Underdamped spring formula changed

Migration is necessary only for springs with damping ratios less than 1 and masses other than 1.

To restore previous animation behavior, update your spring parameters accordingly. You can calculate the required parameter adjustments using the provided [JSFiddle for migration][]. Detailed formulas and explanations follow in the next sections.

Default constructor

If the SpringDescription was built with the default constructor with mass m, stiffness k, and damping c, then it should be changed with the following formula:

new_m = 1
new_c = c * m
new_k = (4 * (k / m) - (c / m)^2 + (c * m)^2) / 4

Code before migration:

const spring = SpringDescription(
  mass: 20.0,
  stiffness: 10,
  damping: 1,
);

Code after migration:

const spring = SpringDescription(
  mass: 1.0,
  stiffness: 100.499375,
  damping: 20,
);

.withDampingRatio constructor

If the SpringDescription was built with the .withDampingRatio constructor with mass m, stiffness k, and ratio z, then first calculate damping:

c = z * 2 * sqrt(m * k)

Then apply the formula above. Optionally, you might convert the result back to damping ratio with:

new_z = new_c / 2 / sqrt(new_m * new_k)

Code before migration:

const spring = SpringDescription.withDampingRatio(
  mass: 5.0,
  stiffness: 6.0,
  damping: 0.03,
);

Code after migration:

const spring = SpringDescription.withDampingRatio(
  mass: 1,
  stiffness: 1.87392,
  ratio: 0.60017287468545,
);