WebGL shading: diffuse vs. specular

Above you see images of a decorative relief. Move the mouse cursor around: it controls the position of a light source. The relief on the left appears matte, while the relief on the right appears shiny.

The difference is in the shading model. Here are the key lines of the diffuse and specular models:

// Diffuse
vec4 to_light_dir = normalize(light_pos-surface_pos);  
float intensity = dot(normal, to_light_dir);

// Specular
vec4 from_light_dir = normalize(surface_pos-light_pos);  
vec4 reflection_dir = reflect(from_light_dir, normal);
vec4 camera_dir = normalize(vec4(0.0, 0.0, 1.0, 0.0));
float intensity = dot(reflection_dir, camera_dir);

Both models use the surface normal and the light position to calculate the light intensity at a point on the surface. In the diffuse model, a point is lit to the extent that it faces the light source. In the specular model, a point is lit to the extent that it reflects the light source towards the camera.

The specular model uses the built-in function vec4 reflect(vec4 I, vec4 N), which reflects a vector across the normal of a surface. It then uses the dot product to compare this reflection with the camera direction. I don’t know what the formal name is for this shading model!

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