OpenGL進階(十三) - GLSL光照（Lighting）

提要

       在上一篇文章中，我們介紹了簡單的Shading，同時提出了一個光照模型，模擬了一個點光源，但是，關于光的故事還沒有結束... 

       今天要學習的是方向光源（Directional Light），聚光燈，per pixel shading，halfway vector。

      關于光源的原理及數學描述，請參考：光線追蹤(RayTracing)算法理論與實踐(三)光照

方向光源

      方向光源就兩個參數，方向和強度。

      還是簡單的 ambient + diffuse + spec 光照模型。先看shader的代碼。

basic.vert

#version 400 layout (location = 0) in vec3 VertexPosition;   layout (location = 1) in vec3 VertexNormal;    out vec3 LightIntensity;  struct LightInfo{ 	vec4 Direction; 	vec3 Intensity; };  struct MaterialInfo{ 	vec3 Ka; 	vec3 Kd; 	vec3 Ks; 	float Shininess; };  uniform LightInfo Light; uniform	MaterialInfo Material;  uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP;   void getEyeSpace(out vec3 norm, out vec4 position) { 	norm =  normalize(NormalMatrix * VertexNormal); 	position = ModelViewMatrix * vec4(VertexPosition, 1.0); }    vec3 ads(vec4 position, vec3 norm) { 	vec3 s; 	if(Light.Direction.w == 0.0) 		s = normalize(vec3(Light.Direction)); 	else 		s = normalize(vec3(Light.Direction - position)); 	vec3 v = normalize(vec3(-position)); 	vec3 r = reflect(-s, norm);  	return Light.Intensity * (Material.Ka + Material.Kd*max(dot(s,norm), 0.0) +  	       Material.Ks * pow(max(dot(r,v),0.0), Material.Shininess)); }  void main() { 	vec3 eyeNorm; 	vec4 eyePosition; 	getEyeSpace(eyeNorm, eyePosition); 	LightIntensity = ads(eyePosition, eyeNorm); 	 	gl_Position = MVP * vec4( VertexPosition, 1.0); }

接下來的ads用來計算光照模型下頂點的顏色,分別計算三個分量，然后相加。

basic.frag

#version 400  in vec3 LightIntensity;  void main(void) { 	gl_FragColor = vec4(LightIntensity, 1.0); 	//gl_FragColor = vec4(1.0,1.0,0.1, 1.0); }

void CGL::setUniform() {     mat4 projection = glm::perspective(45.0f, 4.0f / 3.0f, 0.1f, 100.0f);     mat4 mv = view * model;      prog.setUniform("Material.Kd", 0.9f, 0.5f, 0.3f);     prog.setUniform("Material.Ka", 0.9f, 0.5f, 0.3f);     prog.setUniform("Material.Ks", 0.8f, 0.8f, 0.8f);     prog.setUniform("Material.Shininess", 100.0f);     prog.setUniform("Light.Direction", vec4(1.0f, 0.0f, 0.0f, 0.0f));     prog.setUniform("Light.Intensity", 1.0f, 1.0f, 1.0f);      prog.setUniform("ModelViewMatrix", mv);     prog.setUniform("NormalMatrix",mat3( vec3(mv[0]), vec3(mv[1]), vec3(mv[2]) ));     prog.setUniform("MVP", projection * mv);  }

聚光燈 Spotlight

        這里的采用一個最簡單的聚光燈模型：

       燈光的屬性有：位置，強度，方向，衰減（exponent），裁剪角度。

       實現起來也比較簡單，在投射角內的物體，渲染方式和點光源的計算一樣，投射角之外的頂點，著色的時候只有ambient。

        還是采用我們比較熟悉的per vertex shading 方式。在vert中定義聚光燈：

//baisc.vert #version 400 layout (location = 0) in vec3 VertexPosition;   layout (location = 1) in vec3 VertexNormal;    out vec3 LightIntensity;  struct SpotLightInfo{ 	vec4 position; 	vec3 direction; 	vec3 intensity; 	float exponent; 	float cutoff; };  struct MaterialInfo{ 	vec3 Ka; 	vec3 Kd; 	vec3 Ks; 	float Shininess; };  uniform SpotLightInfo Spot; uniform	MaterialInfo Material;  uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP;   void getEyeSpace(out vec3 norm, out vec4 position) { 	norm =  normalize(NormalMatrix * VertexNormal); 	position = ModelViewMatrix * vec4(VertexPosition, 1.0); }    vec3 adsSpotLight(vec4 position, vec3 norm) { 	vec3 s = normalize(vec3(Spot.position - position)); 	float angle = acos(dot(-s, normalize(Spot.direction))); 	float cutoff = radians(clamp(Spot.cutoff, 0.0, 90.0)); 	vec3 ambient = Spot.intensity * Material.Ka; 	 	if(angle < cutoff){ 		float spotFactor = pow(dot(-s, normalize(Spot.direction)), Spot.exponent); 		vec3 v = normalize(vec3(-position)); 		vec3 h = normalize(v + s); 		return ambient + spotFactor * Spot.intensity * (Material.Kd * max(dot(s, norm),0.0) 		      + Material.Ks * pow(max(dot(h,norm), 0.0),Material.Shininess));  	} 	else 	{ 		return ambient;  	} 	        }  void main() { 	vec3 eyeNorm; 	vec4 eyePosition; 	getEyeSpace(eyeNorm, eyePosition); 	 	LightIntensity = adsSpotLight(eyePosition, eyeNorm); 	 	gl_Position = MVP * vec4( VertexPosition, 1.0); }

genType clamp( genType x, genType minVal, genType maxVal);

