OpenGL实现多光源（平行光、点光源、聚光灯）光照效果
发表于2017-05-17
常规的光源类型有三种。最简单的自然是平行光，稍复杂些的为点光源，最复杂的为聚光灯。总体说来，在可编程管线中原理是一样的。在OpenGL代码中传入必要的参数，在shader中进行相关的计算绘制出效果。本文分别介绍三种光源效果的shader实现。
平行光
平行光我们已经写的轻车熟路了。它只需要一个光源方向就够了。通常我们会定义一个光源点，它与坐标轴原点的连线就是光线方向。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | //GL Code float lightPos[] = { 0.0f,1.5f,0.0f,0.0f }; float diffuseLightColor[] = { 1.0f,1.0f,1.0f,1.0f }; glUniformMatrix4fv(gpuProgram.GetLocation("M"), 1,GL_FALSE, glm::value_ptr(model)); glUniformMatrix4fv(gpuProgram.GetLocation("V"), 1, GL_FALSE, identity); glUniformMatrix4fv(gpuProgram.GetLocation("P"), 1, GL_FALSE, glm::value_ptr(projectionMatrix)); glUniformMatrix4fv(gpuProgram.GetLocation("NM"), 1, GL_FALSE, glm::value_ptr(normalMatrix)); glUniform4fv(gpuProgram.GetLocation("U_LightPos"), 1, lightPos); glUniform4fv(gpuProgram.GetLocation("U_DiffuseLightColor"), 1, diffuseLightColor); //fs void main(){ //... vec3 n = normalize(V_Normal); vec3 L = U_LightPos.xyz; float diffuseIntensity=max(0.0,dot(L,n)); vec4 diffuseColor=U_DiffuseLightColor*diffuseIntensity; //... gl_FragColor=ambientColor+diffuseColor; } |
获取的光线方向直接与法线点乘，即为光照强度。
点光源
在平行光的基础上，根据模型与光源之间的距离进行衰减。衰减为与距离相关的二次函数。因此需要加入三个因数，分别为expFactor、linearFactor、constantFactor。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 | //GL Code float constantFactor = 1.0; float linearFactor = 0.2; float expFactor=0.0; glUniform1f(gpuProgram.GetLocation("U_ConstantFactorn"), 1, constantFactor); glUniform1f(gpuProgram.GetLocation("U_LinearFactor"), 1, linearFactor); glUniform1f(gpuProgram.GetLocation("U_ExpFactor"), 1, expFactor); //fs float attenuation = 1.0; void main(){ //... vec3 n = normalize(V_Normal); vec3 L = U_LightPos.xyz - V_WorldPos; float distance = length(L); attenuation = 1.0/(U_ExpFactor*distance*distance + U_LinearFactor*distance + U_ConstantFactor); float diffuseIntensity = max(0.0,dot(L,n)); vec4 diffuseColor = U_DiffuseLightColor*attenuation*diffuseIntensity; //... gl_FragColor=ambientColor+diffuseColor; } |
聚光灯
在点光源的基础上，添加一个带角度的锥形区域的检测。只有在区域内的部分才有照亮处理。因此需要传入shader两个参数，分别为光照方向和范围角度。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 | //GL Code //... float spotLightDirection[] = { 0.0f,-1.0f,0.0f,128.0f }; float spotLightCutoff = 15.0f; glUniform1f(gpuProgram.GetLocation("U_Cutoff"), spotLightCutoff); glUniform4fv(gpuProgram.GetLocation("U_LightDirection"), 1, spotLightDirection); //... //fs float attenuation = 1.0; void main(){ //... float radianCutoff = U_Cutoff*3.14/180; float cosThta = cos(radianCutoff); vec3 spotLightDirection = normalize(U_LightDirection.xyz); vec3 n = normalize(V_Normal); vec3 L = U_LightPos.xyz - V_WorldPos; float distance = length(L); attenuation = 1.0/(U_ExpFactor*distance*distance + U_LinearFactor*distance + U_ConstantFactor); float currentCosThta=max(0.0,dot(-L,spotLightDirection)); float diffuseIntensity = 0.0; if(currentCosThta > cosThta) { if(dot(L,n) >0.0) { diffuseIntensity = pow(currentCosThta,U_LightDirection.w); } } vec4 diffuseColor = U_DiffuseLightColor*attenuation*diffuseIntensity; //... gl_FragColor=ambientColor+diffuseColor; } |
总结
以上为三种光源的OpenGL实现，可以看出，计算量最大的为聚光灯效果，而且为了让边缘柔和，需要pow的大量计算。移动端跑实时运算估计要优化算法了。