// luma.gl
// SPDX-License-Identifier: MIT
// Copyright (c) vis.gl contributors

// Attribution:
// MIT license, Copyright (c) 2016-2017 Mohamad Moneimne and Contributors

// This fragment shader defines a reference implementation for Physically Based Shading of
// a microfacet surface material defined by a glTF model.

// TODO - better do the checks outside of shader

export const fs = /* glsl */ `\
precision highp float;

uniform pbrMaterialUniforms {
  // Material is unlit
  bool unlit;

  // Base color map
  bool baseColorMapEnabled;
  vec4 baseColorFactor;

  bool normalMapEnabled;  
  float normalScale; // #ifdef HAS_NORMALMAP

  bool emissiveMapEnabled;
  vec3 emissiveFactor; // #ifdef HAS_EMISSIVEMAP

  vec2 metallicRoughnessValues;
  bool metallicRoughnessMapEnabled;

  bool occlusionMapEnabled;
  float occlusionStrength; // #ifdef HAS_OCCLUSIONMAP
  
  bool alphaCutoffEnabled;
  float alphaCutoff; // #ifdef ALPHA_CUTOFF
  
  // IBL
  bool IBLenabled;
  vec2 scaleIBLAmbient; // #ifdef USE_IBL
  
  // debugging flags used for shader output of intermediate PBR variables
  // #ifdef PBR_DEBUG
  vec4 scaleDiffBaseMR;
  vec4 scaleFGDSpec;
  // #endif
} pbrMaterial;

// Samplers
#ifdef HAS_BASECOLORMAP
uniform sampler2D pbr_baseColorSampler;
#endif
#ifdef HAS_NORMALMAP
uniform sampler2D pbr_normalSampler;
#endif
#ifdef HAS_EMISSIVEMAP
uniform sampler2D pbr_emissiveSampler;
#endif
#ifdef HAS_METALROUGHNESSMAP
uniform sampler2D pbr_metallicRoughnessSampler;
#endif
#ifdef HAS_OCCLUSIONMAP
uniform sampler2D pbr_occlusionSampler;
#endif
#ifdef USE_IBL
uniform samplerCube pbr_diffuseEnvSampler;
uniform samplerCube pbr_specularEnvSampler;
uniform sampler2D pbr_brdfLUT;
#endif

// Inputs from vertex shader

in vec3 pbr_vPosition;
in vec2 pbr_vUV;

#ifdef HAS_NORMALS
#ifdef HAS_TANGENTS
in mat3 pbr_vTBN;
#else
in vec3 pbr_vNormal;
#endif
#endif

// Encapsulate the various inputs used by the various functions in the shading equation
// We store values in this struct to simplify the integration of alternative implementations
// of the shading terms, outlined in the Readme.MD Appendix.
struct PBRInfo {
  float NdotL;                  // cos angle between normal and light direction
  float NdotV;                  // cos angle between normal and view direction
  float NdotH;                  // cos angle between normal and half vector
  float LdotH;                  // cos angle between light direction and half vector
  float VdotH;                  // cos angle between view direction and half vector
  float perceptualRoughness;    // roughness value, as authored by the model creator (input to shader)
  float metalness;              // metallic value at the surface
  vec3 reflectance0;            // full reflectance color (normal incidence angle)
  vec3 reflectance90;           // reflectance color at grazing angle
  float alphaRoughness;         // roughness mapped to a more linear change in the roughness (proposed by [2])
  vec3 diffuseColor;            // color contribution from diffuse lighting
  vec3 specularColor;           // color contribution from specular lighting
  vec3 n;                       // normal at surface point
  vec3 v;                       // vector from surface point to camera
};

const float M_PI = 3.141592653589793;
const float c_MinRoughness = 0.04;

vec4 SRGBtoLINEAR(vec4 srgbIn)
{
#ifdef MANUAL_SRGB
#ifdef SRGB_FAST_APPROXIMATION
  vec3 linOut = pow(srgbIn.xyz,vec3(2.2));
#else // SRGB_FAST_APPROXIMATION
  vec3 bLess = step(vec3(0.04045),srgbIn.xyz);
  vec3 linOut = mix( srgbIn.xyz/vec3(12.92), pow((srgbIn.xyz+vec3(0.055))/vec3(1.055),vec3(2.4)), bLess );
#endif //SRGB_FAST_APPROXIMATION
  return vec4(linOut,srgbIn.w);;
#else //MANUAL_SRGB
  return srgbIn;
#endif //MANUAL_SRGB
}

