How to make player move only along the line

Question

I am using Line Renderer to draw a line on 2D scene using a path of multiple Transforms. And I want player to move along the line without the ability to get off it, like if he was on a rail. The simplest method I can see is setting a number of invisible walls along the line, so the player couldn't physically move away from the line. But maybe there's better method?

Answer 1

You have a line with a starting point pStart and an end point pEnd.

To get any point on a line, use the linear function equation:

y = m * x + b

Now you have to get the slope of the line which is simply m = (pEnd.x - pStart.x) / (pEnd.y - pStart.y). Then, you can calculate every x and y pair on the line by simply putting in a value for x. So far for the mathematical background, now to the practical implementation (this is all pseudocode).

In your case, you can build a slope vector for the line

slopeX = pEnd.x - pStart.x
slopeY = pEnd.y - pStart.y

and then normalize it (get it to a lenght of one):

slopeLength = squareroot((slopeX * slopeX) + (slopeY * slopeY))
slopeNormX = slopeX / slopeLength
slopeNormY = slopeY / slopeLength

Now you know in which direction you have to move your player each time step. To move him, simply do the following:

playerX += slopeNormX * deltaTime
playerY += slopeNormY * deltaTime

Also, don't forget to check wether the player is still on the line:

playerX*playerX > pEnd.x*pEnd.x && playerY*playerY > pEnd.y*pEnd.y

We have to square the positions here because we don't know wether the coordinates are positive or negative (abs() would be too slow).

Cheers