iOS Developer's Cookbook: Painting Sprite Kit
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Sprite Kit lives in a country of its own. It sports its own currency, customs, and language. It's not exactly UIKit. It's not exactly AppKit. It's something different. Although certain classes and macros appear familiar, they often present unexpected difficulties. For example, consider SKColor. The SKColor macro resolves to UIColor on iOS and NSColor on OS X. It provides a convenient way to simplify cross-platform development. But (you knew there was a "but" coming) this cross-platform support works only with solid colors in the RGBA device color space.
#if TARGET_OS_IPHONE #define SKColor UIColor #else #define SKColor NSColor #endif
So what happens when you want to paint in paisley? Many Sprite Kit node classes support some sort of color attribute, but to leverage UIColor's colorWithPatternImage: API (as in Figure 1) requires a little work.
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Figure 1 To paint using a UIColor pattern involves a sprite-based workaround
Before showing the solution, take a quick look at the code that generated the right scene in Figure 1. In particular, notice how the color is applied. The paintedBackgroundColor property belongs to a custom SKNode category.
- (void) setup
{
SKScene *scene = [SKScene sceneWithSize:skview.frame.size];
UIImage *pattern = [UIImage imageNamed:@"Purple Paisley"];
scene.paintedBackgroundColor = [UIColor colorWithPatternImage:pattern];
ShadowLabelNode *label = [ShadowLabelNode labelNodeWithText:@"I am an SKScene"];
label.fontSize = 20;
label.fontColor = [UIColor purpleColor];
[scene addChild:label];
[label centerNodeInScene];
[skview presentScene:scene];
}
Leveraging UIKit Drawing
Behind this property is the following code. It consists of a helper function that draws an image, creates a child sprite node with that image, and matches it to the size of the parent node.
NSString *const SceneryBackdrop = @"SceneryBackdrop";
- (void) updateBackgroundWithBlock: (void (^)()) block
{
// Remove any existing background
SKNode *background = [self childNodeWithName:SceneryBackdrop];
if (background) [background removeFromParent];
// Paint or draw
UIGraphicsBeginImageContextWithOptions(self.frame.size, NO, 0);
if (block) block();
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// Build a sprite node to present the texture
SKTexture *texture = [SKTexture textureWithImage:image];
SKSpriteNode *node = [SKSpriteNode spriteNodeWithTexture:texture size:image.size];
node.name = SceneryBackdrop;
node.anchorPoint = CGPointMake(0, 0);
node.position = CGPointZero;
// Add it at the back
[self insertChild:node atIndex:0];
node.zposition = -1; // adjust if you want it further back
}
- (void) setPaintedBackgroundColor:(UIColor *)SceneryBackdropColor
{
[self updateBackgroundWithBlock:^(){
[SceneryBackdropColor set];
UIRectFill((CGRect){.size = self.frame.size});
}];
}
To paint the background color, the setPaintedBackgroundColor: method passes a block with drawing instructions. The block sets the UIKit color and draws it using the UIKit UIRectFill() function. When drawn, the method converts the UIKit image to a Sprite Kit texture and creates a sprite node, making sure to name it with the reserved scenery backdrop keyword. The backdrop is always added at index 0, so it floats behind any other children in the node.
Keep in mind that Sprite Kit textures do not use screen-scale coding. A retina image produces a texture twice as big as its 1x scale sibling. This method uses the image size to scale the sprite node, ensuring that the point size of the sprite matches the point size of the source node. Although nodes offer xScale and yScale properties (and you can retrieve the calculateAccumulatedFrame property) I generally find it easier to work directly with the expected size.
The block-based approach used here makes it easy to add other drawn properties using UIKit calls. The following methods assign an image to be centered in the background (setBackgroundImage:) and one that fills the node's background (setScenery:). The node creation work is done by the helper method, so these methods concern themselves only with drawing operations. Because these blocks are quickly run and then disposed, they do not use any self-capture workarounds.
- (void) setScenery:(UIImage *)scenery
{
[self updateBackgroundWithBlock:^() {
CGRect rect = (CGRect){.size = scenery.size};
CGRect bounds = (CGRect){.size = self.frame.size};
rect = SKRectByFillingRect(rect, bounds);
[scenery drawInRect:rect];
}];
}
- (void) setBackgroundImage:(UIImage *)backgroundImage
{
[self updateBackgroundWithBlock:^() {
CGFloat screenScale = UIScreen.mainScreen.scale;
CGFloat imageScale = backgroundImage.scale;
CGSize size = backgroundImage.size;
if ((imageScale == 1) && (screenScale != imageScale))
size = CGSizeMake(backgroundImage.size.width / screenScale,
backgroundImage.size.height / screenScale);
CGRect rect = SKRectCenteredInRect((CGRect){.size = size},
(CGRect){.size = self.frame.size});
[backgroundImage drawInRect:rect];
}];
}
Painting Shape Nodes
You tweak this approach slightly to account for the SKShapeNode class. The first modification in the following method retrieves the instance's path. It uses it to clip its drawing, as in the first of the two screen shots in Figure 1. Clipping to the path enables the drawing to take place only within its edges. When working with complex shapes, remember to enable the path's even-odd fill rule for best results. (Tangentially, I did consider using a crop node here instead but found it wasn't really worth the extra effort.)
