Evaluation by time doesn't take in account tangents X-axis

[arkaht]

The method Curve::evaluate_by_time doesn't take in account the X-axis of tangents, making it useless inside the editor.

As the code suggests, it isn't implemented yet, some tries have been made but the results are not satisfying enough:

cpp-curve-x/src/curve.cpp

Lines 72 to 78 in d91b871

	// Compute tangents Y-positions
	/*const float y1 = p0.y + atan2f( t1.y / m1, t1.x / m1 ) * time_diff * m1 / 3.0f;
	const float y2 = p3.y + atan2f( t2.y / m2, t2.x / m2 ) * time_diff * m2 / 3.0f;*/
	const float y1 = p0.y + t1.y;
	const float y2 = p3.y + t2.y;
	return Utils::bezier_interp( p0.y, y1, y2, p3.y, t );

I have looked into Unreal's code and to A Primer on Bézier Curves but I still can't wrap my head around the mathematics behind their implementations to make the tangent weights working as expected.

As a side note, this small yet major change would force any existing timed-based curves to be re-made to correspond to the new behavior.