opengl 教程(21) 聚光灯

原帖地址：http://ogldev.atspace.co.uk/www/tutorial21/tutorial21.html

     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。

主要代码：

lighting_technique.h

struct SpotLight : public PointLight
{
Vector3f Direction;
float Cutoff;
SpotLight()
{
Direction = Vector3f(0.0f, 0.0f, 0.0f);
Cutoff = 0.0f;
}
};

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

struct SpotLight
{
struct PointLight Base;
vec3 Direction;
float Cutoff;
};
...
uniform int gNumSpotLights;
...
uniform SpotLight gSpotLights[MAX_SPOT_LIGHTS];


定义多个聚光灯光源。

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;
}

修改点光源shader代码，使用结构作为参数。

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);
}
}

上面是计算聚光灯效果的函数。

...
for (int i = 0 ; i < gNumSpotLights ; i++) {
TotalLight += CalcSpotLight(gSpotLights[i], Normal);
}
...

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。

lighting_technique.cpp

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);
}
}

上面是给uniform变量赋值。

程序执行后效果如下：