// Find the normal for this fragment, pulling either from a predefined normal map
// or from the interpolated mesh normal and tangent attributes.
vec3 getNormal()
{
  // Retrieve the tangent space matrix
#ifndef HAS_TANGENTS
  vec3 pos_dx = dFdx(pbr_vPosition);
  vec3 pos_dy = dFdy(pbr_vPosition);
  vec3 tex_dx = dFdx(vec3(pbr_vUV, 0.0));
  vec3 tex_dy = dFdy(vec3(pbr_vUV, 0.0));
  vec3 t = (tex_dy.t * pos_dx - tex_dx.t * pos_dy) / (tex_dx.s * tex_dy.t - tex_dy.s * tex_dx.t);

#ifdef HAS_NORMALS
  vec3 ng = normalize(pbr_vNormal);
#else
  vec3 ng = cross(pos_dx, pos_dy);
#endif

  t = normalize(t - ng * dot(ng, t));
  vec3 b = normalize(cross(ng, t));
  mat3 tbn = mat3(t, b, ng);
#else // HAS_TANGENTS
  mat3 tbn = pbr_vTBN;
#endif

#ifdef HAS_NORMALMAP
  vec3 n = texture(pbr_normalSampler, pbr_vUV).rgb;
  n = normalize(tbn * ((2.0 * n - 1.0) * vec3(pbrMaterial.normalScale, pbrMaterial.normalScale, 1.0)));
#else
  // The tbn matrix is linearly interpolated, so we need to re-normalize
  vec3 n = normalize(tbn[2].xyz);
#endif

  return n;
}

// Calculation of the lighting contribution from an optional Image Based Light source.
// Precomputed Environment Maps are required uniform inputs and are computed as outlined in [1].
// See our README.md on Environment Maps [3] for additional discussion.
#ifdef USE_IBL
vec3 getIBLContribution(PBRInfo pbrInfo, vec3 n, vec3 reflection)
{
  float mipCount = 9.0; // resolution of 512x512
  float lod = (pbrInfo.perceptualRoughness * mipCount);
  // retrieve a scale and bias to F0. See [1], Figure 3
  vec3 brdf = SRGBtoLINEAR(texture(pbr_brdfLUT,
    vec2(pbrInfo.NdotV, 1.0 - pbrInfo.perceptualRoughness))).rgb;
  vec3 diffuseLight = SRGBtoLINEAR(texture(pbr_diffuseEnvSampler, n)).rgb;

#ifdef USE_TEX_LOD
  vec3 specularLight = SRGBtoLINEAR(texture(pbr_specularEnvSampler, reflection, lod)).rgb;
#else
  vec3 specularLight = SRGBtoLINEAR(texture(pbr_specularEnvSampler, reflection)).rgb;
#endif

  vec3 diffuse = diffuseLight * pbrInfo.diffuseColor;
  vec3 specular = specularLight * (pbrInfo.specularColor * brdf.x + brdf.y);

  // For presentation, this allows us to disable IBL terms
  diffuse *= pbrMaterial.scaleIBLAmbient.x;
  specular *= pbrMaterial.scaleIBLAmbient.y;

  return diffuse + specular;
}
#endif

// Basic Lambertian diffuse
// Implementation from Lambert's Photometria https://archive.org/details/lambertsphotome00lambgoog
// See also [1], Equation 1
vec3 diffuse(PBRInfo pbrInfo)
{
  return pbrInfo.diffuseColor / M_PI;
}

// The following equation models the Fresnel reflectance term of the spec equation (aka F())
// Implementation of fresnel from [4], Equation 15
vec3 specularReflection(PBRInfo pbrInfo)
{
  return pbrInfo.reflectance0 +
    (pbrInfo.reflectance90 - pbrInfo.reflectance0) *
    pow(clamp(1.0 - pbrInfo.VdotH, 0.0, 1.0), 5.0);
}

// This calculates the specular geometric attenuation (aka G()),
// where rougher material will reflect less light back to the viewer.
// This implementation is based on [1] Equation 4, and we adopt their modifications to
// alphaRoughness as input as originally proposed in [2].
float geometricOcclusion(PBRInfo pbrInfo)
{
  float NdotL = pbrInfo.NdotL;
  float NdotV = pbrInfo.NdotV;
  float r = pbrInfo.alphaRoughness;

  float attenuationL = 2.0 * NdotL / (NdotL + sqrt(r * r + (1.0 - r * r) * (NdotL * NdotL)));
  float attenuationV = 2.0 * NdotV / (NdotV + sqrt(r * r + (1.0 - r * r) * (NdotV * NdotV)));
  return attenuationL * attenuationV;
}

