BansheeEngine/Data/Raw/Engine/Includes/LightingCommon.bslinc

#include "$ENGINE$\SurfaceData.bslinc"

Technique
 : base("LightingCommon")
 : inherits("SurfaceData") =
{
	Pass =
	{
		Common = 
		{
			// Arbitrary limit, increase if needed
            #define MAX_LIGHTS 512
		
			#define PI 3.1415926
			#define HALF_PI 1.5707963
			
			struct LightData
			{
				float3 position;
				float attRadius;
				float srcRadius;
				float3 direction;
				float luminance;
				float3 spotAngles;
				float attRadiusSqrdInv;
				float3 color;
				float3 shiftedLightPosition;
			};
			
			float3 calcMicrofacetFresnelShlick(float3 F0, float LoH)
			{
				return F0 + (1.0f - F0) * pow(1.0f - LoH, 5.0f);
			}

			float calcMicrofacetShadowingSmithGGX(float roughness4, float NoV, float NoL)
			{
				// Note: It's probably better to use the joint shadowing + masking version of this function

				// Note: Original GGX G1 multiplied by NoV & NoL (respectively), so that the microfacet function divisor gets canceled out
				// Original formula being (ignoring the factor for masking negative directions):
				//   G1(v) = 2 / (1 + sqrt(1 + roughness^4 * tan^2(v)))
				//
				// Using trig identities: tan = sin/cos & sin^2 + cos^2 = 1
				//   G1(v) = 2 / (1 + sqrt(1 + roughness^4 * (1 - cos^2(v))/cos^2(v)))
				//
				// Multiply by cos(v) so that we cancel out the (NoL * NoV) factor in the microfacet formula divisor
				//   G1(v) = 2 * cos(v) / (cos^2(v) + sqrt(cos^2 + roughness^4 - roughness^4 * cos^2(v)))
				// 
				// Actually do the cancellation:
				//    G1(v) = 2 / (cos^2(v) + sqrt(cos^2 + roughness^4 - roughness^4 * cos^2(v)))
				//
				// Also cancel out the 2 and the 4:
				//    G1(v) = 1 / (cos^2(v) + sqrt(cos^2 + roughness^4 - roughness^4 * cos^2(v)))
				//
				// Final equation being:
				//    G(v, l) = G1(v) * G1(l)
				//
				// Where cos(v) is NoV or NoL
				
				float g1V = NoV + sqrt(NoV * (NoV - NoV * roughness4) + roughness4);
				float g1L = NoL + sqrt(NoL * (NoL - NoL * roughness4) + roughness4);
				return rcp(g1V * g1L);
			}
			
			float calcMicrofacetDistGGX(float roughness4, float NoH)
			{
				float d = (NoH * roughness4 - NoH) * NoH + 1.0f;
				return roughness4 / (PI * d * d);
			}
			
			float3 calcDiffuseLambert(float3 color)
			{
				return color * (1.0f / PI);
			}
			
			float getSpotAttenuation(float3 toLight, LightData lightData)
			{
				float output = saturate((dot(toLight, -lightData.direction) - lightData.spotAngles.y) * lightData.spotAngles.z);
				return output * output;
			}

			// Window function to ensure the light contribution fades out to 0 at attenuation radius
			float getRadialAttenuation(float distance2, LightData lightData)
			{
				float radialAttenuation = distance2 * lightData.attRadiusSqrdInv;
				radialAttenuation *= radialAttenuation;
				radialAttenuation = saturate(1.0f - radialAttenuation);
				radialAttenuation *= radialAttenuation;
				
				return radialAttenuation;
			}			
						
			// Calculates illuminance from a non-area point light
			float illuminancePointLight(float distance2, float NoL, LightData lightData)
			{
				return (lightData.luminance * NoL) / max(distance2, 0.01f*0.01f);
			}
			
