n_lightDirection); \n\
//vec4 specular = vec4(lightSpecular * materialSpecular * pow(max(dot(n_normal, reflection), 0.0), shininess), 1.0); \n\
\n\
gl_FragColor = color * (ambient + diffuse + specular); \n\
} \n\
";

效果：

// 在片段着色器中处理光照计算（环境光+漫反射+镜面反射+衰减系数）
const char* ccPositionTextureColorForLight3_v = " \
uniform vec2 translate; \n\
\n\
attribute vec4 a_position; \n\
attribute vec4 a_color; \n\
attribute vec2 a_texCoord; \n\
attribute vec3 a_normal; \n\
\n\
#ifdef GL_ES \n\
varying mediump vec4 v_position; \n\
varying mediump vec4 v_color; \n\
varying mediump vec2 v_texCoord; \n\
varying mediump vec3 v_normal; \n\
varying float v_distance; \n\
#else \n\
varying vec4 v_position; \n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
varying vec3 v_normal; \n\
varying float v_distance; \n\
#endif \n\
\n\
void main() { \n\
vec4 position = CC_MVMatrix * a_position; \n\
position.x += translate.x; \n\
position.y += translate.y; \n\
gl_Position = CC_PMatrix * position; \n\
\n\
v_position = position; \n\
v_color = a_color; \n\
v_texCoord = a_texCoord; \n\
v_normal = (CC_MVMatrix * vec4(a_normal, 1.0)).xyz; \n\
} \n\
";
const char* ccPositionTextureColorForLight3_f = " \
const vec3 lightAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightSpecular = vec3(1.0, 1.0, 1.0); \n\
const float lightAttenuationConst = 1.0; \n\
const float lightAttenuationLinear = 0.0002; \n\
const float lightAttenuationQuadratic = 0.000001; \n\
const vec3 materialAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialSpecular = vec3(1.0, 1.0, 1.0); \n\
const int shininess = 8; // 0-128 \n\
\n\
const vec3 lightPosition = vec3(1024.0, 512.0, 100.0); \n\
const vec3 eyePosition = vec3(512.0, 256.0, 442.0); \n\
\n\
#ifdef GL_ES \n\
precision lowp float; \n\
#endif \n\
\n\
varying vec4 v_position; \n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
varying vec3 v_normal; \n\
varying float v_distance; \n\
\n\
void main() { \n\
vec4 color = v_color * texture2D(CC_Texture0, v_texCoord); \n\
color.a = 1.0; \n\
\n\
vec3 lightDirection = lightPosition - v_position.xyz; \n\
float distance = length(lightDirection); \n\
float atten = 1.0 / (lightAttenuationConst + lightAttenuationLinear * distance + lightAttenuationQuadratic * distance * distance); \n\
\n\
vec3 normal = v_normal; \n\
vec3 n_normal = normalize(normal); \n\
vec3 n_lightDirection = normalize(lightDirection); \n\
float cosNL = max(dot(n_normal, n_lightDirection), 0.0); \n\
vec4 diffuse = vec4(lightDiffuse * materialDiffuse, 1.0) * cosNL; \n\
\n\
vec4 ambient = vec4(lightAmbient * materialAmbient, 1.0); \n\
\n\
vec3 n_eyeDirection = normalize(eyePosition - v_position.xyz); \n\
vec3 reflection = 2 * max(dot(n_normal, n_lightDirection), 0.0) * n_normal - n_lightDirection; \n\
vec4 specular = vec4(lightSpecular * materialSpecular * pow(max(dot(n_eyeDirection, reflection), 0.0), shininess), 1.0); \n\
//vec3 reflection = normalize(n_eyeDirection + n_lightDirection); \n\
//vec4 specular = vec4(lightSpecular * materialSpecular * pow(max(dot(n_normal, reflection), 0.0), shininess), 1.0); \n\
\n\
float colorA = color.a; \n\
gl_FragColor = vec4(color.rgb * (ambient + diffuse + specular).xyz * atten, colorA); \n\
} \n\
";

