diff --git a/toonz/sources/common/tvrender/tinbetween.cpp b/toonz/sources/common/tvrender/tinbetween.cpp
index fce82be..2318b10 100644
--- a/toonz/sources/common/tvrender/tinbetween.cpp
+++ b/toonz/sources/common/tvrender/tinbetween.cpp
@@ -229,8 +229,8 @@ static void detectCorners(const TStroke *stroke, double minDegree,
     }
   }
 
-  const double ratioLen   = 2.5;
-  const double ratioAngle = 0.2;
+  const double ratioLen                            = 2.5;
+  const double ratioAngle                          = 0.2;
   std::vector<std::pair<int, double>>::iterator it = corners.begin();
 
   for (j = 1; j < (int)quadCount1;
@@ -241,8 +241,9 @@ static void detectCorners(const TStroke *stroke, double minDegree,
       continue;
     }
 
-    if (j - 2 >= 0 && (corners.empty() || it == corners.begin() ||
-                       j - 1 != (*(it - 1)).first) &&
+    if (j - 2 >= 0 &&
+        (corners.empty() || it == corners.begin() ||
+         j - 1 != (*(it - 1)).first) &&
         j + 1 < (int)quadCount1 &&
         (corners.empty() || it == corners.end() || j + 1 != (*it).first)) {
       speed1 = stroke->getChunk(j - 2)->getSpeed(1);
@@ -255,7 +256,7 @@ static void detectCorners(const TStroke *stroke, double minDegree,
         if (tan1 * tan2 < 0) {
           angle = 180 - asin(tcrop(vectorialProduct, -1.0, 1.0)) * M_180_PI;
 
-          metaCornerLen = ratioLen * (stroke->getChunk(j - 1)->getLength() +
+          metaCornerLen  = ratioLen * (stroke->getChunk(j - 1)->getLength() +
                                       stroke->getChunk(j)->getLength());
           partialLen     = 0;
           bool goodAngle = false;
@@ -1480,8 +1481,8 @@ TVectorImageP TInbetween::Imp::tween(double t) const {
             len1 += step1;
             len2 += step2;
           }
-          point2 = subStroke2->getThickPointAtLength(totalLen2);
-          point2 = TThickPoint(m_transformation[i].m_inverse *
+          point2     = subStroke2->getThickPointAtLength(totalLen2);
+          point2     = TThickPoint(m_transformation[i].m_inverse *
                                    subStroke2->getThickPointAtLength(totalLen2),
                                point2.thick);
           finalPoint = subStroke1->getThickPointAtLength(totalLen1) * (1 - t) +
@@ -1554,3 +1555,20 @@ void TInbetween::Imp::transferColor(const TVectorImageP &destination) const {
 TVectorImageP TInbetween::tween(double t) const { return m_imp->tween(t); }
 
 //-------------------------------------------------------------------
+
+double TInbetween::interpolation(double t, enum TweenAlgorithm algorithm) {
+  // in tutte le interpolazioni : s(0) = 0, s(1) = 1
+  switch (algorithm) {
+  case EaseInInterpolation:  // s'(1) = 0
+    return t * (2 - t);
+  case EaseOutInterpolation:  // s'(0) = 0
+    return t * t;
+  case EaseInOutInterpolation:  // s'(0) = s'(1) = 0
+    return t * t * (3 - 2 * t);
+  case LinearInterpolation:
+  default:
+    return t;
+  }
+}
+
+//-------------------------------------------------------------------
diff --git a/toonz/sources/include/tinbetween.h b/toonz/sources/include/tinbetween.h
index 123ca7b..ce89e10 100644
--- a/toonz/sources/include/tinbetween.h
+++ b/toonz/sources/include/tinbetween.h
@@ -24,6 +24,15 @@ class DVAPI TInbetween {
   std::unique_ptr<Imp> m_imp;
 
 public:
+  enum TweenAlgorithm {
+    LinearInterpolation,
+    EaseInInterpolation,
+    EaseOutInterpolation,
+    EaseInOutInterpolation
+  };
+
+  static double interpolation(double t, enum TweenAlgorithm);
+
   TInbetween(const TVectorImageP firstImage, const TVectorImageP lastImage);
 
   virtual ~TInbetween();
diff --git a/toonz/sources/toonz/filmstripcommand.cpp b/toonz/sources/toonz/filmstripcommand.cpp
index dc79cef..c768bab 100644
--- a/toonz/sources/toonz/filmstripcommand.cpp
+++ b/toonz/sources/toonz/filmstripcommand.cpp
@@ -2499,25 +2499,27 @@ void FilmstripCmd::inbetweenWithoutUndo(
   TVectorImageP img1 = sl->getFrame(fid1, false);
   if (!img0 || !img1) return;
 
+  enum TInbetween::TweenAlgorithm algorithm;
+  switch (interpolation) {
+  case II_Linear:
+    algorithm = TInbetween::LinearInterpolation;
+    break;
+  case II_EaseIn:
+    algorithm = TInbetween::EaseInInterpolation;
+    break;
+  case II_EaseOut:
+    algorithm = TInbetween::EaseOutInterpolation;
+    break;
+  case II_EaseInOut:
+    algorithm = TInbetween::EaseInOutInterpolation;
+    break;
+  }
+
   TInbetween inbetween(img0, img1);
   int i;
   for (i = ia + 1; i < ib; i++) {
     double t = (double)(i - ia) / (double)(ib - ia);
-    double s = t;
-    // in tutte le interpolazioni : s(0) = 0, s(1) = 1
-    switch (interpolation) {
-    case II_Linear:
-      break;
-    case II_EaseIn:
-      s = t * (2 - t);
-      break;  // s'(1) = 0
-    case II_EaseOut:
-      s = t * t;
-      break;  // s'(0) = 0
-    case II_EaseInOut:
-      s = t * t * (3 - 2 * t);
-      break;  // s'(0) = s'(1) = 0
-    }
+    double s = TInbetween::interpolation(t, algorithm);
 
     TVectorImageP vi = inbetween.tween(s);
     sl->setFrame(fids[i], vi);