float3 WrinkledNormal(Texture2D baseTex,
                      Texture2D wrinkleTex,
                      Texture2D maskTex[2],
                      float4 weights[2],
                      float2 texcoord) {
  // Fetch and unpack base normal (3Dc format):
  float4 base;
  base.xy = baseTex.Sample(AnisotropicSampler16, texcoord).gr;
  base.xy = -1.0 + 2.0 * base.xy;

  #ifdef WRINKLES
  // Fetch and unpack wrinkles normal (3Dc format):
  // (a linear sampler is enough for the low-frequency wrinkles information)
  float2 wrinkles = wrinkleTex.Sample(LinearSampler, texcoord).gr;
  wrinkles = -1.0 + 2.0 * wrinkles;

  // We're going accumulate wrinkles on xy and zw simultaneously, so reset zw
  // to zero:
  base.zw = 0.0;

  // Fetch the first mask and apply the weights:
  float4 mask1 = maskTex[0].Sample(LinearSampler, texcoord);
  mask1 *= weights[0];

  // Accumulate the first mask wrinkles:
  base += mask1.rrgg * wrinkles.xyxy;
  base += mask1.bbaa * wrinkles.xyxy;

  // Fetch the second mask and apply the weights:
  float4 mask2 = maskTex[1].Sample(LinearSampler, texcoord);
  mask2 *= weights[1];

  // Accumulate the second mask wrinkles:
  base += mask2.rrgg * wrinkles.xyxy;
  base += mask2.bbaa * wrinkles.xyxy;

  // Accumulate the wrinkles:
  base.xy += base.zw;
  #endif

  // Normalize the result:
  base.z = 1.0;
  return normalize(base.xyz);
}

float3 WrinkledNormal(Texture2D baseTex,
                      Texture2D normalDiffTex,
                      Texture2D maskTex[2],
                      float4 weights[2],
                      float2 texcoord) {
  // Fetch and unpack base normal (3Dc format):
  float3 base;
  base.xy = baseTex.Sample(AnisotropicSampler16, texcoord).gr;
  base.xy = -1.0 + 2.0 * base.xy;
  base.z = sqrt(1.0 - base.x * base.x - base.y * base.y);

  #ifdef WRINKLES
  // Fetch and unpack normal diffrence (swizzled DXT5 format):
  float3 diff = normalDiffTex.Sample(LinearSampler, texcoord).agb;
  diff = -1.0 + 2.0 * diff;

  // Multiply by 2.0 as the difference wrinkle map was calculated using values
  // in the range 0..1, and now we are working in the range -1..1:
  diff *= 2.0;

  // Fetch the masks and apply the weights:
  float4 mask1 = maskTex[0].Sample(LinearSampler, texcoord);
  float4 mask2 = maskTex[1].Sample(LinearSampler, texcoord);
  mask1 *= weights[0];
  mask2 *= weights[1];

  // Accumulate the wrinkles from different zones:
  base += mask1.r * diff;
  base += mask1.g * diff;
  base += mask1.b * diff;
  base += mask1.a * diff;
  base += mask2.r * diff;
  base += mask2.g * diff;
  base += mask2.b * diff;
  base += mask2.a * diff;
  #endif

  return normalize(base);
}

@article{JIMENEZ2009_TAP,
    author = {Jorge Jimenez and Veronica Sundstedt and Diego Gutierrez},
    title = {Screen-space perceptual rendering of human skin},
    journal = {ACM Transactions on Applied Perception},
    volume = {6},
    number = {4},
    year = {2009},
    pages = {23:1--23:15},
}

@inbook{JIMENEZ2010_GPUPRO,
    author = {Jorge Jimenez and Diego Gutierrez},
    editor = {Wolfgang Engel},
    title = {GPU Pro: Advanced Rendering Techniques},
    chapter = {Screen-Space Subsurface Scattering},
    publisher = {{AK} Peters Ltd.},
    year = {2010},
    pages = {335--351},
}

@article{JIMENEZ2010_IEEE,
    author = {Jorge Jimenez and David Whelan and Veronica Sundstedt and Diego Gutierrez},
    title = {Real-Time Realistic Skin Translucency},
    journal = {IEEE Computer Graphics and Applications},
    volume = {30},
    number = {4},
    year = {2010},
    pages = {32--41},
}

@article{JIMENEZ2010_TOG,
    author = {Jorge Jimenez and Timothy Scully and Nuno Barbosa and Craig Donner and Xenxo Alvarez and
              Teresa Vieira and Paul Matts and Verónica Orvalho and Diego Gutierrez and Tim Weyrich},
    title = {A practical appearance model for dynamic facial color},
    journal = {ACM Transactions on Graphics (Proc. SIGGRAPH Asia)},
    year = {2010},
    volume = {29},
    number = {6},
    pages = {141:1--141:10},
}

@article{JIMENEZ2015_CGF,
    author = {Jorge Jimenez and Károly Zsolnai and Adrian Jarabo and Christian Freude and Thomas Auzinger and
              Xian-Chun Wu and Javier von der Pahlen and Michael Wimmer and Diego Gutierrez},
    title = {Separable Subsurface Scattering},
    journal = {Computer Graphics Forum},
    year = {2015},
}