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在iOS不同的Framework中,使用的是不同的坐标系:
UIKit : Y轴向下/Core Graphics(Quartz) : y轴向上/OpenGL ES : y轴向上
UIKit是iPhone SDK的Cocoa Touch层的核心framework，是iPhone应用程序图形界面和事件驱动的基础，它和传统的windows桌面一样，坐标系是y轴向下的;
Core Graphics(Quartz)一个基于2D的图形绘制引擎，它的坐标系则是y轴向上的；而OpenGL ES是iPhone SDK的2D和3D绘制引擎，它使用左手坐标系，它的坐
标系也是y轴向上的，如果不考虑z轴，在二维下它的坐标系和Quartz是一样的。
另外从相机导入的图片,需要先进行方向处理!用相机拍摄出来的照片含有EXIF信息，UIImage的imageOrientation属性指的就是EXIF中的orientation信息。如
果我们忽略orientation信息，而直接对照片进行像素处理或者drawInRect等操作，得到的结果是翻转或者旋转90之后的样子。这是因为我们执行像素处理或者
drawInRect等操作之后，imageOrientaion信息被删除了，imageOrientaion被重设为0，造成照片内容和imageOrientaion不匹配。所以，在对照片进行处理之前，
先将照片旋转到正确的方向，并且返回的imageOrientaion为0。
列举3中方法:
1.判断图片方向,进行图片旋转
1 - (UIImage *)fixOrientation:(UIImage *)aImage { 2 3 // No-op if the orientation is already correct 4 if (aImage.imageOrientation == UIImageOrientationUp) 5 return aImage; 6 7 CGAffineTransform transform = CGAffineTransformIdentity; 8 9 switch (aImage.imageOrientation) { 10 case UIImageOrientationDown: 11 case UIImageOrientationDownMirrored: 12 transform = CGAffineTransformTranslate(transform, aImage.size.width, aImage.size.height); 13 transform = CGAffineTransformRotate(transform, M_PI); 14 break; 15 16 case UIImageOrientationLeft: 17 case UIImageOrientationLeftMirrored: 18 transform = CGAffineTransformTranslate(transform, aImage.size.width, 0); 19 transform = CGAffineTransformRotate(transform, M_PI_2); 20 break; 21 22 case UIImageOrientationRight: 23 case UIImageOrientationRightMirrored: 24 transform = CGAffineTransformTranslate(transform, 0, aImage.size.height); 25 transform = CGAffineTransformRotate(transform, -M_PI_2); 26 break; 27 default: 28 break; 29 } 30 31 switch (aImage.imageOrientation) { 32 case UIImageOrientationUpMirrored: 33 case UIImageOrientationDownMirrored: 34 transform = CGAffineTransformTranslate(transform, aImage.size.width, 0); 35 transform = CGAffineTransformScale(transform, -1, 1); 36 break; 37 38 case UIImageOrientationLeftMirrored: 39 case UIImageOrientationRightMirrored: 40 transform = CGAffineTransformTranslate(transform, aImage.size.height, 0); 41 transform = CGAffineTransformScale(transform, -1, 1); 42 break; 43 default: 44 break; 45 } 46 47 // Now we draw the underlying CGImage into a new context, applying the transform 48 // calculated above. 49 CGContextRef ctx = CGBitmapContextCreate(NULL, aImage.size.width, aImage.size.height, 50 CGImageGetBitsPerComponent(aImage.CGImage), 0, 51 CGImageGetColorSpace(aImage.CGImage), 52 CGImageGetBitmapInfo(aImage.CGImage)); 53 CGContextConcatCTM(ctx, transform); 54 switch (aImage.imageOrientation) { 55 case UIImageOrientationLeft: 56 case UIImageOrientationLeftMirrored: 57 case UIImageOrientationRight: 58 case UIImageOrientationRightMirrored: 59 // Grr... 60 CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.height,aImage.size.width), aImage.CGImage); 61 break; 62 63 default: 64 CGContextDrawImage(ctx, CGRectMake(0,0,aImage.size.width,aImage.size.height), aImage.CGImage); 65 break; 66 } 67 68 // And now we just create a new UIImage from the drawing context 69 CGImageRef cgimg = CGBitmapContextCreateImage(ctx); 70 UIImage *img = [UIImage imageWithCGImage:cgimg]; 71 CGContextRelease(ctx); 72 CGImageRelease(cgimg); 73 return img; 74 }
2.使用UIImage的drawInRect函数，该函数内部能自动处理图片的正确方向
- (UIImage *)normalizedImage:(UIImage *)noImage { if (noImage.imageOrientation == UIImageOrientationUp) return noImage; UIGraphicsBeginImageContextWithOptions(noImage.size, NO, noImage.scale); [noImage drawInRect:(CGRect){0, 0, noImage.size}]; UIImage *normalizedImage = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); return normalizedImage; }
3.使用CGcontext绘制图片
CGContextTranslateCTM(context, 0, height); CGContextScaleCTM(context, 1.0, -1.0); CGContextDrawImage(context, CGRectMake(0, 0, width, height), uiImage.CGImage);
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原文地址：http://www.cnblogs.com/jinyaowei/p/4718309.html
