How can you create a glow around a sprite via SKEffectNode

Question

I have a SKSpriteNode that I'd like to have a blue glow around it's edges for highlighting purposes. I am guessing that I would need to make my sprite a child of a SKEffectNode and then create/apply a filter of some sort.

UPDATE : I have investigated this quite a it with the chosen answer's approach, and discovered that SKEffectNode has a sizeable hit on performance even if you have it set to shouldRasterize and 'no filter' defined. My conclusion is that if your game requires more than 10 moving objects at one time, they can't involve a SKEffectNode even if rasterized.

My solution will likely involve pre-rendered glow images/animations, as SKEffectNode is not going to cut it for my requirements.

If someone has insight as to anything I am missing, I'd appreciate hearing whatever you know!

I am accepting an answer because it does achieve what I asked for, but wanted to add these notes to anyone looking to go this route, so you can be aware of some of the issues with using SKEffectNode.

Answer 1

@rickster's answer is great. Since I have low rep, I'm apparently not allowed to add this code as a comment to his. I hope this doesn't break stackoverflow rules of propriety. I'm not trying to userp his rep in any way.

Here's code that does what he's describing in his answer:

Header:

//  ENHGlowFilter.h
#import <CoreImage/CoreImage.h>

@interface ENHGlowFilter : CIFilter

@property (strong, nonatomic) UIColor *glowColor;
@property (strong, nonatomic) CIImage *inputImage;
@property (strong, nonatomic) NSNumber *inputRadius;
@property (strong, nonatomic) CIVector *inputCenter;

@end

//Based on ASCGLowFilter from Apple

Implementation:

#import "ENHGlowFilter.h"

@implementation ENHGlowFilter

-(id)init
{
    self = [super init];
    if (self)
    {
        _glowColor = [UIColor whiteColor];
    }
    return self;
}

- (NSArray *)attributeKeys {
    return @[@"inputRadius", @"inputCenter"];
}

- (CIImage *)outputImage {
    CIImage *inputImage = [self valueForKey:@"inputImage"];
    if (!inputImage)
        return nil;

    // Monochrome
    CIFilter *monochromeFilter = [CIFilter filterWithName:@"CIColorMatrix"];
    CGFloat red = 0.0;
    CGFloat green = 0.0;
    CGFloat blue = 0.0;
    CGFloat alpha = 0.0;
    [self.glowColor getRed:&red green:&green blue:&blue alpha:&alpha];
    [monochromeFilter setDefaults];
    [monochromeFilter setValue:[CIVector vectorWithX:0 Y:0 Z:0 W:red] forKey:@"inputRVector"];
    [monochromeFilter setValue:[CIVector vectorWithX:0 Y:0 Z:0 W:green] forKey:@"inputGVector"];
    [monochromeFilter setValue:[CIVector vectorWithX:0 Y:0 Z:0 W:blue] forKey:@"inputBVector"];
    [monochromeFilter setValue:[CIVector vectorWithX:0 Y:0 Z:0 W:alpha] forKey:@"inputAVector"];
    [monochromeFilter setValue:inputImage forKey:@"inputImage"];
    CIImage *glowImage = [monochromeFilter valueForKey:@"outputImage"];

    // Scale
    float centerX = [self.inputCenter X];
    float centerY = [self.inputCenter Y];
    if (centerX > 0) {
        CGAffineTransform transform = CGAffineTransformIdentity;
        transform = CGAffineTransformTranslate(transform, centerX, centerY);
        transform = CGAffineTransformScale(transform, 1.2, 1.2);
        transform = CGAffineTransformTranslate(transform, -centerX, -centerY);

        CIFilter *affineTransformFilter = [CIFilter filterWithName:@"CIAffineTransform"];
        [affineTransformFilter setDefaults];
        [affineTransformFilter setValue:[NSValue valueWithCGAffineTransform:transform] forKey:@"inputTransform"];
        [affineTransformFilter setValue:glowImage forKey:@"inputImage"];
        glowImage = [affineTransformFilter valueForKey:@"outputImage"];
    }

    // Blur
    CIFilter *gaussianBlurFilter = [CIFilter filterWithName:@"CIGaussianBlur"];
    [gaussianBlurFilter setDefaults];
    [gaussianBlurFilter setValue:glowImage forKey:@"inputImage"];
    [gaussianBlurFilter setValue:self.inputRadius ?: @10.0 forKey:@"inputRadius"];
    glowImage = [gaussianBlurFilter valueForKey:@"outputImage"];

    // Blend
    CIFilter *blendFilter = [CIFilter filterWithName:@"CISourceOverCompositing"];
    [blendFilter setDefaults];
    [blendFilter setValue:glowImage forKey:@"inputBackgroundImage"];
    [blendFilter setValue:inputImage forKey:@"inputImage"];
    glowImage = [blendFilter valueForKey:@"outputImage"];

    return glowImage;
}


@end

In use:

@implementation ENHMyScene //SKScene subclass

-(id)initWithSize:(CGSize)size {    
    if (self = [super initWithSize:size]) {
        /* Setup your scene here */
        [self setAnchorPoint:(CGPoint){0.5, 0.5}];
        self.backgroundColor = [SKColor colorWithRed:0.15 green:0.15 blue:0.3 alpha:1.0];

        SKEffectNode *effectNode = [[SKEffectNode alloc] init];
        ENHGlowFilter *glowFilter = [[ENHGlowFilter alloc] init];
        [glowFilter setGlowColor:[[UIColor redColor] colorWithAlphaComponent:0.5]];
        [effectNode setShouldRasterize:YES];
        [effectNode setFilter:glowFilter];
        [self addChild:effectNode];
        _effectNode = effectNode;
    }
    return self;
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    /* Called when a touch begins */

    for (UITouch *touch in touches) {
        CGPoint location = [touch locationInNode:self];
        SKSpriteNode *sprite = [SKSpriteNode spriteNodeWithImageNamed:@"Spaceship"];
        sprite.position = location;
        [self.effectNode addChild:sprite];
    }
}