Odd behavior when decrementing a CCSprite velocity
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07-09-2020 - |
Question
My CCSprite subclass has two float properties called, velX and velY to keep track of the velocity of my player. When it gets swiped in the game layer, I figure out the vector direction of the swipe and start the player moving that way. In the -update:(ccTime)dt
method of my game layer, I call [player update:(ccTime)dt
(I know, same name) and that method in the CCSprite class has this code.
// move
self.position = ccp(self.position.x + self.velX * dt, self.position.y + self.velY * dt);
// do edge checking
// decelerate
#define kDeceleration .95
NSLog(@">velX = %1.1f, velY = %1.1f, fabs(velX) = %1.1f, fabs(velY) = %1.1f", self.velX, self.velY, fabs(velX), fabs(velY));
self.velX *= kDeceleration;
if (fabs(self.velX < 1.0)) self.velX = 0.0;
self.velY *= kDeceleration;
if (fabs(self.velY < 1.0)) self.velY = 0.0;
This gets called every time the game layer calls update
and passes in the same time increment that the game loop uses.
It works great if I swipe in a positive direction, but if either of the swipe components are negative (left or down) then they go straight to zero. If I take out the conditionals, everything works, I just figured that multiplying by zero should be cheaper than constantly multiplying by very small decimals.
I must be using the fabs()
function incorrectly...
EDIT: Some sample output from two swipes, one positive in both axes, and one negative in both.
>velX = 21.8, velY = 20.6, fabs(velX) = 21.8, fabs(velY) = 20.6
>velX = 20.7, velY = 19.6, fabs(velX) = 20.7, fabs(velY) = 19.6
>velX = 19.7, velY = 18.6, fabs(velX) = 19.7, fabs(velY) = 18.6
>velX = 18.7, velY = 17.7, fabs(velX) = 18.7, fabs(velY) = 17.7
>velX = 17.8, velY = 16.8, fabs(velX) = 17.8, fabs(velY) = 16.8
>velX = -23.0, velY = -19.3, fabs(velX) = 23.0, fabs(velY) = 19.3
>velX = 0.0, velY = 0.0, fabs(velX) = 0.0, fabs(velY) = 0.0
>velX = 0.0, velY = 0.0, fabs(velX) = 0.0, fabs(velY) = 0.0
>velX = 0.0, velY = 0.0, fabs(velX) = 0.0, fabs(velY) = 0.0
>velX = 0.0, velY = 0.0, fabs(velX) = 0.0, fabs(velY) = 0.0
Solution
I believe this block is a syntax typo:
self.velX *= kDeceleration;
if (fabs(self.velX < 1.0)) self.velX = 0.0;
self.velY *= kDeceleration;
if (fabs(self.velY < 1.0)) self.velY = 0.0;
It should be this instead:
self.velX *= kDeceleration;
if (fabs(self.velX) < 1.0) self.velX = 0.0;
self.velY *= kDeceleration;
if (fabs(self.velY) < 1.0) self.velY = 0.0;
You need to close the fabs
parentheses before the comparison operator
p/s: it's okay, everyone does this kind of mistake all the time :)