CGbitmapcontext 和 alpha 的问题
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【中文标题】CGbitmapcontext 和 alpha 的问题【英文标题】:Problems with CGbitmapcontext and alpha 【发布时间】:2014-05-01 19:31:11 【问题描述】:我正在尝试使用 Core Graphics 开发绘图应用程序。我想让背景处于 alpha 状态,但它是黑色的。我尝试使用所有不同类型的 bitmapinfo 类型,但没有成功。 kCGImageAlphaPremultipliedLast 也不起作用。有谁知道如何解决这个问题?
- (BOOL) initContext:(CGSize)size
int bitmapByteCount;
int bitmapBytesPerRow;
// 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.
cacheBitmap = malloc( bitmapByteCount );
if (cacheBitmap == NULL)
return NO;
CGBitmapInfo bitmapInfo = kCGImageAlphaNoneSkipFirst;
cacheContext = CGBitmapContextCreate (cacheBitmap, size.width, size.height, 8, bitmapBytesPerRow, CGColorSpaceCreateDeviceRGB(), bitmapInfo);
CGContextSetRGBFillColor(cacheContext, 0, 0, 0, 0);
CGContextFillRect(cacheContext, (CGRect)CGPointZero, size);
return YES;
【问题讨论】:
这里的代码没有指定 alpha 通道(因此 AlphaNone)我想你要找的可能是kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrderDefault
另外,正如所写,我相信你正在泄漏CGColorSpaceCreateDeviceRGB()
分配的内存
你能解释更多关于泄漏内存的内容吗?
还是不行。
你如何确定它不起作用?
【参考方案1】:
我正在使用此代码来完成您所要求的几乎所有操作。我已将颜色设置为红色,alpha 为 50% 而不是 0%,因此您实际上可以看到 alpha 通道就在那里。
@implementation ViewController
CGContextRef cacheContext;
void* cacheBitmap;
__weak IBOutlet UIImageView* _imageView;
- (void)viewDidLoad
[super viewDidLoad];
-(void)viewDidAppear:(BOOL)animated
[super viewDidAppear:animated];
[self setupContext:self.view.bounds.size];
CGImageRef cgImage = CGBitmapContextCreateImage(cacheContext);
_imageView.image = [UIImage imageWithCGImage:cgImage];
CGImageRelease(cgImage);
// Name changed to avoid using the magic word "init"
- (BOOL) setupContext:(CGSize)size
int bitmapByteCount;
int bitmapBytesPerRow;
// 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.
cacheBitmap = malloc( bitmapByteCount );
if (cacheBitmap == NULL)
return NO;
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrderDefault;
// Create and define the color space
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
cacheContext = CGBitmapContextCreate (cacheBitmap, size.width, size.height, 8, bitmapBytesPerRow, colorSpace, bitmapInfo);
CGContextSetRGBFillColor(cacheContext, 1., 0, 0, 0.5);
CGContextFillRect(cacheContext, (CGRect)CGPointZero, size);
// Release the color space so memory doesn't leak
CGColorSpaceRelease(colorSpace);
return YES;
@end
【讨论】:
谢谢!还发现我没有将视图设置为 [UIColor ClearColor]以上是关于CGbitmapcontext 和 alpha 的问题的主要内容,如果未能解决你的问题,请参考以下文章