I have just solved the problem the distance between two points on the circumference of an ellipse following the outer curve after I read the following article.
Then I want to solve this problem in another view.
Please watch the example below which link of a question of the owner I have pasted above.
example ←here
This ellipse's equation is $\frac{x^2}{2^2} + \frac{y^2}{1^2} = 1$ (Just what original question has mentioned).
So my question is, suppose we know arc-length between two points $x_A$ and $x_B$ and we know Coordinate of $x_A$, how to calculate coordinate of $x_B$?If this solution is impossible, we can assume that $x_A$ = (0, 1).In this case, the equation has very strong Symmetry, so I think we can solve the problem.