How to limit the distance of a point?

Question

I'm making an Angry Birds like game and I need to limit the distance that the user can pull back the slingshot, how do I do this?

Answer 1

Get the length of the vector from the slingshot to the mouse/finger/whatever.

If it's more than your allowed length then normalise the vector (to make its length 1) then multiply it by the maximum length.

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Note, to avoid a square-root (costly operation) in the first bit compare the length squared with the max length squared.

// Get a vector from slingPos to targetPos
// (you may actually want the other way round in your game,
// as the angry object will be thrown towards the slingPos)
var orignalVector = targetPos - slingPos;
var originalLengthSq = originalVector.X * originalVector.X +
                       originalVector.Y * originalVector.Y;
if (originalLengthSq > maxLengthSq)
{
    // I have already done the first bit of normalisation (getting the length squared)
    // so we re-use that calculation.
    var normalisedVector = originalVector / sqrt(orignalLengthSq);
    originalVector = normalisedVector * maxLength;
}