Thread safe UIImage resizing? - iphone

Question

I made this UIImage extension to get a rescaled copy:
-(UIImage*)scaleByRatio:(float) scaleRatio
{
CGSize scaledSize = CGSizeMake(self.size.width * scaleRatio, self.size.height * scaleRatio);
//The output context.
UIGraphicsBeginImageContext(scaledSize);
CGContextRef context = UIGraphicsGetCurrentContext();
//Percent (101%)
#define SCALE_OVER_A_BIT 1.01
//Scale.
CGContextScaleCTM(context, scaleRatio * SCALE_OVER_A_BIT, scaleRatio * SCALE_OVER_A_BIT);
[self drawAtPoint:CGPointZero];
//End?
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return scaledImage;
}
It works in the most cases, but in my recent project it outputs the original image (I save to disk right after scaling using UIImageJPEGRepresentation and imageWithData:).
I invoke the method within a background thread. Could this be the problem? How can I rewrite this to be thread safe (supposing the problem is caused by threading).

Answer 1

-(UIImage*)scaleByRatio:(float) scaleRatio
{
CGSize scaledSize = CGSizeMake(self.size.width * scaleRatio, self.size.height * scaleRatio);
CGColorSpaceRef colorSpace;
int bitmapBytesPerRow;
bitmapBytesPerRow = (size.width * 4);
//The output context.
UIGraphicsBeginImageContext(scaledSize);
CGContextRef context = context = CGBitmapContextCreate (NULL,
scaledSize .width,
scaledSize .height,
8, // bits per component
bitmapBytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedLast);
//Percent (101%)
#define SCALE_OVER_A_BIT 1.01
//Scale.
CGContextScaleCTM(context, scaleRatio * SCALE_OVER_A_BIT, scaleRatio * SCALE_OVER_A_BIT);
[self drawAtPoint:CGPointZero];
//End?
UIImage *scaledImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return scaledImage;
}
In-short you have to Create CGContextRef using CGBitmapContextCreate not using UIGraphicsGetCurrentContext(); becuase UIGraphicsGetCurrentContext(); is not thrad safe.
Hope, this will help you...enjoy