Cycles: switch Cycles triangle barycentric convention to match Embree/OptiX

Simplifies intersection code a little and slightly improves precision regarding
self intersection.

The parametric texture coordinate in shader nodes is still the same as before
for compatibility.

2 files changed, 12 insertions, 7 deletions

diff --git a/intern/cycles/kernel/integrator/init_from_bake.h b/intern/cycles/kernel/integrator/init_from_bake.h
index bf3f41b52b9..dd26215bcd2 100644
--- a/intern/cycles/kernel/integrator/init_from_bake.h
+++ b/intern/cycles/kernel/integrator/init_from_bake.h
@@ -155,6 +155,11 @@ ccl_device bool integrator_init_from_bake(KernelGlobals kg,
                                  1.0f - u);
   }
 
+  /* Convert from Blender to Cycles/Embree/OptiX barycentric convention. */
+  const float tmp = u;
+  u = v;
+  v = 1.0f - tmp - v;
+
   /* Position and normal on triangle. */
   const int object = kernel_data.bake.object_index;
   float3 P, Ng;
diff --git a/intern/cycles/kernel/integrator/mnee.h b/intern/cycles/kernel/integrator/mnee.h
index 7a6f866b1a0..aa72b93c9d2 100644
--- a/intern/cycles/kernel/integrator/mnee.h
+++ b/intern/cycles/kernel/integrator/mnee.h
@@ -186,7 +186,7 @@ ccl_device_forceinline void mnee_setup_manifold_vertex(KernelGlobals kg,
     triangle_vertices_and_normals(kg, sd_vtx->prim, verts, normals);
 
     /* Compute refined position (same code as in triangle_point_from_uv). */
-    sd_vtx->P = isect->u * verts[0] + isect->v * verts[1] + (1.f - isect->u - isect->v) * verts[2];
+    sd_vtx->P = (1.f - isect->u - isect->v) * verts[0] + isect->u * verts[1] + isect->v * verts[2];
     if (!(sd_vtx->object_flag & SD_OBJECT_TRANSFORM_APPLIED)) {
       const Transform tfm = object_get_transform(kg, sd_vtx);
       sd_vtx->P = transform_point(&tfm, sd_vtx->P);
@@ -213,8 +213,8 @@ ccl_device_forceinline void mnee_setup_manifold_vertex(KernelGlobals kg,
   }
 
   /* Tangent space (position derivatives) WRT barycentric (u, v). */
-  float3 dp_du = verts[0] - verts[2];
-  float3 dp_dv = verts[1] - verts[2];
+  float3 dp_du = verts[1] - verts[0];
+  float3 dp_dv = verts[2] - verts[0];
 
   /* Geometric normal. */
   vtx->ng = normalize(cross(dp_du, dp_dv));
@@ -223,16 +223,16 @@ ccl_device_forceinline void mnee_setup_manifold_vertex(KernelGlobals kg,
 
   /* Shading normals: Interpolate normals between vertices. */
   float n_len;
-  vtx->n = normalize_len(normals[0] * sd_vtx->u + normals[1] * sd_vtx->v +
-                             normals[2] * (1.0f - sd_vtx->u - sd_vtx->v),
+  vtx->n = normalize_len(normals[0] * (1.0f - sd_vtx->u - sd_vtx->v) + normals[1] * sd_vtx->u +
+                             normals[2] * sd_vtx->v,
                          &n_len);
 
   /* Shading normal derivatives WRT barycentric (u, v)
    * we calculate the derivative of n = |u*n0 + v*n1 + (1-u-v)*n2| using:
    * d/du [f(u)/|f(u)|] = [d/du f(u)]/|f(u)| - f(u)/|f(u)|^3 <f(u), d/du f(u)>. */
   const float inv_n_len = 1.f / n_len;
-  float3 dn_du = inv_n_len * (normals[0] - normals[2]);
-  float3 dn_dv = inv_n_len * (normals[1] - normals[2]);
+  float3 dn_du = inv_n_len * (normals[1] - normals[0]);
+  float3 dn_dv = inv_n_len * (normals[2] - normals[0]);
   dn_du -= vtx->n * dot(vtx->n, dn_du);
   dn_dv -= vtx->n * dot(vtx->n, dn_dv);