前言
一般我们处理图片大小有两种方式：Crop和Resize，即裁剪和缩放，前者会对原图进行裁剪，即会丢失一些图片区域，而后者只是对图片进行放大缩小，并不对图片进行裁剪。
知识点
imageOrientation
imageOrientation表示图片的方向信息，大部分的时候其实你是不用管它的，因为你存储在相册里或者一些本地的图片在导入之前会被自动调整好方向，即都是正常的UIImageOrientationUp方向。
只有在iphone刚拍摄图片时，Exif中存储的图片方向为当前拍摄的方向，iphone中默认横屏，home键在右边时，图片的Orientation为UIImageOrientationUp。所以在很多时候，我们在iOS程序里对刚拍摄的照片进行处理是需要修正方向，代码如下：
- (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;
}
具体的关于图片中的Orientation可以参考这篇文章: 如何处理iOS中照片的方向
Crop和Resize
Crop
在Crop和Resize图片的时候需要注意裁剪或缩放的大小为图片大小还是像素大小。比如图片裁剪的时候代码如下：
//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;
}
大部分时候我们都会使用第一种按image.size大小裁剪，比如我们裁剪图片的上半部分的时候，我们会使用image.size：
UIImage *image2 = [image croppedImageWithRect:CGRectMake(0, 0, image.size.width, image.size.height/2)];
最近有一个需求是对图片进行裁剪，得到对应像素大小的图片，然后传入图片识别模型识别。我当时使用的就是第一种方法，比如我当时想得到一个{100,100}像素大小的图片，我用的是[image croppedImageWithRect:CGRectMake(0, 0, 100, 100)];，其实这时得到的图片像素大小在二倍屏上为{200,200}，三倍屏上为{300,300}。
所以我们在裁剪或缩放图片时要注意需要的大小为图片大小还是像素大小，在iOS中像素和image.size的关系：
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输出大小为（20，20);
UIImage *image = [UIImage imageNamed:@"test@2x.png"];
image.size输出大小为（20，20);
UIImage *image = [UIImage imageNamed:@"test@3x.png"];
image.size输出大小为（20，20);
image.size输出的大小会自动识别图片是几倍的，如果是3倍的输出的结果就是像素除以3，2倍的像素除以2。
Resize
缩放图片大小也类似，关键就是UIGraphicsBeginImageContextWithOptions(dstSize, NO, isPixelSize?1:self.scale);这句代码，在创建画布时，如果按像素大小来缩放不乘以self.scale即可。
-(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;
}
