前言
一般我們處理圖片大小有兩種方式：Crop和Resize遥赚，即裁剪和縮放，前者會對原圖進行裁剪毕籽，即會丟失一些圖片區(qū)域，而后者只是對圖片進行放大縮小旧烧，并不對圖片進行裁剪影钉。
知識點
imageOrientation
imageOrientation表示圖片的方向信息，大部分的時候其實你是不用管它的掘剪，因為你存儲在相冊里或者一些本地的圖片在導入之前會被自動調(diào)整好方向平委，即都是正常的UIImageOrientationUp方向。
只有在iphone剛拍攝圖片時夺谁，Exif中存儲的圖片方向為當前拍攝的方向廉赔，iphone中默認橫屏，home鍵在右邊時匾鸥，圖片的Orientation為UIImageOrientationUp蜡塌。所以在很多時候，我們在iOS程序里對剛拍攝的照片進行處理是需要修正方向勿负，代碼如下：
- (UIImage *)fixOrientation {
// No-op if the orientation is already correct
if (self.imageOrientation == UIImageOrientationUp) return self;
// 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 (self.imageOrientation) {
case UIImageOrientationDown:
case UIImageOrientationDownMirrored:
transform = CGAffineTransformTranslate(transform, self.size.width, self.size.height);
transform = CGAffineTransformRotate(transform, M_PI);
break;
case UIImageOrientationLeft:
case UIImageOrientationLeftMirrored:
transform = CGAffineTransformTranslate(transform, self.size.width, 0);
transform = CGAffineTransformRotate(transform, M_PI_2);
break;
case UIImageOrientationRight:
case UIImageOrientationRightMirrored:
transform = CGAffineTransformTranslate(transform, 0, self.size.height);
transform = CGAffineTransformRotate(transform, -M_PI_2);
break;
case UIImageOrientationUp:
case UIImageOrientationUpMirrored:
break;
}
switch (self.imageOrientation) {
case UIImageOrientationUpMirrored:
case UIImageOrientationDownMirrored:
transform = CGAffineTransformTranslate(transform, self.size.width, 0);
transform = CGAffineTransformScale(transform, -1, 1);
break;
case UIImageOrientationLeftMirrored:
case UIImageOrientationRightMirrored:
transform = CGAffineTransformTranslate(transform, self.size.height, 0);
transform = CGAffineTransformScale(transform, -1, 1);
break;
case UIImageOrientationUp:
case UIImageOrientationDown:
case UIImageOrientationLeft:
case UIImageOrientationRight:
break;
}
// Now we draw the underlying CGImage into a new context, applying the transform
// calculated above.
CGContextRef ctx = CGBitmapContextCreate(NULL, self.size.width, self.size.height,
CGImageGetBitsPerComponent(self.CGImage), 0,
CGImageGetColorSpace(self.CGImage),
CGImageGetBitmapInfo(self.CGImage));
CGContextConcatCTM(ctx, transform);
switch (self.imageOrientation) {
case UIImageOrientationLeft:
case UIImageOrientationLeftMirrored:
case UIImageOrientationRight:
case UIImageOrientationRightMirrored:
// Grr...
CGContextDrawImage(ctx, CGRectMake(0,0,self.size.height,self.size.width), self.CGImage);
break;
default:
CGContextDrawImage(ctx, CGRectMake(0,0,self.size.width,self.size.height), self.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;
}
具體的關(guān)于圖片中的Orientation可以參考這篇文章: 如何處理iOS中照片的方向
Crop和Resize
Crop
在Crop和Resize圖片的時候需要注意裁剪或縮放的大小為圖片大小還是像素大小馏艾。比如圖片裁剪的時候代碼如下：
//按image.size大小裁剪
- (UIImage *)croppedImageWithRect:(CGRect)bounds {
if (self.scale > 1.0f) {
bounds = CGRectMake(bounds.origin.x * self.scale,
bounds.origin.y * self.scale,
bounds.size.width * self.scale,
bounds.size.height * self.scale);
}
CGImageRef imageRef = CGImageCreateWithImageInRect(self.CGImage, bounds);
UIImage *result = [UIImage imageWithCGImage:imageRef scale:self.scale orientation:self.imageOrientation];
CGImageRelease(imageRef);
return result;
}
//按像素大小裁剪
- (UIImage *)croppedImageWithPixelRect:(CGRect)bounds {
CGImageRef imageRef = CGImageCreateWithImageInRect([self CGImage], bounds);
UIImage *croppedImage = [UIImage imageWithCGImage:imageRef];
CGImageRelease(imageRef);
return croppedImage;
}
大部分時候我們都會使用第一種按image.size大小裁剪，比如我們裁剪圖片的上半部分的時候奴愉，我們會使用image.size：
UIImage *image2 = [image croppedImageWithRect:CGRectMake(0, 0, image.size.width, image.size.height/2)];
最近有一個需求是對圖片進行裁剪琅摩，得到對應像素大小的圖片，然后傳入圖片識別模型識別锭硼。我當時使用的就是第一種方法房资，比如我當時想得到一個{100,100}像素大小的圖片，我用的是[image croppedImageWithRect:CGRectMake(0, 0, 100, 100)];檀头，其實這時得到的圖片像素大小在二倍屏上為{200,200}轰异，三倍屏上為{300,300}。
所以我們在裁剪或縮放圖片時要注意需要的大小為圖片大小還是像素大小暑始，在iOS中像素和image.size的關(guān)系：