			// Calculates illuminance scale for a sphere or a disc area light, while also handling the case when
			// parts of the area light are below the horizon.
			// Input NoL must be unclamped.
			// Sphere solid angle = arcsin(r / d)
			// Right disc solid angle = atan(r / d)
			//   - To compensate for oriented discs, multiply by dot(diskNormal, -L)
			float illuminanceScaleSphereDiskAreaLight(float unclampedNoL, float sinSolidAngleSqrd)
			{
				// Handles parts of the area light below the surface horizon
				// See https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf for reference
				float sinSolidAngle = sqrt(sinSolidAngleSqrd);
				if(unclampedNoL < sinSolidAngle)
				{
					// Hermite spline approximation (see reference for exact formula)
					unclampedNoL = max(unclampedNoL, -sinSolidAngle);
					return ((sinSolidAngle + unclampedNoL) * (sinSolidAngle + unclampedNoL)) / (4 * sinSolidAngle);
				}
				else
					return PI * sinSolidAngleSqrd * saturate(unclampedNoL);
			}

			// Calculates illuminance from a sphere area light.
			float illuminanceSphereAreaLight(float unclampedNoL, float distToLight2, LightData lightData)
			{
				float radius2 = lightData.srcRadius * lightData.srcRadius;
				
				// Squared sine of the sphere solid angle
				float sinSolidAngle2 = radius2 / distToLight2;

				// Prevent divide by zero
				sinSolidAngle2 = min(sinSolidAngle2, 0.9999f);
				
				return lightData.luminance * illuminanceScaleSphereDiskAreaLight(unclampedNoL, sinSolidAngle2);	
			}
			
			// Calculates illuminance from a disc area light.
			float illuminanceDiscAreaLight(float unclampedNoL, float distToLight2, float3 L, LightData lightData)
			{
				// Solid angle for right disk = atan (r / d)
				//  atan (r / d) = asin((r / d)/sqrt((r / d)^2+1))
				//  sinAngle = (r / d)/sqrt((r / d)^2 + 1)
				//  sinAngle^2 = (r / d)^2 / (r / d)^2 + 1
				//             = r^2 / (d^2 + r^2)
			
				float radius2 = lightData.srcRadius * lightData.srcRadius;
				
				// max() to prevent light penetrating object
				float sinSolidAngle2 = saturate(radius2 / (radius2 + max(radius2, distToLight2)));
				
				// Multiply by extra term to somewhat handle the case of the oriented disc (formula above only works
				// for right discs).
				return lightData.luminance * illuminanceScaleSphereDiskAreaLight(unclampedNoL, sinSolidAngle2 * saturate(dot(lightData.direction, -L)));	
			}
		
			// With microfacet BRDF the BRDF lobe is not centered around the reflected (mirror) direction.
			// Because of NoL and shadow-masking terms the lobe gets shifted toward the normal as roughness
			// increases. This is called the "off-specular peak". We approximate it using this function.
			float3 getSpecularDominantDir(float3 N, float3 R, float roughness)
			{
				// Note: Try this formula as well:
				//  float smoothness = 1 - roughness;
				//  return lerp(N, R, smoothness * (sqrt(smoothness) + roughness));
			
				float r2 = roughness * roughness;
				return normalize(lerp(N, R, (1 - r2) * (sqrt(1 - r2) + r2)));
			}		
			
			float3 getSurfaceShading(float3 V, float3 L, float specLobeEnergy, SurfaceData surfaceData)
			{
				float3 N = surfaceData.worldNormal.xyz;

				float3 H = normalize(V + L);
				float LoH = saturate(dot(L, H));
				float NoH = saturate(dot(N, H));
				float NoV = saturate(dot(N, V));
				float NoL = saturate(dot(N, L));
				
				float3 diffuseColor = lerp(surfaceData.albedo.rgb, float3(0.0f, 0.0f, 0.0f), 1.0f - surfaceData.metalness);
				
