用相機拍攝出來的照片含有EXIF信息，UIImage的imageOrientation屬性指的就是EXIF中的orientation信息。
如果我們忽略orientation信息，而直接對照片進行像素處理或者drawInRect等操作，得到的結果是翻轉或者旋轉90之後的樣子。這是因爲我們執行像素處理或者drawInRect等操作之後，imageOrientaion信息被刪除了，imageOrientaion被重設爲0，造成照片內容和imageOrientaion不匹配。
所以，在對照片進行處理之前，先將照片旋轉到正確的方向，並且返回的imageOrientaion爲0。
下面這個方法就是一個UIImage category中的方法，用它可以達到以上目的。
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(UIImage *)fixOrientation:(UIImage *)aImage { //
No-op if the orientation is already correct if
(aImage.imageOrientation == UIImageOrientationUp) return
aImage; //
We need to calculate the proper transformation to make the image upright. //
We do it in 2 steps: Rotate if Left/Right/Down, and then flip if Mirrored. CGAffineTransform
transform = CGAffineTransformIdentity; switch
(aImage.imageOrientation) { case
UIImageOrientationDown: case
UIImageOrientationDownMirrored: transform
= CGAffineTransformTranslate(transform, aImage.size.width, aImage.size.height); transform
= CGAffineTransformRotate(transform, M_PI); break; case
UIImageOrientationLeft: case
UIImageOrientationLeftMirrored: transform
= CGAffineTransformTranslate(transform, aImage.size.width, 0); transform
= CGAffineTransformRotate(transform, M_PI_2); break; case
UIImageOrientationRight: case
UIImageOrientationRightMirrored: transform
= CGAffineTransformTranslate(transform, 0,
aImage.size.height); transform
= CGAffineTransformRotate(transform, -M_PI_2); break; default: break; } switch
(aImage.imageOrientation) { case
UIImageOrientationUpMirrored: case
UIImageOrientationDownMirrored: transform
= CGAffineTransformTranslate(transform, aImage.size.width, 0); transform
= CGAffineTransformScale(transform, -1,
1); break; case
UIImageOrientationLeftMirrored: case
UIImageOrientationRightMirrored: transform
= CGAffineTransformTranslate(transform, aImage.size.height, 0); transform
= CGAffineTransformScale(transform, -1,
1); break; default: break; } //
Now we draw the underlying CGImage into a new context, applying the transform //
calculated above. CGContextRef
ctx = CGBitmapContextCreate(NULL, aImage.size.width, aImage.size.height, CGImageGetBitsPerComponent(aImage.CGImage),
0, CGImageGetColorSpace(aImage.CGImage), CGImageGetBitmapInfo(aImage.CGImage)); CGContextConcatCTM(ctx,
transform); switch
(aImage.imageOrientation) { case
UIImageOrientationLeft: case
UIImageOrientationLeftMirrored: case
UIImageOrientationRight: case
UIImageOrientationRightMirrored: //
Grr... CGContextDrawImage(ctx,
CGRectMake(0,0,aImage.size.height,aImage.size.width),
aImage.CGImage); break; default: CGContextDrawImage(ctx,
CGRectMake(0,0,aImage.size.width,aImage.size.height),
aImage.CGImage); break; } //
And now we just create a new UIImage from the drawing context CGImageRef
cgimg = CGBitmapContextCreateImage(ctx); UIImage
*img = [UIImage imageWithCGImage:cgimg]; CGContextRelease(ctx); CGImageRelease(cgimg); return
img;} |