How to calculate direction from initial point and another point?

Question

I'm making a simple game where I shoot things from a certain point on screen (A).

I tap the screen and shoot the projectile from initial point(A) to the tap point(B).

But I want the projectile to move along the same path instead and fly out of bounds of the screen.

How do I calculate a point that is on the same line that these two points, but further away?

This is a simple math, but I can't figure it out.

Icy Defiance · Accepted Answer · 2014-08-19 20:31:01Z

12

If A and B are both 2D vectors, then...

direction = normalize(B - A)
any point on the line = direction * distance + A

Or you can just take normalize(B - A) * speed and add that to the projectile's position every frame.

answered Aug 19, 2014 at 20:31

Icy Defiance

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David Starkey · Accepted Answer · 2014-08-19 20:18:26Z

The equation of a line is y=mx+b where m is the slope, b is the offset, and x and y are variables (coordinates on the grid).

To get this equation from 2 points, you can use the point-slope formula:

y - A.y = m(x - A.x)

To calculate the slope, m, you need to figure out the change in y values over the change in x values:

(B.y-A.y)/(B.x-A.x) = m

To get the offset, b, you need to plug in the slope and simplify the formula:

y = m*x - m*A.x + A.y

So b = m*A.x + A.y

After you have m and b, you have the equation for the line. Plug in x and y values and it will be on that line.

2 Answers 2