				// Note: Using a fixed F0 value of 0.04 (plastic) for dielectrics, and using albedo as specular for conductors.
				// For more customizability allow the user to provide separate albedo/specular colors for both types.
				float3 specularColor = lerp(float3(0.04f, 0.04f, 0.04f), surfaceData.albedo.rgb, surfaceData.metalness);
				
				float3 diffuse = calcDiffuseLambert(diffuseColor);
				
				float roughness = max(surfaceData.roughness, 0.04f); // Prevent NaNs
				float roughness2 = roughness * roughness;
				float roughness4 = roughness2 * roughness2;
				
				float3 specular = calcMicrofacetFresnelShlick(specularColor, LoH) * 
					calcMicrofacetDistGGX(roughness4, NoH) *
					calcMicrofacetShadowingSmithGGX(roughness4, NoV, NoL);
				
				// Note: Need to add energy conservation between diffuse and specular terms?
				return diffuse + specular * specLobeEnergy;
			}	
			
			StructuredBuffer<LightData> gLights;
			
			#ifdef USE_COMPUTE_INDICES
				groupshared uint gLightIndices[MAX_LIGHTS];
			#endif
			#ifdef USE_LIGHT_GRID_INDICES
				Buffer<uint> gLightIndices;
			#endif
			
			float4 getDirectLighting(float3 worldPos, float3 V, float3 R, SurfaceData surfaceData, uint4 lightOffsets)
			{
				float3 N = surfaceData.worldNormal.xyz;
				float roughness2 = max(surfaceData.roughness, 0.08f);
				roughness2 *= roughness2;
				
				float3 outLuminance = 0;
				float alpha = 0.0f;
				if(surfaceData.worldNormal.w > 0.0f)
				{
					// Handle directional lights
					for(uint i = 0; i < lightOffsets.x; ++i)
					{
						LightData lightData = gLights[i];
					
						float3 L = -lightData.direction;
						float NoL = saturate(dot(N, L));
						float specEnergy = 1.0f;
						
						// Distant disk area light. Calculate its contribution analytically by
						// finding the most important (least error) point on the area light and
						// use it as a form of importance sampling.
						if(lightData.srcRadius > 0)
						{
							float diskRadius = sin(lightData.srcRadius);
							float distanceToDisk = cos(lightData.srcRadius);
							
							// Closest point to disk (approximation for distant disks)
							float DoR = dot(L, R);
							float3 S = normalize(R - DoR * L);
							L = DoR < distanceToDisk ? normalize(distanceToDisk * L + S * diskRadius) : R;
						}
						
						float3 surfaceShading = getSurfaceShading(V, L, specEnergy, surfaceData);
						float illuminance = lightData.luminance * NoL;
						outLuminance += lightData.color * illuminance * surfaceShading;
					}
					
					// Handle radial lights
                    for (uint i = lightOffsets.y; i < lightOffsets.z; ++i)
                    {
                        uint lightIdx = gLightIndices[i];
						LightData lightData = gLights[lightIdx];
                        
						float3 toLight = lightData.position - worldPos;
						float distToLightSqrd = dot(toLight, toLight);
						float invDistToLight = rsqrt(distToLightSqrd);
						
						float3 L = toLight * invDistToLight;
						float NoL = dot(N, L);
						
						float specEnergy = 1.0f;
						float illuminance = 0.0f;

						// Sphere area light. Calculate its contribution analytically by
						// finding the most important (least error) point on the area light and
						// use it as a form of importance sampling.
						if(lightData.srcRadius > 0)
						{
							// Calculate illuminance depending on source size, distance and angle
							illuminance = illuminanceSphereAreaLight(NoL, distToLightSqrd, lightData);	

							// Energy conservation:
							//    We are widening the specular distribution by the sphere's subtended angle, 
							//    so we need to handle the increase in energy. It is not enough just to account
							//    for the sphere solid angle, since the energy difference is highly dependent on
							//    specular distribution. By accounting for this energy difference we ensure glossy
							//    reflections have sharp edges, instead of being too blurry.
							//    See http://blog.selfshadow.com/publications/s2013-shading-course/karis/s2013_pbs_epic_notes_v2.pdf for reference
							float sphereAngle = saturate(lightData.srcRadius * invDistToLight);
							
							specEnergy = roughness2 / saturate(roughness2 + 0.5f * sphereAngle);
							specEnergy *= specEnergy;							
						
