This is an implementation of the paper [A Microfacet-based Hair Scattering Model](https://onlinelibrary.wiley.com/doi/full/10.1111/cgf.14588) by Weizhen Huang, Matthias Hullin, and Johannes Hanika.
Original implementation in [Mitsuba 2](https://github.com/RiverIntheSky/roughhair) by Weizhen Huang.
Adapted to Cycles by Olivier Maury D16682 and Christophe Hery.

@@ -319,2 +323,4 @@
           attr_intercept->add(time);
 
+        if (attr_normal) {
+          /* TODO: compute geometry normals. */

@@ -469,3 +468,1 @@
-
-ccl_device_inline float bsdf_principled_hair_albedo_roughness_scale(
-    const float azimuthal_roughness)
+/* Hair Albedo. Also used by `bsdf_hair_microfacet.h` */

@@ -1142,1 +1141,3 @@
-      num_closures += 4;
+    else if (closure_type == CLOSURE_BSDF_HAIR_PRINCIPLED_ID ||
+             closure_type == CLOSURE_BSDF_HAIR_MICROFACET_ID) {
+      num_closures += 2;

@@ -0,0 +16,4 @@
+
+typedef struct MicrofacetHairExtra {
+  /* Optional modulation factors. */
+  float R, TT, TRT;

@@ -0,0 +66,4 @@
+ * \{ */
+
+/* Returns `sin(theta)` of the given direction. */
+ccl_device_inline float sin_theta(const float3 w)

@@ -0,0 +479,4 @@
+
+    const float weight = (i == 0 || i == intervals) ? 0.5f : (i % 2 + 1);
+
+    const Spectrum A_t = exp(mu_a / cos_theta(wt) *

@@ -0,0 +608,4 @@
+  *eta = bsdf->eta;
+
+  /* Treat as transparent material if intersection lies outside of the projected radius. */
+  if (fabsf(bsdf->h) > bsdf->extra->radius) {

@@ -0,0 +616,4 @@
+    return LABEL_TRANSMIT | LABEL_TRANSPARENT;
+  }
+
+  if (bsdf->extra->R <= 0.0f && bsdf->extra->TT <= 0.0f && bsdf->extra->TRT <= 0.0f) {

@@ -0,0 +643,4 @@
+
+  const float h = sample_h * 2.0f - 1.0f;
+  const float gamma_mi = is_circular ?
+                             asinf(h) :

@@ -0,0 +771,4 @@
+      fast_sincosf(phi_o, &sin_phi_o, &cos_phi_o);
+
+      /* Compute outgoing direction. */
+      wtrrt = make_float3(sin_phi_o * cos_theta_o, sin_theta_o, cos_phi_o * cos_theta_o);

@@ -0,0 +853,4 @@
+  const float3 local_I = bsdf->extra->wi;
+  const float3 local_O = make_float3(dot(wo, X), dot(wo, Y), dot(wo, Z));
+
+  /* TODO: better estimation of the pdf */

@@ -58,2 +58,3 @@
     /* All closures contribute to the normal feature, but only diffuse-like ones to the albedo. */
-    normal += sc->N * sc->sample_weight;
+    /* If far-field hair, use fiber tangent as feature instead of normal. */
+    normal += (sc->type == CLOSURE_BSDF_HAIR_MICROFACET_ID ? safe_normalize(sd->dPdu) : sc->N) *

@@ -90,3 +95,3 @@
   }
 
-  BSDF = principled_hair(Normal, sigma, roughness, radial_roughness, m0_roughness, Offset, IOR);
+  normal major_axis = Normal;

@@ -3434,2 +3445,4 @@
 void PrincipledHairBsdfNode::attributes(Shader *shader, AttributeRequestSet *attributes)
 {
+  /* Make sure we have the normal for elliptical cross section tracking. */
+  if (model == NODE_PRINCIPLED_HAIR_HUANG && aspect_ratio != 1.0f) {

@@ -1282,0 +1282,4 @@
+typedef struct NodeShaderHairPrincipled {
+  short model;
+  short parametrization;
+  short cross_section;

@@ -1282,0 +1284,4 @@
+  short parametrization;
+  short cross_section;
+  char _pad[2];
+} NodeShaderHairPrincipled;

@@ -4546,0 +4554,4 @@
+     0,
+     "Far-field Model",
+     "Microfacet-based hair scattering model by Huang et. al 2022, suitable for viewing from a "
+     "distance, supports elliptical cross-sections and has preciser highlight in forward "

@@ -27,0 +47,4 @@
+      .description(
+          "For elliptical hair cross-section, the aspect ratio is the ratio of the minor axis to "
+          "the major axis (the major axis is aligned with the curve normal). Recommended values "
+          "are 0.8~1 for Asian hair, 0.65~0.9 for Caucasian hair, 0.5~0.65 for African hair. Set "