SpriteKit: how to turn touches into vectors of uniform speed?

Question

I am using touches to set SpriteKit nodes in motion. Currently I do this:

 //(previous code turns current touch and last touch into thisP and thatP)...

 CGPoint velocity = CGPointMake(thisP.x - lastP.x, thisP.y - lastP.y);

//If the velocity is non-zero, apply it to the node:

if (!CGPointEqualToPoint(velocity, CGPointZero)) {
    [[MyNode physicsBody] applyForce:CGVectorMake(velocity.x, velocity.y)];
}

This is very effective at getting the nodes to move different speeds depending on how fast the user swiped. Oh, Anna, if only that was what I wanted!

My actual desired effect is for every node to move at a uniform speed, no matter how fast they were swiped. It seems like I'd have to make some kind of calculation that removes magnitude from the vector but preserves direction, which is beyond me.

Is this possible?

First, who's Anna? Second, Have you tried using gesture recognizers and apply a set velocity based on swipe direction? — sangony, Apr 22 '15 at 21:02
@sangony, “Oh, Anna, if only…” sounds like an allusion to the movie Frozen. I highly recommend it. — Noah Witherspoon, Apr 22 '15 at 21:27

Noah Witherspoon · Accepted Answer · 2015-04-23T14:41:27.443

4

You’re on the right track; the thing you’re looking for is called the normalized vector of the touch-movement vector you’re already calculating. You can obtain that by calculating the length of the starting vector and dividing each of its components by that length, then multiplying them by the length (or in this case the speed) that you want the final vector to have. In code:

float length = hypotf(velocity.x, velocity.y); // standard Euclidean distance: √(x^2 + y^2)
float multiplier = 100.0f / length; // 100 is the desired length
velocity.x *= multiplier;
velocity.y *= multiplier;

edited Apr 23 '15 at 14:41

answered Apr 22 '15 at 21:20

Noah Witherspoon

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There is also a `hypot()` function which comes in handy here. – Martin R Apr 22 '15 at 21:50
@MartinR: good comment, sent me searching SO for how to use it. In case anyone needs the reference, it's in a comment by lucius on this page: http://stackoverflow.com/questions/1906511/how-to-find-the-distance-between-two-cg-points – Le Mot Juiced Apr 23 '15 at 13:01
Thanks Martin! Somehow I’ve never encountered that one. Updated the answer. – Noah Witherspoon Apr 23 '15 at 14:41
nice going you guys! SO at its best. – Le Mot Juiced Apr 23 '15 at 15:47

SpriteKit: how to turn touches into vectors of uniform speed?

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