How can I keep my sprites velocity constant? - ios

Question

I'm making a simple space shooter in Sprite Kit. I have asteroid rock sprites that are randomly added to my scene. I give them a random velocity within a certain range. However, once added to the scene their velocity decreases every second and eventually they stop. as they get closer to the bottom of the screen their velocity decreases at a slower rate. I have gravity of my physics world set to (0,0).
below is the method that adds the rocks to the scene
- (void)addRock {
SPRockNode *rock = [SPRockNode rock];
float dy = [SPUtil randomWithMin:SPRockMinSpeed max:SPRockMaxSpeed];
rock.physicsBody.velocity = CGVectorMake(0, -dy);
float y = self.frame.size.height + rock.size.height;
float x = [SPUtil randomWithMin:10 + rock.size.width max:self.frame.size.width - 10 - rock.size.width];
rock.position = CGPointMake(x, y);
[self addChild:rock];
}
I set up the physics world in my scenes initWIthSize method
self.physicsWorld.contactDelegate = self;
self.physicsWorld.gravity = CGVectorMake(0, 0);
below is the code for my rock node class
+ (instancetype)rock {
SPRockNode *rock = [self spriteNodeWithImageNamed:#"rock"];
rock.name = #"Rock";
float scale = [SPUtil randomWithMin:50 max:100] / 100.0f;
rock.xScale = scale;
rock.yScale = scale;
NSInteger randomInt = [SPUtil randomWithMin:1 max:3];
if (randomInt == 1) {
SKAction *oneRevolution = [SKAction rotateByAngle:-M_PI*2 duration: 1.0];
SKAction *repeat = [SKAction repeatActionForever:oneRevolution];
[rock runAction:repeat];
} else {
SKAction *oneRevolution = [SKAction rotateByAngle:M_PI*2 duration: 1.0];
SKAction *repeat = [SKAction repeatActionForever:oneRevolution];
[rock runAction:repeat];
}
[rock setupPhysicsBody];
return rock;
}
- (void)setupPhysicsBody {
self.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:self.frame.size];
self.physicsBody.affectedByGravity = NO;
self.physicsBody.categoryBitMask = SPCollisionCategoryRock;
self.physicsBody.collisionBitMask = 0;
self.physicsBody.contactTestBitMask = SPCollisionCategoryProjectile | SPCollisionCategoryShip;
}
any ideas on what causing them to slow down? I dont modify their velocity anywhere else but the addRock method. is there air resistance or something like that on by default? I just dont understand why their velocity is decreasing.

Answer 1

It sounds like you are having an issue with linear damping.
From SKPhysicsBody reference :
This property is used to simulate fluid or air friction forces on the body. The property must be a value between 0.0 and 1.0. The default value is 0.1. If the value is 0.0, no linear damping is applied to the object.
Try adding this :
rock.physicsBody.linearDamping = 0.0f;
It's worthwhile to give the SKPhysicsBody class reference a reading at least once. It only takes a short while, but can save you a ton of time if you have become acquainted with it's properties and methods.