// The following equation(s) model the distribution of microfacet normals across
// the area being drawn (aka D())
// Implementation from "Average Irregularity Representation of a Roughened Surface
// for Ray Reflection" by T. S. Trowbridge, and K. P. Reitz
// Follows the distribution function recommended in the SIGGRAPH 2013 course notes
// from EPIC Games [1], Equation 3.
float microfacetDistribution(PBRInfo pbrInfo)
{
  float roughnessSq = pbrInfo.alphaRoughness * pbrInfo.alphaRoughness;
  float f = (pbrInfo.NdotH * roughnessSq - pbrInfo.NdotH) * pbrInfo.NdotH + 1.0;
  return roughnessSq / (M_PI * f * f);
}

void PBRInfo_setAmbientLight(inout PBRInfo pbrInfo) {
  pbrInfo.NdotL = 1.0;
  pbrInfo.NdotH = 0.0;
  pbrInfo.LdotH = 0.0;
  pbrInfo.VdotH = 1.0;
}

void PBRInfo_setDirectionalLight(inout PBRInfo pbrInfo, vec3 lightDirection) {
  vec3 n = pbrInfo.n;
  vec3 v = pbrInfo.v;
  vec3 l = normalize(lightDirection);             // Vector from surface point to light
  vec3 h = normalize(l+v);                        // Half vector between both l and v

  pbrInfo.NdotL = clamp(dot(n, l), 0.001, 1.0);
  pbrInfo.NdotH = clamp(dot(n, h), 0.0, 1.0);
  pbrInfo.LdotH = clamp(dot(l, h), 0.0, 1.0);
  pbrInfo.VdotH = clamp(dot(v, h), 0.0, 1.0);
}

void PBRInfo_setPointLight(inout PBRInfo pbrInfo, PointLight pointLight) {
  vec3 light_direction = normalize(pointLight.position - pbr_vPosition);
  PBRInfo_setDirectionalLight(pbrInfo, light_direction);
}

vec3 calculateFinalColor(PBRInfo pbrInfo, vec3 lightColor) {
  // Calculate the shading terms for the microfacet specular shading model
  vec3 F = specularReflection(pbrInfo);
  float G = geometricOcclusion(pbrInfo);
  float D = microfacetDistribution(pbrInfo);

  // Calculation of analytical lighting contribution
  vec3 diffuseContrib = (1.0 - F) * diffuse(pbrInfo);
  vec3 specContrib = F * G * D / (4.0 * pbrInfo.NdotL * pbrInfo.NdotV);
  // Obtain final intensity as reflectance (BRDF) scaled by the energy of the light (cosine law)
  return pbrInfo.NdotL * lightColor * (diffuseContrib + specContrib);
}

vec4 pbr_filterColor(vec4 colorUnused)
{
  // The albedo may be defined from a base texture or a flat color
#ifdef HAS_BASECOLORMAP
  vec4 baseColor = SRGBtoLINEAR(texture(pbr_baseColorSampler, pbr_vUV)) * pbrMaterial.baseColorFactor;
#else
  vec4 baseColor = pbrMaterial.baseColorFactor;
#endif

#ifdef ALPHA_CUTOFF
  if (baseColor.a < pbrMaterial.alphaCutoff) {
    discard;
  }
#endif

  vec3 color = vec3(0, 0, 0);

  if(pbrMaterial.unlit){
    color.rgb = baseColor.rgb;
  }
  else{
    // Metallic and Roughness material properties are packed together
    // In glTF, these factors can be specified by fixed scalar values
    // or from a metallic-roughness map
    float perceptualRoughness = pbrMaterial.metallicRoughnessValues.y;
    float metallic = pbrMaterial.metallicRoughnessValues.x;
#ifdef HAS_METALROUGHNESSMAP
    // Roughness is stored in the 'g' channel, metallic is stored in the 'b' channel.
    // This layout intentionally reserves the 'r' channel for (optional) occlusion map data
    vec4 mrSample = texture(pbr_metallicRoughnessSampler, pbr_vUV);
    perceptualRoughness = mrSample.g * perceptualRoughness;
    metallic = mrSample.b * metallic;
#endif
    perceptualRoughness = clamp(perceptualRoughness, c_MinRoughness, 1.0);
    metallic = clamp(metallic, 0.0, 1.0);
    // Roughness is authored as perceptual roughness; as is convention,
    // convert to material roughness by squaring the perceptual roughness [2].
    float alphaRoughness = perceptualRoughness * perceptualRoughness;

    vec3 f0 = vec3(0.04);
    vec3 diffuseColor = baseColor.rgb * (vec3(1.0) - f0);
    diffuseColor *= 1.0 - metallic;
    vec3 specularColor = mix(f0, baseColor.rgb, metallic);

