Lightning with Sprite Kit

Have you ever tried to implement such effect as lightning for your iOS game? In this article I’m going to describe three ways I found to create a lightning effect with a decent quality:

• SKShapeNode.
• CAShapeLayer.
• SKSpriteNodes.

For each of them we need to compose a path for our lightning bolt. I searched the web and found this article about procedural generation of lightning bolt. This approach uses Midpoint Displacement algorithm. The result looks pretty realistic. Here is Objective-C code for Midpoint Displacement algorithm:

void createBolt(float x1, float y1, float x2, float y2, float displace, UIBezierPath *path) {
    if (displace < 1.8f) {
        CGPoint point = CGPointMake(x2, y2);
        [path MoveToPoint:point];
    }
    else {
        float mid_x = (x2+x1)*0.5f;
        float mid_y = (y2+y1)*0.5f;
        mid_x += (arc4random_uniform(100)*0.01f-0.5f)*displace;
        mid_y += (arc4random_uniform(100)*0.01f-0.5f)*displace;
        createBolt(x1, y1, mid_x, mid_y, displace*0.5f, path);
        createBolt(mid_x, mid_y, x2, y2, displace*0.5f, path);
    }
}

Ok, we got the path, how about drawing it? “We can define SKShapeNode with our path!” — that’s how I thought.

SKShapeNode.

Let’s create SKShapeNode with UIBezierPath, created from our pathArray:

- (void)addBoltWithStartPoint:(CGPoint)startPoint endPoint:(CGPoint)endPoint {
    // Dynamically calculating displace
    // Distance between two points
    float hypot = hypotf(fabsf(endPoint.x - startPoint.x), fabsf(endPoint.y - startPoint.y));
    // hypot/displace = 4/1
    float displace = hypot*0.25;
 
    UIBezierPath *path = [UIBezierPath bezierPath];
    [path moveToPoint:startPoint];        
 
    createBoltPath(startPoint.x, startPoint.y, endPoint.x, endPoint.y, displace, path);
 
    SKShapeNode *bolt = [SKShapeNode node];
    bolt.path = path.CGPath;
    bolt.strokeColor = [SKColor whiteColor];
    bolt.lineWidth = 0.5f;
    bolt.antialiased = NO;
    [self addChild:bolt];
 
    SKShapeNode *shadowNode = [[SKShapeNode alloc] init];
    shadowNode.path = path.CGPath;
    shadowNode.strokeColor = [SKColor colorWithRed:0.702 green:0.745 blue:1 alpha:1.0];
    shadowNode.lineWidth = 0.5f;
    shadowNode.alpha = 0.4;
    shadowNode.glowWidth = 5.f;
    [self addChild:shadowNode];
}
 
- (void)createBoltWithPath:(UIBezierPath*)path {    
    BoltNode *bolt = [[BoltNode alloc] initWithBezierPath:path lifetime:0.5f];
    [self addChild:bolt];
}

Defining SKShapeNode with UIBezierPath seems very simple, and result looks nice and realistic, but if you use a lot of such nodes, FPS dramatically drops. That didn’t suit me, so I came to the second approach: using CAShapeLayer.

CAShapeLayer.

CAShapeLayer allows us to draw a path on our view’s CALayer. Moreover, it offers a shadow setting for the path out of the box: by using shadowColor, shadowOffset, shadowRadius and shadowOpacity properties. Ok, let’s try that!

- (void)addBoltWithStartPoint:(CGPoint)startPoint endPoint:(CGPoint)endPoint {
    // Dynamically calculating displace
    // Distance between two points
    float hypot = hypotf(fabsf(endPoint.x - startPoint.x), fabsf(endPoint.y - startPoint.y));
    // hypot/displace = 4/1
    float displace = hypot*0.25;
 
    UIBezierPath *path = [UIBezierPath bezierPath];
    [path moveToPoint:startPoint];        
 
    createBoltPath(startPoint.x, startPoint.y, endPoint.x, endPoint.y, displace, path);
 
    CAShapeLayer *shapeLayer = [CAShapeLayer layer];
    shapeLayer.path = bezierPath.CGPath;
    shapeLayer.strokeColor = [[UIColor whiteColor] CGColor];
    shapeLayer.lineWidth = 1.f;
    shapeLayer.fillColor = [UIColor clearColor].CGColor;
    shapeLayer.zPosition = 20;
    shapeLayer.shadowColor = [UIColor colorWithRed:0.702 green:0.745 blue:1 alpha:1.0].CGColor;
    shapeLayer.shadowOffset = CGSizeMake(0, 0);
    shapeLayer.shadowRadius = 7.f;
    shapeLayer.shadowOpacity = 1.f;
    shapeLayer.shouldRasterize = YES;
    [self.view.layer addSublayer:shapeLayer];
}

Using CAShapeLayer seems much more efficient than SKShapeNode:

But there are several cons:

• The shadow looks very pale and unnatural
• You cannot change zPosition of shapeLayer on SKScene. Thus you cannot place any of your SKNodes above shapeLayer.

I googled about other methods of lightning generation and found a great article “How to Generate Shockingly Good 2D Lightning Effects” by Michael Hoffman. He has written his project using C# and XNA, so I decided to make the same trick with using Objective-C and Sprite Kit’s SKSpriteNode!

SKSpriteNode.

