[OpenGL] notes 29. Advanced lighting (specular highlights)

1. technological process

In the traditional Feng's illumination model , The algorithm of specular illumination can achieve better results ：


But the illumination model also has limitations , You can see , An obvious fault appears at the edge of the specular highlight area in the figure below . The reason for this problem is that the angle between the observation vector and the reflection vector cannot be greater than 90 degree . If the result of the dot product is negative , The specular component becomes 0.0.

When calculating diffuse illumination , The angle between the illumination direction and the normal vector exceeds 90° It means that the light comes from the back , It is correct not to show the light at this time , But the calculation of specular light must not be like this , Because the angle between the incident angle of light and the angle of view is greater than 90° It's not against common sense to see specular highlights , Even if the angle of view is far from the direction of reflection , As long as the specular reflection coefficient is small enough , Then its reflection component cannot be ignored , As shown above , To solve this part of the lighting defects , We need to think of another way to approximate ：

That is, as shown in the above figure , The half range vector obtained by adding the angle vector and the incident vector and the normal vector are point multiplied , So the included angle is greater than 90° The light is really invisible , give the result as follows , The edges of the visible mirror light are indeed much smoother ：

Here is a simple example of shader illumination calculation ：

vec3 CalcDirLight(DirLight light, vec3 normal, vec3 viewDir)
{
    
    vec3 lightDir = normalize(-light.direction);
    //  Diffuse shading 
    float diff = max(dot(normal, lightDir), 0.0);
    //  Specular shading 
    vec3 halfDir = normalize(lightDir + viewDir);
    float spec = pow(max(dot(halfDir, normal), 0.0), material.shininess);
    //  Consolidated results 
    vec3 ambient  = light.ambient  * vec3(texture(material.diffuse, TexCoords));
    vec3 diffuse  = light.diffuse  * diff * vec3(texture(material.diffuse, TexCoords));
    vec3 specular = light.specular * spec * vec3(texture(material.specular, TexCoords));
    return (ambient + diffuse + specular);
}