我正在尝试在后台线程中更改图像的颜色 Apple doc说UIGraphicsBeginImageContext只能从主线程调用，而我正在尝试使用CGBitmapContextCreate：
context = CGBitmapContextCreate （位图数据， pixelsWide， pixelsHigh， 8，//每个组件的比特
bitmapBytesPerRow, colorSpace, kCGImageAlphaPremultipliedFirst);
我有两个版本的“changeColor”，第一个使用UIGraphisBeginImageContext，第二个使用CGBitmapContextCreate。
第一个正确改变颜色，但第二个没有 这是为什么？
- (UIImage*) changeColor: (UIColor*) aColor
{
if(aColor == nil)
return self;
UIGraphicsBeginImageContext(self.size);
CGRect bounds;
bounds.origin = CGPointMake(0,0);
bounds.size = self.size;
[aColor set];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextTranslateCTM(context, 0, self.size.height);
CGContextScaleCTM(context, 1.0, -1.0);
CGContextClipToMask(context, bounds, [self CGImage]);
CGContextFillRect(context, bounds);
UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return img;
}
- (UIImage*) changeColor: (UIColor*) aColor
{
if(aColor == nil)
return self;
CGContextRef context = CreateARGBBitmapContext(self.size);
CGRect bounds;
bounds.origin = CGPointMake(0,0);
bounds.size = self.size;
CGColorRef colorRef = aColor.CGColor;
const CGFloat *components = CGColorGetComponents(colorRef);
float red = components[0];
float green = components[1];
float blue = components[2];
CGContextSetRGBFillColor(context, red, green, blue, 1);
CGContextClipToMask(context, bounds, [self CGImage]);
CGContextFillRect(context, bounds);
CGImageRef imageRef = CGBitmapContextCreateImage(context);
UIImage* img = [UIImage imageWithCGImage: imageRef];
unsigned char* data = (unsigned char*)CGBitmapContextGetData (context);
CGContextRelease(context);
free(data);
return img;
}
CGContextRef CreateARGBBitmapContext(CGSize size)
{
CGContextRef context = NULL;
CGColorSpaceRef colorSpace;
void * bitmapData;
int bitmapByteCount;
int bitmapBytesPerRow;
// Get image width, height. We'll use the entire image.
size_t pixelsWide = size.width;
size_t pixelsHigh = size.height;
// 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 = (pixelsWide * 4);
bitmapByteCount = (bitmapBytesPerRow * pixelsHigh);
// Use the generic RGB color space.
colorSpace = CGColorSpaceCreateDeviceRGB();
if (colorSpace == NULL)
{
fprintf(stderr, "Error allocating color space\n");
return NULL;
}
// 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)
{
fprintf (stderr, "Memory not allocated!");
CGColorSpaceRelease( colorSpace );
return NULL;
}
// Create the bitmap context. We want pre-multiplied ARGB, 8-bits
// per component. Regardless of what the source image format is
// (CMYK, Grayscale, and so on) it will be converted over to the format
// specified here by CGBitmapContextCreate.
context = CGBitmapContextCreate (bitmapData,
pixelsWide,
pixelsHigh,
8, // bits per component
bitmapBytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedFirst);
if (context == NULL)
{
free (bitmapData);
fprintf (stderr, "Context not created!");
}
// Make sure and release colorspace before returning
CGColorSpaceRelease( colorSpace );
return context;
}
答案 0 :(得分：3)
你的第二种方法是做第一种从未做过的工作。这是对第二种方法的调整，以便更接近第一种方法：
- (UIImage*) changeColor: (UIColor*) aColor
{
if(aColor == nil)
return self;
CGContextRef context = CreateARGBBitmapContext(self.size);
CGRect bounds;
bounds.origin = CGPointMake(0,0);
bounds.size = self.size;
CGContextSetFillColorWithColor(aColor.CGColor);
CGContextClipToMask(context, bounds, [self CGImage]);
CGContextFillRect(context, bounds);
CGImageRef imageRef = CGBitmapContextCreateImage(context);
UIImage* img = [UIImage imageWithCGImage: imageRef];
CGContextRelease(context);
return img;
}
我做的两个更改是将其转换为使用CGContextSetFillColorWithColor()，并删除了位图上下文的后备数据的危险且不正确的free()。如果此代码段的行为与第一个代码段的行为不同，那么您必须查看CreateARGBBitmapContext()的实现，以验证它是否正确。
当然，正如Brad Larson在评论中提到的，如果您的目标是iOS 4.0及更高版本，UIKit图形方法（根据发行说明）是线程安全的，您应该能够使用第一种方法细