    // Compute reflectance.
    float reflectance = max(max(specularColor.r, specularColor.g), specularColor.b);

    // For typical incident reflectance range (between 4% to 100%) set the grazing
    // reflectance to 100% for typical fresnel effect.
    // For very low reflectance range on highly diffuse objects (below 4%),
    // incrementally reduce grazing reflecance to 0%.
    float reflectance90 = clamp(reflectance * 25.0, 0.0, 1.0);
    vec3 specularEnvironmentR0 = specularColor.rgb;
    vec3 specularEnvironmentR90 = vec3(1.0, 1.0, 1.0) * reflectance90;

    vec3 n = getNormal();                          // normal at surface point
    vec3 v = normalize(pbrProjection.camera - pbr_vPosition);  // Vector from surface point to camera

    float NdotV = clamp(abs(dot(n, v)), 0.001, 1.0);
    vec3 reflection = -normalize(reflect(v, n));

    PBRInfo pbrInfo = PBRInfo(
      0.0, // NdotL
      NdotV,
      0.0, // NdotH
      0.0, // LdotH
      0.0, // VdotH
      perceptualRoughness,
      metallic,
      specularEnvironmentR0,
      specularEnvironmentR90,
      alphaRoughness,
      diffuseColor,
      specularColor,
      n,
      v
    );


#ifdef USE_LIGHTS
    // Apply ambient light
    PBRInfo_setAmbientLight(pbrInfo);
    color += calculateFinalColor(pbrInfo, lighting.ambientColor);

    // Apply directional light
    for(int i = 0; i < lighting.directionalLightCount; i++) {
      if (i < lighting.directionalLightCount) {
        PBRInfo_setDirectionalLight(pbrInfo, lighting_getDirectionalLight(i).direction);
        color += calculateFinalColor(pbrInfo, lighting_getDirectionalLight(i).color);
      }
    }

    // Apply point light
    for(int i = 0; i < lighting.pointLightCount; i++) {
      if (i < lighting.pointLightCount) {
        PBRInfo_setPointLight(pbrInfo, lighting_getPointLight(i));
        float attenuation = getPointLightAttenuation(lighting_getPointLight(i), distance(lighting_getPointLight(i).position, pbr_vPosition));
        color += calculateFinalColor(pbrInfo, lighting_getPointLight(i).color / attenuation);
      }
    }
#endif

    // Calculate lighting contribution from image based lighting source (IBL)
#ifdef USE_IBL
    if (pbrMaterial.IBLenabled) {
      color += getIBLContribution(pbrInfo, n, reflection);
    }
#endif

 // Apply optional PBR terms for additional (optional) shading
#ifdef HAS_OCCLUSIONMAP
    if (pbrMaterial.occlusionMapEnabled) {
      float ao = texture(pbr_occlusionSampler, pbr_vUV).r;
      color = mix(color, color * ao, pbrMaterial.occlusionStrength);
    }
#endif

#ifdef HAS_EMISSIVEMAP
    if (pbrMaterial.emissiveMapEnabled) {
      vec3 emissive = SRGBtoLINEAR(texture(pbr_emissiveSampler, pbr_vUV)).rgb * pbrMaterial.emissiveFactor;
      color += emissive;
    }
#endif

    // This section uses mix to override final color for reference app visualization
    // of various parameters in the lighting equation.
#ifdef PBR_DEBUG
    // TODO: Figure out how to debug multiple lights

    // color = mix(color, F, pbr_scaleFGDSpec.x);
    // color = mix(color, vec3(G), pbr_scaleFGDSpec.y);
    // color = mix(color, vec3(D), pbr_scaleFGDSpec.z);
    // color = mix(color, specContrib, pbr_scaleFGDSpec.w);

    // color = mix(color, diffuseContrib, pbr_scaleDiffBaseMR.x);
    color = mix(color, baseColor.rgb, pbrMaterial.scaleDiffBaseMR.y);
    color = mix(color, vec3(metallic), pbrMaterial.scaleDiffBaseMR.z);
    color = mix(color, vec3(perceptualRoughness), pbrMaterial.scaleDiffBaseMR.w);
#endif

  }

  return vec4(pow(color,vec3(1.0/2.2)), baseColor.a);
}
`;