CGAffineTransformMake(a,b,c,d,tx,ty) 矩阵运算的原理
//UIImageOrientation的定义，定义了如下几种变换typedef enum { UIImageOrientationUp, // default orientation UIImageOrientationDown, // 180 deg rotation UIImageOrientationLeft, // 90 deg CCW UIImageOrientationRight, // 90 deg CW UIImageOrientationUpMirrored, // as above but image mirrored along other axis. horizontal flip UIImageOrientationDownMirrored, // horizontal flip UIImageOrientationLeftMirrored, // vertical flip UIImageOrientationRightMirrored, // vertical flip} UIImageOrientation;//按照UIImageOrientation的定义，利用矩阵自定义实现对应的变换；-(UIImage *)transformImage:(UIImage *)aImage { CGImageRef imgRef = aImage.CGImage; CGFloat width = CGImageGetWidth(imgRef); CGFloat height = CGImageGetHeight(imgRef); CGAffineTransform transform = CGAffineTransformIdentity; CGRect bounds = CGRectMake(0, 0, width, height); CGFloat scaleRatio = 1; CGFloat boundHeight; UIImageOrientation orient = aImage.imageOrientation; switch(UIImageOrientationLeftMirrored) { case UIImageOrientationUp: transform = CGAffineTransformIdentity; break; case UIImageOrientationUpMirrored: transform = CGAffineTransformMakeTranslation(width, 0.0); transform = CGAffineTransformScale(transform, -1.0, 1.0); //沿y轴向左翻 break; case UIImageOrientationDown: transform = CGAffineTransformMakeTranslation(width, height); transform = CGAffineTransformRotate(transform, M_PI); break; case UIImageOrientationDownMirrored: transform = CGAffineTransformMakeTranslation(0.0, height); transform = CGAffineTransformScale(transform, 1.0, -1.0); break; case UIImageOrientationLeft: boundHeight = bounds.size.height; bounds.size.height = bounds.size.width; bounds.size.width = boundHeight; transform = CGAffineTransformMakeTranslation(0.0, width); transform = CGAffineTransformRotate(transform, 3.0 * M_PI / 2.0); break; case UIImageOrientationLeftMirrored: boundHeight = bounds.size.height; bounds.size.height = bounds.size.width; bounds.size.width = boundHeight; transform = CGAffineTransformMakeTranslation(height, width); transform = CGAffineTransformScale(transform, -1.0, 1.0); transform = CGAffineTransformRotate(transform, 3.0 * M_PI / 2.0); break; case UIImageOrientationRight: //EXIF = 8 boundHeight = bounds.size.height; bounds.size.height = bounds.size.width; bounds.size.width = boundHeight; transform = CGAffineTransformMakeTranslation(height, 0.0); transform = CGAffineTransformRotate(transform, M_PI / 2.0); break; case UIImageOrientationRightMirrored: boundHeight = bounds.size.height; bounds.size.height = bounds.size.width; bounds.size.width = boundHeight; transform = CGAffineTransformMakeScale(-1.0, 1.0); transform = CGAffineTransformRotate(transform, M_PI / 2.0); break; default: [NSException raise:NSInternalInconsistencyException format:@"Invalid image orientation"]; } UIGraphicsBeginImageContext(bounds.size); CGContextRef context = UIGraphicsGetCurrentContext(); if (orient == UIImageOrientationRight || orient == UIImageOrientationLeft) { CGContextScaleCTM(context, -scaleRatio, scaleRatio); CGContextTranslateCTM(context, -height, 0); } else { CGContextScaleCTM(context, scaleRatio, -scaleRatio); CGContextTranslateCTM(context, 0, -height); } CGContextConcatCTM(context, transform); CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, width, height), imgRef); UIImage *imageCopy = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); return imageCopy; }
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参考：
https://developer.apple.com/library/ios/documentation/GraphicsImaging/Conceptual/drawingwithquartz2d/dq_affine/dq_affine.html
