CGLayer绘图随着时间的推移变慢

Question

任何人都可以解释为什么重绘到离线CGLayer会导致渲染随着时间的推移而变慢？让我给你展示一个我为了说明问题而创建的测试。

@implementation DrawView 


- (id)initWithFrame:(CGRect)frame { 
    self = [super initWithFrame:frame]; 
    if (self) { 
     //setup frame rate monitoring 
     fps = [[UITextField alloc] initWithFrame:CGRectMake(0, 0, 100, 20)]; 
     fps.textColor = [UIColor whiteColor]; 
     fps.font = [UIFont boldSystemFontOfSize:15]; 
     fps.text = @"0 fps"; 
     [self addSubview:fps]; 
     frames = 0; 
     lastRecord = [NSDate timeIntervalSinceReferenceDate]; 

     //create a cglayer and draw the background graphic to it 
     CGContextRef context = UIGraphicsGetCurrentContext(); 
     cacheLayer = CGLayerCreateWithContext(context, self.bounds.size, NULL); 

     CGImageRef background = [[UIImage imageNamed:@"background.jpg"] CGImage]; 
     CGContextRef cacheContext = CGLayerGetContext(cacheLayer); 
     CGContextDrawImage(cacheContext, CGRectMake(0, 0, 768, 1024), background); 

     //initialize cgimage stamp 
     stamp = [[UIImage imageNamed:@"stamp.png"] CGImage]; 
     stampTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/60 target:self selector:@selector(stamp) userInfo:nil repeats:YES]; 
    } 
    return self; 
} 

- (void) stamp { 
    //calculate fps 
    NSTimeInterval interval = [NSDate timeIntervalSinceReferenceDate]; 
    NSTimeInterval diff = interval-lastRecord; 
    if (diff > 1.0) { 
     float rate = frames/diff; 
     frames = 0; 
     lastRecord = [NSDate timeIntervalSinceReferenceDate]; 
     fps.text = [NSString stringWithFormat:@"%0.1f fps", rate]; 
    } 
    //stamp the offscreen cglayer with the cgimage graphic 
    CGRect stampRect = CGRectMake(0, 0, 200, 200); 
    CGContextRef cacheContext = CGLayerGetContext(cacheLayer); 
    CGContextDrawImage(cacheContext, stampRect, stamp); 
    [self setNeedsDisplayInRect:stampRect]; 
} 

- (void)drawRect:(CGRect)dirtyRect { 
    CGContextRef context = UIGraphicsGetCurrentContext(); 
    CGContextDrawLayerInRect(context, self.bounds, cacheLayer); 
    frames++; 
} 

当运行该测试在ipad的模拟器或设备它开始于40 fps的，并以恒定的速率在10秒的过程中，直到它在像3 fps的运行下降。这是为什么发生？不应该以恒定的帧率运行吗？什么样的解决方案可以让我一次又一次地“印”图像，同时保持恒定的帧率？

Answer 1

你的问题是你创建了一个NULL上下文的图层。 UIGraphicsGetCurrentContext()仅在绘制循环中有效。你把它叫做draw循环外部，所以它是NULL，所以图层不能缓存任何东西。它仍然令我感到惊讶，随着时间的推移，这种表现会如何糟糕。我怀疑CoreGraphics中可能存在一个错误;你会认为这会很慢;不会“放慢速度”。但是，它的设计目的不是这样。

如果您创建位图上下文并将其用于图层，则会获得60fps。你不需要在这里放弃CGLayer。

我所做的一切，回到60fps的是替换这样的：

CGContextRef context = UIGraphicsGetCurrentContext(); 

与此：

CGContextRef context = CreateBitmapContext(self.bounds.size); 

哪里CreateBitmapContext()是最多返回同样的事情你createBitmapContext集的功能。

Answer 2

我发现了这个解决方案，虽然看起来与直觉相反，但创建一个缓存的位图上下文并且然后从中复制一个图像以绘制到当前上下文会更快。

- (void) createBitmapContext { 
    // Create the bitmap context 
    void *   bitmapData; 
    int    bitmapByteCount; 
    int    bitmapBytesPerRow; 
    CGSize   size = self.bounds.size; 

    // Declare the number of bytes per row. Each pixel in the bitmap in this 
    // example is represented by 4 bytes; 8 bits each of red, green, blue, and 
    // alpha. 
    bitmapBytesPerRow = (size.width * 4); 
    bitmapByteCount  = (bitmapBytesPerRow * size.height); 

    // Allocate memory for image data. This is the destination in memory 
    // where any drawing to the bitmap context will be rendered. 
    bitmapData = malloc(bitmapByteCount); 
    if (bitmapData == NULL) 
    { 
     //return nil; 
    } 
    cacheContext = CGBitmapContextCreate (bitmapData, size.width, size.height,8,bitmapBytesPerRow, 
              CGColorSpaceCreateDeviceRGB(),kCGImageAlphaNoneSkipFirst); 
} 


- (id)initWithFrame:(CGRect)frame { 
    self = [super initWithFrame:frame]; 
    if (self) { 
     //setup frame rate monitoring 
     fps = [[UITextField alloc] initWithFrame:CGRectMake(0, 0, 100, 20)]; 
     fps.textColor = [UIColor whiteColor]; 
     fps.font = [UIFont boldSystemFontOfSize:15]; 
     fps.text = @"0 fps"; 
     [self addSubview:fps]; 
     frames = 0; 
     lastRecord = [NSDate timeIntervalSinceReferenceDate]; 

     //create a bitmap context and draw the background graphic to it 
     [self createBitmapContext]; 

     CGImageRef background = [[UIImage imageNamed:@"background.jpg"] CGImage]; 
     CGContextDrawImage(cacheContext, CGRectMake(0, 0, 768, 1024), background); 

     //initialize cgimage stamp 
     stamp = [[UIImage imageNamed:@"stamp.png"] CGImage]; 
     stampTimer = [NSTimer scheduledTimerWithTimeInterval:1.0/60 target:self selector:@selector(stamp) userInfo:nil repeats:YES]; 
    } 
    return self; 
} 

- (void) stamp { 
    //calculate fps 
    NSTimeInterval interval = [NSDate timeIntervalSinceReferenceDate]; 
    NSTimeInterval diff = interval-lastRecord; 
    if (diff > 1.0) { 
     float rate = frames/diff; 
     frames = 0; 
     lastRecord = [NSDate timeIntervalSinceReferenceDate]; 
     fps.text = [NSString stringWithFormat:@"%0.1f fps", rate]; 
    } 
    //stamp the offscreen bitmap context with the cgimage graphic 
    CGRect stampRect = CGRectMake(0, 0, 200, 200); 
    CGContextDrawImage(cacheContext, stampRect, stamp); 
    [self setNeedsDisplayInRect:stampRect]; 
} 

- (void)drawRect:(CGRect)dirtyRect { 
    CGContextRef context = UIGraphicsGetCurrentContext(); 
    CGImageRef cacheImage = CGBitmapContextCreateImage(cacheContext); 
    CGContextDrawImage(context, self.bounds, cacheImage); 
    CGImageRelease(cacheImage); 
    frames++; 
}