本篇教程中，我们来学习聚光灯的的光照效果，聚光灯有光源位置，也会随着传播距离增加而衰减，还有照射方向，另外聚光灯增加的特性是，它的照射范围在一个圆锥内，类似探照灯的效果。
下图显示了聚光灯的效果:
聚光灯的方向用黑色的箭头L表示，而我们的光照效果将会限制在两条红线内。我们可以通过光线方向，和红线的夹角来定义光照范围，对于视线向量V，我们可以用L和V进行点积，如果结果小于cone的角，则不会有聚光灯效果。
lighting_technique.h
<code>struct SpotLight : public PointLight { Vector3f Direction; float Cutoff; SpotLight() { Direction = Vector3f(0.0f, 0.0f, 0.0f); Cutoff = 0.0f; } };</code>
在聚光灯类中，我们定义了2个新的属性，光照方向以及光照圆锥夹角。
lighting_technique.cpp
<code>struct SpotLight { struct PointLight Base; vec3 Direction; float Cutoff; }; ... uniform int gNumSpotLights; ... uniform SpotLight gSpotLights[MAX_SPOT_LIGHTS];</code>
定义多个聚光灯光源。
<code>vec4 CalcPointLight(struct PointLight l, vec3 Normal) { vec3 LightDirection = WorldPos0 - l.Position; float Distance = length(LightDirection); LightDirection = normalize(LightDirection); vec4 Color = CalcLightInternal(l.Base, LightDirection, Normal); float Attenuation = l.Atten.Constant + l.Atten.Linear * Distance + l.Atten.Exp * Distance * Distance; return Color / Attenuation; }</code>
修改点光源shader代码，使用结构作为参数。
<code>vec4 CalcSpotLight(struct SpotLight l, vec3 Normal) { vec3 LightToPixel = normalize(WorldPos0 - l.Base.Position); float SpotFactor = dot(LightToPixel, l.Direction); if (SpotFactor > l.Cutoff) { vec4 Color = CalcPointLight(l.Base, Normal); return Color * (1.0 - (1.0 - SpotFactor) * 1.0/(1.0 - l.Cutoff)); } else { return vec4(0,0,0,0); } }</code>
上面是计算聚光灯效果的函数。
<code>... for (int i = 0 ; i < gNumSpotLights ; i++) { TotalLight += CalcSpotLight(gSpotLights[i], Normal); } ...</code>
和点光源的光照的计算方式相同，对于多光源，我们可以累加聚光灯的效果，从而得到最终像素的光照颜色。
<code>void LightingTechnique::SetSpotLights(unsigned int NumLights, const SpotLight* pLights) { glUniform1i(m_numSpotLightsLocation, NumLights); for (unsigned int i = 0 ; i < NumLights ; i++) { glUniform3f(m_spotLightsLocation[i].Color, pLights[i].Color.x, pLights[i].Color.y, pLights[i].Color.z); glUniform1f(m_spotLightsLocation[i].AmbientIntensity, pLights[i].AmbientIntensity); glUniform1f(m_spotLightsLocation[i].DiffuseIntensity, pLights[i].DiffuseIntensity); glUniform3f(m_spotLightsLocation[i].Position, pLights[i].Position.x, pLights[i].Position.y, pLights[i].Position.z); Vector3f Direction = pLights[i].Direction; Direction.Normalize(); glUniform3f(m_spotLightsLocation[i].Direction, Direction.x, Direction.y, Direction.z); glUniform1f(m_spotLightsLocation[i].Cutoff, cosf(ToRadian(pLights[i].Cutoff))); glUniform1f(m_spotLightsLocation[i].Atten.Constant, pLights[i].Attenuation.Constant); glUniform1f(m_spotLightsLocation[i].Atten.Linear, pLights[i].Attenuation.Linear); glUniform1f(m_spotLightsLocation[i].Atten.Exp, pLights[i].Attenuation.Exp); } }</code>
上面是给uniform变量赋值。
程序执行后效果如下：