The second modification is specific to the way I personally create shape nodes. My shapes always center around CGPointZero. This ensures that position properties correspond to the center of the drawn shape. Updating the sprite node's anchor point to (0.5, 0.5) ensures the drawn image remains centered in its parent.
I use background drawing almost exclusively for SKScene and SKShapeNode instances, so this method might need adapting for other classes.
- (void) updateBackgroundWithBlock: (void (^)()) block
{
// Remove any existing background
SKNode *background = [self childNodeWithName:SceneryBackdrop];
if (background) [background removeFromParent];
// Paint or draw
UIGraphicsBeginImageContextWithOptions(self.frame.size, NO, 0);
if ([self isKindOfClass:[SKShapeNode class]])
{
SKShapeNode *node = (SKShapeNode *) self;
// Fetch a drawing path
UIBezierPath *path = [UIBezierPath bezierPathWithCGPath:node.path];
[path applyTransform:CGAffineTransformMakeTranslation(
-path.bounds.origin.x, -path.bounds.origin.y)];
[path addClip];
}
if (block) block();
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
// Build a sprite node to present the texture
SKTexture *texture = [SKTexture textureWithImage:image];
SKSpriteNode *node = [SKSpriteNode spriteNodeWithTexture:texture size:image.size];
node.name = SceneryBackdrop;
node.anchorPoint = CGPointMake(0, 0);
if ([self isKindOfClass:[SKShapeNode class]]) node.anchorPoint = CGPointMake(0.5, 0.5);
node.position = CGPointZero;
// Add it at the back
[self insertChild:node atIndex:0];
node.zposition = -1; // adjust if you want it further back
}
Building a Debugging Backdrop
Figure 2 shows my standard debugging backdrop. It consists of two grids. A fine grid lays out 8-point texture blocks, enabling easy (if not exact) measurements to edges and between sprites. A second overlay, the 2-by-2 gray blocks, splits the SKScene into quadrants. This helps focus position checks on boundaries and centerlines.
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Figure 2 A gridded backdrop enables you to check alignment and estimate point positions
The following method draws this placement backdrop. It uses the custom scenery property defined earlier in this write-up to add it to the SKScene instance. As you can see, this method takes advantage of UIKit drawing to produce an artifact that's quite useful for the Sprite Kit world.
- (void) drawPlacementBackdrop
{
CGRect rect = self.frame;
CGSize size = rect.size;
CGSize small = CGSizeMake(16, 16);
CGSize smaller = CGSizeMake(8, 8);
// Create 8x8 grid pattern color
UIGraphicsBeginImageContextWithOptions(small, NO, 0);
[[UIColor whiteColor] set];
UIRectFill((CGRect){.size = small});
[[UIColor lightGrayColor] set];
UIRectFill((CGRect){.size = smaller});
UIRectFill((CGRect){.size = smaller, .origin = CGPointMake(smaller.width, smaller.height)});
UIColor *patternColor = [UIColor colorWithPatternImage:
UIGraphicsGetImageFromCurrentImageContext()];
UIGraphicsEndImageContext();
// Draw backdrop
UIGraphicsBeginImageContextWithOptions(size, NO, 0);
UIColor *light = [[UIColor lightGrayColor] colorWithAlphaComponent:0.25];
UIColor *dark = [[UIColor grayColor] colorWithAlphaComponent:0.25];
// Fill bg with white
[[UIColor whiteColor] set];
UIRectFill(rect);
// Lightly draw pattern over it
UIBezierPath *path = [UIBezierPath bezierPathWithRect:rect];
[[patternColor colorWithAlphaComponent:0.15] set];
[path fill];
// Add left backsplash
path = [UIBezierPath bezierPathWithRect:CGRectMake(0, 0, size.width / 2, size.height)];
[light set];
[path fill];
// Add right backsplash
path = [UIBezierPath bezierPathWithRect:
CGRectMake(0, size.height / 2, size.width, size.height / 2)];
[dark set];
[path fill];
UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
self.scenery = image;
}
Wrap-Up
Applying patterns and easily drawing backdrops simplifies basic Sprite Kit development. Building this object category enabled me to move away from the fussy details of wallpapering my world to help focus on actual game design and debugging.