First, we need to draw our building blocks: a half-circle image and a lightning segment. Here is what I got so far:

Second, Michael uses his own method to generate a lightning bolt path, but Midpoint Displacement algorithm seems more realistic for me, so I decided to keep using it. We don’t need UIBezierPath with this approach, so let’s slightly change the code of createBolt function:

void createBolt(float x1, float y1, float x2, float y2, float displace, NSMutableArray *pathArray) {
    if (displace < 1.8f) {
        CGPoint point = CGPointMake(x2, y2);
        [pathArray addObject:[NSValue valueWithCGPoint:point]];
    }
    else {
        float mid_x = (x2+x1)*0.5f;
        float mid_y = (y2+y1)*0.5f;
        mid_x += (arc4random_uniform(100)*0.01f-0.5f)*displace;
        mid_y += (arc4random_uniform(100)*0.01f-0.5f)*displace;
        createBolt(x1, y1, mid_x, mid_y, displace*0.5f, pathArray);
        createBolt(mid_x, mid_y, x2, y2, displace*0.5f, pathArray);
    }
}

Third, let’s create a special SKNode called LightningLine, which represents one single line of our path:

@interface LightningLine : SKNode
 
@property (nonatomic) CGPoint startPoint;
@property (nonatomic) CGPoint endPoint;
@property (nonatomic) float thickness;
 
- (instancetype)initWithStartPoint:(CGPoint)startPoint endPoint:(CGPoint)endPoint;
- (void)draw;
+ (void)loadSharedAssets;
 
@end
 
@implementation LightningLine
 
- (instancetype)initWithStartPoint:(CGPoint)startPoint endPoint:(CGPoint)endPoint {
    if (self = [super init]) {
        self.startPoint = startPoint;
        self.endPoint = endPoint;
        self.thickness = 1.3f;
    }
    return self;
}
 
- (void)draw {
    const float imageThickness = 2.f;
    float thicknessScale = self.thickness / imageThickness;
    CGPoint startPointInThisNode = [self convertPoint:self.startPoint fromNode:self.parent];
    CGPoint endPointInThisNode = [self convertPoint:self.endPoint fromNode:self.parent];
    float angle = atan2(endPointInThisNode.y - startPointInThisNode.y,
                        endPointInThisNode.x - startPointInThisNode.x);
    float length = hypotf(fabsf(endPointInThisNode.x - startPointInThisNode.x),
                         fabsf(endPointInThisNode.y - startPointInThisNode.y));
 
    SKSpriteNode *halfCircleA = [SKSpriteNode spriteNodeWithTexture:[self halfCircle]];
    halfCircleA.anchorPoint = CGPointMake(1, 0.5);
    SKSpriteNode *halfCircleB = [SKSpriteNode spriteNodeWithTexture:[self halfCircle]];
    halfCircleB.anchorPoint = CGPointMake(1, 0.5);
    halfCircleB.xScale = -1.f;
    SKSpriteNode *lightningSegment = [SKSpriteNode spriteNodeWithTexture:[self lightningSegment]];
    halfCircleA.yScale = halfCircleB.yScale = lightningSegment.yScale = thicknessScale;
    halfCircleA.zRotation = halfCircleB.zRotation = lightningSegment.zRotation = angle;
    lightningSegment.xScale = length*2;
 
    halfCircleA.blendMode = halfCircleB.blendMode = lightningSegment.blendMode = SKBlendModeAlpha;
 
    halfCircleA.position = startPointInThisNode;
    halfCircleB.position = endPointInThisNode;
    lightningSegment.position = CGPointMake((startPointInThisNode.x + endPointInThisNode.x)*0.5f,
                                            (startPointInThisNode.y + endPointInThisNode.y)*0.5f);
    [self addChild:halfCircleA];
    [self addChild:halfCircleB];
    [self addChild:lightningSegment];
}
 
+ (void)loadSharedAssets {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sHalfCircle = [SKTexture textureWithImageNamed:@"half_circle"];
        sLightningSegment = [SKTexture textureWithImageNamed:@"lightning_segment"];
    });
}
 
static SKTexture *sHalfCircle = nil;
- (SKTexture*)halfCircle {
    return sHalfCircle;
}
 
static SKTexture *sLightningSegment = nil;
- (SKTexture*)lightningSegment {
    return sLightningSegment;
}
 
@end

loadSharedAssets method loads our images to static variables, so it must be called before we initialize a node. The best way to call it is SKScene’s init method.

- (instancetype)initWithSize:(CGSize)size {
    if (self = [super initWithSize:size]) {
        [LightningLine loadSharedAssets];
    }
    return self;
}

Finally, here is a method, that does all magic:

- (void)drawBoltFromPoint:(CGPoint)startPoint toPoint:(CGPoint)endPoint {
    // Dynamically calculating displace
    float hypot = hypotf(fabsf(endPoint.x - startPoint.x), fabsf(endPoint.y - startPoint.y));
    // hypot/displace = 4/1
    float displace = hypot*0.25;
    float angle = atan2(endPoint.x - startPoint.x, endPoint.y - startPoint.y);
 
    NSMutableArray *pathArray = [NSMutableArray array];
    [pathArray addObject:[NSValue valueWithCGPoint:startPoint]];
    createBolt(startPoint.x, startPoint.y, endPoint.x, endPoint.y, displace, pathArray);
    //    NSMutableArray *boltLines = [NSMutableArray array];
    for (int i = 0; i < pathArray.count - 1; i = i + 1) {
        LightningLine *line = [[LightningLine alloc] initWithStartPoint:((NSValue *)pathArray[i]).CGPointValue 
                                                               endPoint:((NSValue *)pathArray[i+1]).CGPointValue];
        [self addChild:line];
        [line draw];
    }
}

I prepared sample Xcode project, which includes all of the approaches described here: https://github.com/andrew8712/SpriteKitLightning

Here is Swift version: https://github.com/andrew8712/SpriteKitLightning-Swift

You can use it however you want to.