Lighting technique notes

notes.md

Schlick's Approximation of the fresnel factor

https://en.wikipedia.org/wiki/Schlick%27s_approximation

http://filmicworlds.com/blog/everything-has-fresnel/

float NdotL = max( 0.0, dot( PixelNormal, LightDir ) );
float fresnelFactor = SpecularColor + ( 1.0 - SpecularColor ) * pow( ( 1.0 - NdotL ), 5.0 );

For microfacet models :

vec3 halfwayVector = normalize(viewDirection + light);
float HdotL = max( 0.0, dot( halfwayVector, light ) );
float fresnelFactor = SpecularColor + ( 1.0 - SpecularColor ) * pow( ( 1.0 - HdotL ), 5.0 );

Lambert Diffuse

https://www.youtube.com/watch?v=gIeCnJbb_UI

https://en.wikipedia.org/wiki/Lambertian_reflectance

float product = dot(normalize(vNormal), light);
float brightness = max(0.0, product);

Half Lambert Diffuse

https://developer.valvesoftware.com/wiki/Half_Lambert

float product = dot(normalize(vNormal), light);
float brightness = pow(product * 0.5 + 0.5, 2.0);

Generalized Lambert Diffuse

float product = dot(normalize(vNormal), light);
float brightness = pow(max(0.0, product * multiply + add), power);

Oren Nayar Diffuse

Model created for rough surfaces (concrete, plaster, sand, cardboard, etc.).

https://en.wikipedia.org/wiki/Oren%E2%80%93Nayar_reflectance_model

http://www1.cs.columbia.edu/CAVE/projects/oren/

float LdotV = dot(light, viewDirection);
float NdotL = dot(light, surfaceNormal);
float NdotV = dot(surfaceNormal, viewDirection);

float s = LdotV - NdotL * NdotV;
float t = mix(1.0, max(NdotL, NdotV), step(0.0, s));
float sigma2 = roughness * roughness;
float A = 1.0 + sigma2 * (albedo / (sigma2 + 0.13) + 0.5 / (sigma2 + 0.33));
float B = 0.45 * sigma2 / (sigma2 + 0.09);

float brightness = albedo + max(0.0, NdotL) * (A + B * s / t) / PI;

// TOCHECK

Blinn-Phong Specular

https://www.youtube.com/watch?v=hYKK4rIAB48

https://en.wikipedia.org/wiki/Blinn%E2%80%93Phong_shading_model

// vector from the surface pointing to the camera
vec3 directionToCamera = normalize(cameraPosition - vPosition);

// vector between the previous surface-to-camera vector and surface-to-light vector
vec3 halfwayVector = normalize(directionToCamera + light);

float specularBrightness = specularAmount * (
  pow(
    max(0.0, dot(vNormal, halfwayVector)),
    specularShininess
  )
);

Phong Specular

// TODO

Torrance-Sparrow Specular

Model designed for rough surfaces.

// TODO

GGX Specular

http://www.neilblevins.com/cg_education/ggx/ggx.htm

http://www.shlyaev.com/rnd/37-cpp-category/54-ggx

https://github.com/glslify/glsl-ggx/blob/master/index.glsl

https://graphics.rwth-aachen.de:9000/Glow/glow-extras/blob/cfce9b4c4f5b806e4981c0ccad7de0e46edac68c/material/shader/glow-material/material-ggx.glsl

http://filmicworlds.com/blog/optimizing-ggx-shaders-with-dotlh/

http://filmicworlds.com/blog/optimizing-ggx-update/

Beckmann Specular

https://en.wikipedia.org/wiki/Specular_highlight#Beckmann_distribution

https://github.com/glslify/glsl-specular-beckmann

vec3 halfwayVector = normalize(directionToCamera + light);
float NdotH = max(dot(surfaceNormal, halfwayVector), 0.0001);
float cos2Alpha = NdotH * NdotH;
float tan2Alpha = (cos2Alpha - 1.0) / cos2Alpha;
float roughness2 = roughness * roughness;
float denom = 3.141592653589793 * roughness2 * cos2Alpha * cos2Alpha;
return exp(tan2Alpha / roughness2) / denom;

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