(sorry for the draw) I try to get the blue angle , i thought it was easy but...
the curve is a Bézier curve. the black dot shows the tangent of D I know position of A B C D.
I tried to get the angle by addition of angle a and angle b
the point F is found by multiplying AB per the time factor (between 0 and 1) the point E is found using the y coordinate of B and x coordinate of A
i get a with Al-Kashi method with the triangle FBD. i get b with Al-Kashi method too with the triangle EBA.
this is for rotation of an object who follow the curve and i always get weird result.
Is there any better way to do it and if not what I'm doing wrong ? thanks