使用Core Image渲染随机噪声CALayer会产生奇怪的伪像

Question

我是以编程方式创建一个CALayer子类,它将一些像素噪声应用于自身.代码的工作原理是它在图层中呈现噪声,但图像上有一个奇怪的工件,我无法确定根本原因.

这是一个示例图像,noiseOpacity调高后可以使问题更加明显.

粉红色的盒子是UANoisyGradientLayer,一个CAGradientLayer具有以下位的子类:

@interface UANoisyGradientLayer ()
    @property (nonatomic, retain) CIContext *noiseContext;
    @property (nonatomic, retain) CIFilter  *noiseGenerator;
    @property (nonatomic, retain) CIImage   *noiseImage;
@end

@implementation UANoisyGradientLayer

@synthesize noiseOpacity = _noiseOpacity, noiseImage;

- (id)init {
    self = [super init];
    if (self) {
        self.noiseOpacity = 0.10;
        self.noiseContext = [CIContext contextWithOptions:nil];         
        self.noiseGenerator = [CIFilter filterWithName:@"CIColorMonochrome"];
        [self.noiseGenerator setValue:[[CIFilter filterWithName:@"CIRandomGenerator"] valueForKey:@"outputImage"] forKey:@"inputImage"];
        [self.noiseGenerator setDefaults];

        self.noiseImage = [self.noiseGenerator outputImage];

    }
    return self;
}

- (void)drawInContext:(CGContextRef)ctx {

    [super drawInContext:ctx];

    CGRect extentRect = [self.noiseImage extent];
    if (CGRectIsInfinite(extentRect) || CGRectIsEmpty(extentRect)) {
        extentRect = self.bounds;
    }

    CGImageRef cgimg = [self.noiseContext createCGImage:self.noiseImage fromRect:extentRect];
    CGContextSetBlendMode(ctx, kCGBlendModeOverlay);
    CGContextSetAlpha(ctx, self.noiseOpacity);
    CGContextDrawImage(ctx, self.bounds, cgimg);
    CGImageRelease(cgimg);
}

基本上,我CIImage使用CIRandomGeneratoras作为CIColorMonochrome过滤器的输入来创建in init .然后,当需要绘制它时,我CGImageRef使用self.bounds(范围总是 infinite或0)创建一个它,并将其绘制到上下文中.

结果大部分都很好,但正如你在图像中看到的那样,似乎有一些延伸.这里发生了什么？

Answer 1

虽然没有解决原来的问题,但我从不同的角度解决了这个问题,并且重复了输出.self.bounds我现在正在生成一个只有64x64的图像,然后使用它来平铺它,而不是尝试生成大小的单个图像CGContextDrawTiledImage.因为我现在正在调整固定大小,我可以从drawInContext:方法中提取一些代码.最后,因为在绘制方法中没有更多的图像生成,我能够使它成为一个静态var,所以它只生成一次!这是完整的课程:

static CGImageRef   __noiseImage        = nil;
static CGFloat      __noiseImageWidth   = 0.0;
static CGFloat      __noiseImageHeight  = 0.0;

@implementation UANoisyGradientLayer

@synthesize noiseOpacity = _noiseOpacity;

- (id)init {
    self = [super init];
    if (self) {
        self.noiseOpacity = 0.1f;
        self.needsDisplayOnBoundsChange = YES;

        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            CIContext *noiseContext = [CIContext contextWithOptions:nil];

            CIFilter *noiseGenerator = [CIFilter filterWithName:@"CIColorMonochrome"];
            [noiseGenerator setValue:[[CIFilter filterWithName:@"CIRandomGenerator"] valueForKey:@"outputImage"] forKey:@"inputImage"];
            [noiseGenerator setDefaults];

            CIImage *ciImage = [noiseGenerator outputImage];

            CGRect extentRect = [ciImage extent];
            if (CGRectIsInfinite(extentRect) || CGRectIsEmpty(extentRect)) {
                extentRect = CGRectMake(0, 0, 64, 64);
            }

            __noiseImage = [noiseContext createCGImage:ciImage fromRect:extentRect];
            __noiseImageWidth = CGImageGetWidth(__noiseImage);
            __noiseImageHeight = CGImageGetHeight(__noiseImage);
        });
    }

    return self;
}

- (void)drawInContext:(CGContextRef)ctx {

    [super drawInContext:ctx];

    if (self.noiseOpacity > 0) {

        CGContextSaveGState(ctx);
        CGPathRef path = [[UIBezierPath bezierPathWithRoundedRect:self.bounds cornerRadius:self.cornerRadius] CGPath];
        CGContextAddPath(ctx, path);
        CGContextClip(ctx);
        CGContextSetBlendMode(ctx, kCGBlendModeOverlay);
        CGContextSetAlpha(ctx, self.noiseOpacity);


        CGContextDrawTiledImage(ctx, CGRectMake(0, 0, __noiseImageWidth, __noiseImageHeight), __noiseImage);

        CGContextRestoreGState(ctx);
    }
}

@end

注意:CIColorMonochrome并且CIRandomGenerator需要iOS 6(或更新版本).确保包含所需的框架(CoreImage.framework和QuartzCore.framework).