Question

I writing a simple XNA demo in which a sprite is supposed to move along a line (defined with two Vector2 points (Ax,Ay) and (Bx,By) ) at a given speed V (understood as a distance on the line traveled in a unit of time).
I understand that i’d need to calculate x and y position in a 2D space using equations similar to these:

dx = V*dt*cos(alpha)
dy = V*dt*sin(alpha)

but taking negative V into consideration (like when moving “backwards”)
But i wonder if i am not reinventing the wheel, maybe xna offers solution to that?

Answer 1

Create a 2D vector from point A to point B by subtracting A from B. Normalize it by dividing the X and Y conponents by the length of the vector. Multiply the X and Y components by the speed. Multiply this by the time slice (i.e. 20/1000 for 20 milliseconds). Add these X and Y values to your position.

Example:

A is (2, 5)  B is (1, 8)
Speed is 2 (move 2 units in one second)
Time slice is 20 milliseconds since last update

C will be (-1, 3)
Normalize C by dividing by distance (sqrt(-1 * -1 + 3 * 3) = 3.1622777)
Normalized is (-0.316277, 0.9486833) (to move 1 unit that direction)
Multiply by speed (2): (-0.632554, 1.8973666)
For movement this time period, multiply by 20 and divide by 1000:
(-0.01265108, 0.037947332), move sprite this far this frame