							// Find closest point on sphere to ray
							float3 closestPointOnRay = dot(toLight, R) * R;
							float3 centerToRay = closestPointOnRay - toLight;
							float invDistToRay = rsqrt(dot(centerToRay, centerToRay));
							float3 closestPointOnSphere = toLight + centerToRay * saturate(lightData.srcRadius * invDistToRay);
							
							toLight = closestPointOnSphere;
							L = normalize(toLight);
						}
						else
						{
							NoL = saturate(NoL);
							illuminance = illuminancePointLight(distToLightSqrd, NoL, lightData);
						}
						
						float attenuation = getRadialAttenuation(distToLightSqrd, lightData);
						float3 surfaceShading = getSurfaceShading(V, L, specEnergy, surfaceData);
							
						outLuminance += lightData.color * illuminance * attenuation * surfaceShading;
                    }

					// Handle spot lights
					for(uint i = lightOffsets.z; i < lightOffsets.w; ++i)
                    {
                        uint lightIdx = gLightIndices[i];
						LightData lightData = gLights[lightIdx];
						
						float3 toLight = lightData.position - worldPos;
						float distToLightSqrd = dot(toLight, toLight);
						float invDistToLight = rsqrt(distToLightSqrd);
						
						float3 L = toLight * invDistToLight;
						float NoL = dot(N, L);
						
						float specEnergy = 1.0f;
						float illuminance = 0.0f;
						float spotAttenuation = 1.0f;
						
						// Disc area light. Calculate its contribution analytically by
						// finding the most important (least error) point on the area light and
						// use it as a form of importance sampling.
						if(lightData.srcRadius > 0)
						{
							// Calculate illuminance depending on source size, distance and angle
							illuminance = illuminanceDiscAreaLight(NoL, distToLightSqrd, L, lightData);	
						
							// Energy conservation: Similar case as with radial lights
							float rightDiscAngle = saturate(lightData.srcRadius * invDistToLight);
							
							// Account for disc orientation somewhat
							float discAngle = rightDiscAngle * saturate(dot(lightData.direction, -L));
							
							specEnergy = roughness2 / saturate(roughness2 + 0.5f * discAngle);
							specEnergy *= specEnergy;							
						
							// Find closest point on disc to ray
							float3 discNormal = -lightData.direction;
							float distAlongLightDir = max(dot(R, discNormal), 1e-6f);
							float t = dot(toLight, discNormal) / distAlongLightDir;
							float3 closestPointOnPlane = R * t; // Relative to shaded world point
							
							float3 centerToRay = closestPointOnPlane - toLight;
							float invDistToRay = rsqrt(dot(centerToRay, centerToRay));
							float3 closestPointOnDisc = toLight + centerToRay * saturate(lightData.srcRadius * invDistToRay);

							toLight = closestPointOnDisc;
							L = normalize(toLight);
							
							// Expand spot attenuation by disc radius (not physically based)
							float3 toSpotEdge = normalize(lightData.shiftedLightPosition - worldPos);
							spotAttenuation = getSpotAttenuation(toSpotEdge, lightData);
							
							// TODO - Spot attenuation fades out the specular highlight in a noticeable way
						}
						else
						{
							NoL = saturate(NoL);
							illuminance = illuminancePointLight(distToLightSqrd, NoL, lightData);
							
							spotAttenuation = getSpotAttenuation(L, lightData);
						}
						
						float radialAttenuation = getRadialAttenuation(distToLightSqrd, lightData);
						float attenuation = spotAttenuation * radialAttenuation;
						float3 surfaceShading = getSurfaceShading(V, L, specEnergy, surfaceData);
							
						outLuminance += lightData.color * illuminance * attenuation * surfaceShading;
                    }
					
					// Ambient term for in-editor visualization, not used in actual lighting
					outLuminance += surfaceData.albedo.rgb * gAmbientFactor / PI;
					alpha = 1.0f;
				}
				
				return float4(outLuminance, alpha);
			}
		};
	};
};