獲取三個數中第二大的數。

genType radians(genType degrees);

將角度轉換成弧度。

adsSpotLight是主要的函數，先計算頂點和光源方向之間的夾角，判斷頂點是否在照射的區域，然后分別求得最終的顏色。

片段shader還是那樣:

#version 400  in vec3 LightIntensity;  void main(void) { 	gl_FragColor = vec4(LightIntensity, 1.0); }

void CGL::setUniform() {     //model = glm::rotate(this->model, 10.0f, vec3(0.0f,1.0f,0.0f));     mat4 projection = glm::perspective(45.0f, 4.0f / 3.0f, 0.1f, 100.0f);     mat4 mv = view * model;     mat3 normalMatrix = mat3( vec3(view[0]), vec3(view[1]), vec3(view[2]) );      prog.setUniform("Material.Kd", 0.9f, 0.5f, 0.3f);     prog.setUniform("Material.Ka", 0.9f, 0.5f, 0.3f);     prog.setUniform("Material.Ks", 0.8f, 0.8f, 0.8f);     prog.setUniform("Material.Shininess", 100.0f);      vec4 lightPos = vec4(1.0f, 5.0f, 20.0f, 1.0f);    // prog.setUniform("Spot.position", lightPos);     prog.setUniform("Spot.position", view * lightPos);     prog.setUniform("Spot.direction", normalMatrix * vec3(-10.0,0.0,-40.0) );     //prog.setUniform("Spot.direction", vec3(10.9f,10.9f,10.9f)  );     prog.setUniform("Spot.intensity", vec3(0.9f,0.9f,0.9f) );     prog.setUniform("Spot.exponent", 30.0f );     prog.setUniform("Spot.cutoff", 15.0f );      prog.setUniform("ModelViewMatrix", mv);     prog.setUniform("NormalMatrix",normalMatrix);     prog.setUniform("MVP", projection * mv);  } 

最終效果如下：

逐像素著色 per pixel shading

        相對與前面將主要計算工作放在頂點shader中的 per vertex shading ，per pixel shading 指的是將計算放到片段shader中，這樣可以帶來更加真實可感的渲染效果。

basic2.vert

#version 400 layout (location = 0) in vec3 VertexPosition;   layout (location = 1) in vec3 VertexNormal;    out vec4 Position; out vec3 Normal;   uniform mat4 ModelViewMatrix; uniform mat3 NormalMatrix; uniform mat4 ProjectionMatrix; uniform mat4 MVP;   void getEyeSpace(out vec3 norm, out vec4 position) { 	norm =  normalize(NormalMatrix * VertexNormal); 	position = ModelViewMatrix * vec4(VertexPosition, 1.0); }   void main() { 	getEyeSpace(Normal, Position); 	gl_Position = MVP * vec4( VertexPosition, 1.0); }

#version 400  in vec4 Position; in vec3 Normal;  struct SpotLightInfo{ 	vec4 position; 	vec3 direction; 	vec3 intensity; 	float exponent; 	float cutoff; };  struct MaterialInfo{ 	vec3 Ka; 	vec3 Kd; 	vec3 Ks; 	float Shininess; };  uniform SpotLightInfo Spot; uniform	MaterialInfo Material;  vec3 adsSpotLight(vec4 position, vec3 norm) { 	vec3 s = normalize(vec3(Spot.position - position)); 	float angle = acos(dot(-s, normalize(Spot.direction))); 	float cutoff = radians(clamp(Spot.cutoff, 0.0, 90.0)); 	vec3 ambient = Spot.intensity * Material.Ka; 	 	if(angle < cutoff){ 		float spotFactor = pow(dot(-s, normalize(Spot.direction)), Spot.exponent); 		vec3 v = normalize(vec3(-position)); 		vec3 h = normalize(v + s); 		return ambient + spotFactor * Spot.intensity * (Material.Kd * max(dot(s, norm),0.0) 		      + Material.Ks * pow(max(dot(h,norm), 0.0),Material.Shininess));  	} 	else 	{ 		return ambient;  	} 	        }  void main(void) { 	gl_FragColor = vec4(adsSpotLight(Position, Normal), 1.0); 	//gl_FragColor = vec4(1.0,1.0,0.5, 1.0); }

最終的效果還是有一些差別，特別是光線交界的地方。

代碼下載

OpenGLPro13

參考

OpenGL 4.0 Shading Language Cookbook