2 files changed, 26 insertions, 3 deletions

diff --git a/src/video_core/regs_lighting.h b/src/video_core/regs_lighting.h
index fbfebc0a7..f383b8b4f 100644
--- a/src/video_core/regs_lighting.h
+++ b/src/video_core/regs_lighting.h
@@ -84,7 +84,7 @@ struct LightingRegs {
         NV = 2, // Cosine of the angle between the normal and the view vector
         LN = 3, // Cosine of the angle between the light and the normal vectors
         SP = 4, // Cosine of the angle between the light and the inverse spotlight vectors
-        CP = 5, // TODO: document and implement
+        CP = 5, // Cosine of the angle between the tangent and projection of half-angle vectors
     };
 
     enum class LightingBumpMode : u32 {
diff --git a/src/video_core/renderer_opengl/gl_shader_gen.cpp b/src/video_core/renderer_opengl/gl_shader_gen.cpp
index db53710aa..89977a62b 100644
--- a/src/video_core/renderer_opengl/gl_shader_gen.cpp
+++ b/src/video_core/renderer_opengl/gl_shader_gen.cpp
@@ -534,18 +534,24 @@ static void WriteLighting(std::string& out, const PicaShaderConfig& config) {
                 "(1.0 - (surface_normal.x*surface_normal.x + surface_normal.y*surface_normal.y))";
             out += "surface_normal.z = sqrt(max(" + val + ", 0.0));\n";
         }
+
+        // The tangent vector is not perturbed by the normal map and is just a unit vector.
+        out += "vec3 surface_tangent = vec3(1.0, 0.0, 0.0);\n";
     } else if (lighting.bump_mode == LightingRegs::LightingBumpMode::TangentMap) {
         // Bump mapping is enabled using a tangent map
         LOG_CRITICAL(HW_GPU, "unimplemented bump mapping mode (tangent mapping)");
         UNIMPLEMENTED();
     } else {
-        // No bump mapping - surface local normal is just a unit normal
+        // No bump mapping - surface local normal and tangent are just unit vectors
         out += "vec3 surface_normal = vec3(0.0, 0.0, 1.0);\n";
+        out += "vec3 surface_tangent = vec3(1.0, 0.0, 0.0);\n";
     }
 
     // Rotate the surface-local normal by the interpolated normal quaternion to convert it to
     // eyespace.
-    out += "vec3 normal = quaternion_rotate(normalize(normquat), surface_normal);\n";
+    out += "vec4 normalized_normquat = normalize(normquat);\n";
+    out += "vec3 normal = quaternion_rotate(normalized_normquat, surface_normal);\n";
+    out += "vec3 tangent = quaternion_rotate(normalized_normquat, surface_tangent);\n";
 
     // Gets the index into the specified lookup table for specular lighting
     auto GetLutIndex = [&lighting](unsigned light_num, LightingRegs::LightingLutInput input,
@@ -573,6 +579,23 @@ static void WriteLighting(std::string& out, const PicaShaderConfig& config) {
             index = std::string("dot(light_vector, spot_dir)");
             break;
 
+        case LightingRegs::LightingLutInput::CP:
+            // CP input is only available with configuration 7
+            if (lighting.config == LightingRegs::LightingConfig::Config7) {
+                // Note: even if the normal vector is modified by normal map, which is not the
+                // normal of the tangent plane anymore, the half angle vector is still projected
+                // using the modified normal vector.
+                std::string half_angle_proj = half_angle +
+                                              " - normal / dot(normal, normal) * dot(normal, " +
+                                              half_angle + ")";
+                // Note: the half angle vector projection is confirmed not normalized before the dot
+                // product. The result is in fact not cos(phi) as the name suggested.
+                index = "dot(" + half_angle_proj + ", tangent)";
+            } else {
+                index = "0.0";
+            }
+            break;
+
         default:
             LOG_CRITICAL(HW_GPU, "Unknown lighting LUT input %d\n", (int)input);
             UNIMPLEMENTED();