test.png (像素 20*20) test@2x.png(像素40*40) test@3x.png(像素 60*60)
UIImage *image = [UIImageimageNamed:@"test.png"];
image.size輸出大小為（20搭独，20);
UIImage *image = [UIImage imageNamed:@"test@2x.png"];
image.size輸出大小為（20，20);
UIImage *image = [UIImage imageNamed:@"test@3x.png"];
image.size輸出大小為（20廊镜，20);
image.size輸出的大小會自動識別圖片是幾倍的牙肝，如果是3倍的輸出的結(jié)果就是像素除以3，2倍的像素除以2。
Resize
縮放圖片大小也類似惊奇，關(guān)鍵就是UIGraphicsBeginImageContextWithOptions(dstSize, NO, isPixelSize?1:self.scale);這句代碼，在創(chuàng)建畫布時播赁，如果按像素大小來縮放不乘以self.scale即可颂郎。
-(UIImage*)resizedImageToSize:(CGSize)dstSize isPixelSize:(BOOL)isPixelSize
{
CGImageRef imgRef = self.CGImage;
// the below values are regardless of orientation : for UIImages from Camera, width>height (landscape)
CGSize srcSize = CGSizeMake(CGImageGetWidth(imgRef), CGImageGetHeight(imgRef)); // not equivalent to self.size (which is dependant on the imageOrientation)!
/* Don't resize if we already meet the required destination size. */
if (CGSizeEqualToSize(srcSize, dstSize)) {
return self;
}
CGFloat scaleRatio = dstSize.width / srcSize.width;
UIImageOrientation orient = self.imageOrientation;
CGAffineTransform transform = CGAffineTransformIdentity;
switch(orient) {
case UIImageOrientationUp: //EXIF = 1
transform = CGAffineTransformIdentity;
break;
case UIImageOrientationUpMirrored: //EXIF = 2
transform = CGAffineTransformMakeTranslation(srcSize.width, 0.0);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
break;
case UIImageOrientationDown: //EXIF = 3
transform = CGAffineTransformMakeTranslation(srcSize.width, srcSize.height);
transform = CGAffineTransformRotate(transform, M_PI);
break;
case UIImageOrientationDownMirrored: //EXIF = 4
transform = CGAffineTransformMakeTranslation(0.0, srcSize.height);
transform = CGAffineTransformScale(transform, 1.0, -1.0);
break;
case UIImageOrientationLeftMirrored: //EXIF = 5
dstSize = CGSizeMake(dstSize.height, dstSize.width);
transform = CGAffineTransformMakeTranslation(srcSize.height, srcSize.width);
transform = CGAffineTransformScale(transform, -1.0, 1.0);
transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2);
break;
case UIImageOrientationLeft: //EXIF = 6
dstSize = CGSizeMake(dstSize.height, dstSize.width);
transform = CGAffineTransformMakeTranslation(0.0, srcSize.width);
transform = CGAffineTransformRotate(transform, 3.0 * M_PI_2);
break;
case UIImageOrientationRightMirrored: //EXIF = 7
dstSize = CGSizeMake(dstSize.height, dstSize.width);
transform = CGAffineTransformMakeScale(-1.0, 1.0);
transform = CGAffineTransformRotate(transform, M_PI_2);
break;
case UIImageOrientationRight: //EXIF = 8
dstSize = CGSizeMake(dstSize.height, dstSize.width);
transform = CGAffineTransformMakeTranslation(srcSize.height, 0.0);
transform = CGAffineTransformRotate(transform, M_PI_2);
break;
default:
[NSException raise:NSInternalInconsistencyException format:@"Invalid image orientation"];
}
/////////////////////////////////////////////////////////////////////////////
// The actual resize: draw the image on a new context, applying a transform matrix
UIGraphicsBeginImageContextWithOptions(dstSize, NO, isPixelSize?1:self.scale);
CGContextRef context = UIGraphicsGetCurrentContext();
if (!context) {
return nil;
}
if (orient == UIImageOrientationRight || orient == UIImageOrientationLeft) {
CGContextScaleCTM(context, -scaleRatio, scaleRatio);
CGContextTranslateCTM(context, -srcSize.height, 0);
} else {
CGContextScaleCTM(context, scaleRatio, -scaleRatio);
CGContextTranslateCTM(context, 0, -srcSize.height);
}
CGContextConcatCTM(context, transform);
// we use srcSize (and not dstSize) as the size to specify is in user space (and we use the CTM to apply a scaleRatio)
CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, srcSize.width, srcSize.height), imgRef);
UIImage* resizedImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return resizedImage;
}
