public float strokeSizeFor(int elementIndex) {
    return mStartStrokeSize - (mStartStrokeSize - mEndStrokeSize) * elementIndex * mTrailFraction;
  }
@Test
  public void testStrokeSizeFor() {
    GestureTrailTheme underTest = new GestureTrailTheme(Color.BLACK, Color.BLACK, 120f, 20f, 50);

    Assert.assertEquals(120f, underTest.strokeSizeFor(0), 0.1f);
    Assert.assertEquals(20f, underTest.strokeSizeFor(50), 0.1f);
    Assert.assertEquals(70f, underTest.strokeSizeFor(25), 0.1f);
  }