Fix T97983 EEVEE: Tangent Normal of Curves info behaves differently in Eevee

Curve tangent was correctly mistaken with curve normal.

This patch fixes the name of the output in the glsl function and make curve
attributes more explicit (with `curve_` prefix).

This also improve the normal computation by making it per pixel to match
cycles.

Also ports the changes to eevee-next.

4 files changed, 15 insertions, 3 deletions

diff --git a/source/blender/draw/engines/eevee/shaders/surface_lib.glsl b/source/blender/draw/engines/eevee/shaders/surface_lib.glsl
index 1f2f7cb65cc..696e5d4c97b 100644
--- a/source/blender/draw/engines/eevee/shaders/surface_lib.glsl
+++ b/source/blender/draw/engines/eevee/shaders/surface_lib.glsl
@@ -112,6 +112,7 @@ GlobalData init_globals(void)
     surf.N = -surf.N;
   }
 #  ifdef HAIR_SHADER
+  vec3 V = cameraVec(surf.P);
   /* Shade as a cylinder. */
   vec3 B = normalize(cross(worldNormal, hairTangent));
   float cos_theta;
@@ -125,7 +126,10 @@ GlobalData init_globals(void)
   }
   float sin_theta = sqrt(max(0.0, 1.0 - cos_theta * cos_theta));
   surf.N = safe_normalize(worldNormal * sin_theta + B * cos_theta);
-  surf.T = hairTangent;
+  surf.curve_T = -hairTangent;
+  /* Costly, but follows cycles per pixel tangent space (not following curve shape). */
+  surf.curve_B = cross(V, surf.curve_T);
+  surf.curve_N = safe_normalize(cross(surf.curve_T, surf.curve_B));
   surf.is_strand = true;
   surf.hair_time = hairTime;
   surf.hair_thickness = hairThickness;
@@ -134,7 +138,7 @@ GlobalData init_globals(void)
   surf.barycentric_coords = hair_resolve_barycentric(hairBary);
 #    endif
 #  else
-  surf.T = vec3(0.0);
+  surf.curve_T = surf.curve_B = surf.curve_N = vec3(0.0);
   surf.is_strand = false;
   surf.hair_time = 0.0;
   surf.hair_thickness = 0.0;
diff --git a/source/blender/draw/engines/eevee_next/shaders/eevee_geom_curves_vert.glsl b/source/blender/draw/engines/eevee_next/shaders/eevee_geom_curves_vert.glsl
index 11f93ad0d14..708bd153e84 100644
--- a/source/blender/draw/engines/eevee_next/shaders/eevee_geom_curves_vert.glsl
+++ b/source/blender/draw/engines/eevee_next/shaders/eevee_geom_curves_vert.glsl
@@ -18,7 +18,7 @@ void main()
                               ViewMatrixInverse[3].xyz,
                               ViewMatrixInverse[2].xyz,
                               interp.P,
-                              T,
+                              interp.curves_tangent,
                               interp.curves_binormal,
                               interp.curves_time,
                               interp.curves_thickness,
diff --git a/source/blender/draw/engines/eevee_next/shaders/eevee_surf_lib.glsl b/source/blender/draw/engines/eevee_next/shaders/eevee_surf_lib.glsl
index 0d8644c9901..30b48edaa78 100644
--- a/source/blender/draw/engines/eevee_next/shaders/eevee_surf_lib.glsl
+++ b/source/blender/draw/engines/eevee_next/shaders/eevee_surf_lib.glsl
@@ -42,6 +42,12 @@ void init_globals_curves()
   float sin_theta = sqrt(max(0.0, 1.0 - cos_theta * cos_theta));
   g_data.N = normalize(interp.N * sin_theta + interp.curves_binormal * cos_theta);
 
+  /* Costly, but follows cycles per pixel tangent space (not following curve shape). */
+  vec3 V = cameraVec(g_data.P);
+  g_data.curve_T = -interp.curves_tangent;
+  g_data.curve_B = cross(V, g_data.curve_T);
+  g_data.curve_N = safe_normalize(cross(g_data.curve_T, g_data.curve_B));
+
   g_data.is_strand = true;
   g_data.hair_time = interp.curves_time;
   g_data.hair_thickness = interp.curves_thickness;
@@ -94,6 +100,7 @@ void init_interface()
   interp.P = vec3(0.0);
   interp.N = vec3(0.0);
   interp.barycentric_coords = vec2(0.0);
+  interp.curves_tangent = vec3(0.0);
   interp.curves_binormal = vec3(0.0);
   interp.curves_time = 0.0;
   interp.curves_time_width = 0.0;
diff --git a/source/blender/draw/engines/eevee_next/shaders/infos/eevee_material_info.hh b/source/blender/draw/engines/eevee_next/shaders/infos/eevee_material_info.hh
index e829a41d682..4d6895bcde0 100644
--- a/source/blender/draw/engines/eevee_next/shaders/infos/eevee_material_info.hh
+++ b/source/blender/draw/engines/eevee_next/shaders/infos/eevee_material_info.hh
@@ -54,6 +54,7 @@ GPU_SHADER_INTERFACE_INFO(eevee_surf_iface, "interp")
     .smooth(Type::VEC3, "P")
     .smooth(Type::VEC3, "N")
     .smooth(Type::VEC2, "barycentric_coords")
+    .smooth(Type::VEC3, "curves_tangent")
     .smooth(Type::VEC3, "curves_binormal")
     .smooth(Type::FLOAT, "curves_time")
     .smooth(Type::FLOAT, "curves_time_width")