效果：

// 在片段着色器中处理光照计算（环境光+漫反射+镜面反射+聚光灯+衰减系数）
const char* ccPositionTextureColorForLight4_v = " \
uniform vec2 translate; \n\
const vec3 spotlightPosition = vec3(512.0, 256.0, 120.0); \n\
const vec3 lightPosition = vec3(512.0, 256.0, 442.0); \n\
const vec3 eyePosition = vec3(512.0, 256.0, 442.0); \n\
\n\
attribute vec4 a_position; \n\
attribute vec4 a_color; \n\
attribute vec2 a_texCoord; \n\
attribute vec3 a_normal; \n\
\n\
#ifdef GL_ES \n\
varying mediump vec4 v_position; \n\
varying mediump vec4 v_color; \n\
varying mediump vec2 v_texCoord; \n\
varying mediump vec3 v_normal; \n\
varying mediump float v_distance; \n\
varying mediump vec3 v_lightDirection; \n\
varying mediump vec3 v_spotlightDirection; \n\
#else \n\
varying vec4 v_position; \n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
varying vec3 v_normal; \n\
varying float v_distance; \n\
varying vec3 v_lightDirection; \n\
varying vec3 v_spotlightDirection; \n\
#endif \n\
\n\
void main() { \n\
vec4 position = CC_MVMatrix * a_position; \n\
position.x += translate.x; \n\
position.y += translate.y; \n\
gl_Position = CC_PMatrix * position; \n\
\n\
v_position = position; \n\
v_color = a_color; \n\
v_texCoord = a_texCoord; \n\
v_normal = (CC_MVMatrix * vec4(a_normal, 1.0)).xyz; \n\
\n\
vec3 lightDirection = lightPosition - position.xyz; \n\
vec3 spotlightDirection = spotlightPosition - position.xyz; \n\
v_distance = length(lightDirection); \n\
v_lightDirection = normalize(lightDirection); \n\
v_spotlightDirection = normalize(spotlightDirection); \n\
} \n\
";
const char* ccPositionTextureColorForLight4_f = " \
const vec3 lightAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 lightSpecular = vec3(1.0, 1.0, 1.0); \n\
const float lightAttenuationConst = 1.0; \n\
const float lightAttenuationLinear = 0.001; \n\
const float lightAttenuationQuadratic = 0.000001; \n\
const vec3 materialAmbient = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialDiffuse = vec3(1.0, 1.0, 1.0); \n\
const vec3 materialSpecular = vec3(1.0, 1.0, 1.0); \n\
const int shininess = 8; // 0-128 \n\
const int lightSpotExponent = 1; \n\
const float lightSpotCosCutoff = cos(45.0 / 180.0 * 3.1415926); \n\
\n\
const vec3 lightPosition = vec3(512.0, 256.0, 442.0); \n\
const vec3 eyePosition = vec3(512.0, 256.0, 442.0); \n\
\n\
#ifdef GL_ES \n\
precision lowp float; \n\
#endif \n\
\n\
varying vec4 v_position; \n\
varying vec4 v_color; \n\
varying vec2 v_texCoord; \n\
varying vec3 v_normal; \n\
varying float v_distance; \n\
varying vec3 v_lightDirection; \n\
varying vec3 v_spotlightDirection; \n\
\n\
void main() { \n\
vec4 color = v_color * texture2D(CC_Texture0, v_texCoord); \n\
color.a = 1.0; \n\
\n\
// vec3 lightDirection = lightPosition - v_position.xyz; \n\
vec3 n_spotDirection = normalize(lightPosition - vec3(512.0, 256.0, 0.0)); \n\
vec3 n_spotlightDirection = normalize(v_spotlightDirection); \n\
// float distance = length(lightDirection); \n\
float atten = 1.0 / (lightAttenuationConst + lightAttenuationLinear * v_distance + lightAttenuationQuadratic * v_distance * v_distance); \n\
\n\
vec3 normal = v_normal; \n\
vec3 n_normal = normalize(normal); \n\
vec3 n_lightDirection = normalize(v_lightDirection); \n\
float cosNL = max(dot(n_normal, n_lightDirection), 0.0); \n\
vec4 diffuse = vec4(lightDiffuse * materialDiffuse, 1.0) * cosNL; \n\
\n\
vec4 ambient = vec4(lightAmbient * materialAmbient, 1.0); \n\
\n\
vec3 n_eyeDirection = normalize(eyePosition - v_position.xyz); \n\
vec3 reflection = 2 * max(dot(n_normal, n_lightDirection), 0.0) * n_normal - n_lightDirection; \n\
vec4 specular = vec4(lightSpecular * materialSpecular * pow(max(dot(n_eyeDirection, reflection), 0.0), shininess), 1.0); \n\
//vec3 reflection = normalize(n_eyeDirection + n_lightDirection); \n\
//vec4 specular = vec4(lightSpecular * materialSpecular * pow(max(dot(n_normal, reflection), 0.0), shininess), 1.0); \n\
\n\
float cosSpot = dot(-1 * n_spotDirection, -1 * n_spotlightDirection); \n\
if (cosSpot > lightSpotCosCutoff) \n\
{ \n\
float spotEffect = pow(cosSpot, lightSpotExponent); \n\
atten += spotEffect; \n\
} \n\
float colorA = color.a; \n\
gl_FragColor = vec4(color.rgb * (ambient + diffuse + specular).xyz * atten, colorA); \n\
} \n\
";

效果：