Should use slerp in vertex shader?

[edvakf]

Not sure if necessary.

diff --git a/src/MMD.VertexShaderSource.coffee b/src/MMD.VertexShaderSource.coffee
index 075b18c..1cdb901 100644
--- a/src/MMD.VertexShaderSource.coffee
+++ b/src/MMD.VertexShaderSource.coffee
@@ -39,6 +39,26 @@ MMD.VertexShaderSource = '''
     return v + 2.0 * cross(cross(v, q.xyz) - q.w*v, q.xyz);
   }

+  vec4 qslerp(vec4 quat, vec4 quat2, float slerp) {
+    float cosHalfTheta = dot(quat, quat2);
+
+    if (abs(cosHalfTheta) >= 1.0) {
+        return quat;
+    }
+
+    float halfTheta = acos(cosHalfTheta);
+    float sinHalfTheta = sqrt(1.0 - cosHalfTheta * cosHalfTheta);
+
+    if (abs(sinHalfTheta) < 0.001) {
+        return (quat + quat2) * 0.5;
+    }
+
+    float ratioA = sin((1.0 - slerp) * halfTheta) / sinHalfTheta;
+    float ratioB = sin(slerp * halfTheta) / sinHalfTheta;
+
+    return quat * ratioA + quat2 * ratioB;
+  }
+
   void main() {
     vec3 position = aVertexPosition;
     vec3 normal = aVertexNormal;
@@ -58,10 +78,11 @@ MMD.VertexShaderSource = '''

       position = mix(r2, r1, aBoneWeight);

-      vec3 n1 = qtransform(q1, normal);
-      vec3 n2 = qtransform(q2, normal);
+      //vec3 n1 = qtransform(q1, normal);
+      //vec3 n2 = qtransform(q2, normal);

-      normal = normalize(mix(n2, n1, aBoneWeight));
+      //normal = normalize(mix(n2, n1, aBoneWeight));
+      normal = qtransform(qslerp(q2, q1, aBoneWeight), normal);
     }

     // return vertex point in projection space