iOS应用开发中对UIImage进行截取和缩放的方法详解

截取UIImage指定大小区域
最近遇到这样的需求：从服务器获取到一张照片，只需要显示他的左半部分，或者中间部分等等。也就是截取UIImage指定大小区域。

UIImage扩展：

我的解决方案是对UIImage进行扩展。通过CGImageRef和CGImage完成截取，调用的方法是：CGImageCreateWithImageInRect。扩展类叫UIImage+Crop，具体代码如下：

UIImage+Crop.h

#import <UIKit/UIKit.h>

typedef NS_ENUM(NSInteger, XYCropImageStyle){
  XYCropImageStyleRight        =0,   // 右半部分
  XYCropImageStyleCenter       =1,   // 中间部分
  XYCropImageStyleLeft        =2,   // 左半部分
  XYCropImageStyleRightOneOfThird   =3,   // 右侧三分之一部分
  XYCropImageStyleCenterOneOfThird  =4,   // 中间三分之一部分
  XYCropImageStyleLeftOneOfThird   =5,   // 左侧三分之一部分
  XYCropImageStyleRightQuarter    =6,   // 右侧四分之一部分
  XYCropImageStyleCenterRightQuarter =7,   // 中间右侧四分之一部分
  XYCropImageStyleCenterLeftQuarter  =8,   // 中间左侧四分之一部分
  XYCropImageStyleLeftQuarter     =9,   // 左侧四分之一部分
};

@interface UIImage (Crop)
- (UIImage *)imageByCroppingWithStyle:(XYCropImageStyle)style;

@end
UIImage+Crop.m

#import "UIImage+Crop.h"

@implementation UIImage (Crop)

- (UIImage *)imageByCroppingWithStyle:(XYCropImageStyle)style
{
  CGRect rect;
  switch (style) {
    case XYCropImageStyleLeft:
      rect = CGRectMake(0, 0, self.size.width/2, self.size.height);
      break;
    case XYCropImageStyleCenter:
      rect = CGRectMake(self.size.width/4, 0, self.size.width/2, self.size.height);
      break;
    case XYCropImageStyleRight:
      rect = CGRectMake(self.size.width/2, 0, self.size.width/2, self.size.height);
      break;
    case XYCropImageStyleLeftOneOfThird:
      rect = CGRectMake(0, 0, self.size.width/3, self.size.height);
      break;
    case XYCropImageStyleCenterOneOfThird:
      rect = CGRectMake(self.size.width/3, 0, self.size.width/3, self.size.height);
      break;
    case XYCropImageStyleRightOneOfThird:
      rect = CGRectMake(self.size.width/3*2, 0, self.size.width/3, self.size.height);
      break;
    case XYCropImageStyleLeftQuarter:
      rect = CGRectMake(0, 0, self.size.width/4, self.size.height);
      break;
    case XYCropImageStyleCenterLeftQuarter:
      rect = CGRectMake(self.size.width/4, 0, self.size.width/4, self.size.height);
      break;
    case XYCropImageStyleCenterRightQuarter:
      rect = CGRectMake(self.size.width/4*2, 0, self.size.width/4, self.size.height);
      break;
    case XYCropImageStyleRightQuarter:
      rect = CGRectMake(self.size.width/4*3, 0, self.size.width/4, self.size.height);
      break;
    default:
      break;
  }
  CGImageRef imageRef = self.CGImage;
  CGImageRef imagePartRef = CGImageCreateWithImageInRect(imageRef, rect);
  UIImage *cropImage = [UIImage imageWithCGImage:imagePartRef];
  CGImageRelease(imagePartRef);
  return cropImage;
}

实际运用：

简单测试一下，看看有没有实现我们想要的效果。首先，先加载一个完整的UIImageView。这个应该不难。代码如下：

UIImageView *imgView = [[UIImageView alloc] init];
imgView.frame = CGRectMake((SCREEN.width - 226) / 2, 100, 226, 106);
UIImage *image = [UIImage imageNamed:@"ganggang"];
imgView.image = image;
[self.view addSubview:imgView];

运行一下：

要对UIImage进行裁剪，首先导入头文件：

#import "UIImage+Crop.h"

在上面UIImage *image = [UIImage imageNamed:@"ganggang"];这段代码之后加上下面这句：

image = [image imageByCroppingWithStyle:XYCropImageStyleLeft];

XYCropImageStyleLeft是截取照片的左半部分。效果如下：

截取成功，还可以截取其他区域的，只需要传入不同的XYCropImageStyle即可实现。

UIImage等比缩放
前面讲了截取UIImage指定大小区域，很方便的截取UIImage。今天要和大家分享的是UIImage的缩放。

两种缩放:

• 缩放到指定大小，也就是指定的size.
• 等比缩放。

1.缩放到指定大小

- (UIImage*)imageCompressWithSimple:(UIImage*)image scaledToSize:(CGSize)size
{
  UIGraphicsBeginImageContext(size);
  [image drawInRect:CGRectMake(0,0,size.width,size.height)];
  UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  return newImage;
}

2.等比缩放

（1）通过缩放系数：

- (UIImage*)imageCompressWithSimple:(UIImage*)image scale:(float)scale
{
  CGSize size = image.size;
  CGFloat width = size.width;
  CGFloat height = size.height;
  CGFloat scaledWidth = width * scale;
  CGFloat scaledHeight = height * scale;
  UIGraphicsBeginImageContext(size); // this will crop
  [image drawInRect:CGRectMake(0,0,scaledWidth,scaledHeight)];
  UIImage* newImage= UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  return newImage;
}

scale是缩放系数 。

（2）通过计算得到缩放系数

- (UIImage*)imageByScalingAndCroppingForSize:(CGSize)targetSize
{

  UIImage *sourceImage = [UIImage imageNamed:@"test.jpg"];
  UIImage *newImage = nil;
  CGSize imageSize = sourceImage.size;
  CGFloat width = imageSize.width;
  CGFloat height = imageSize.height;
  CGFloat targetWidth = targetSize.width;
  CGFloat targetHeight = targetSize.height;
  CGFloat scaleFactor = 0.0;
  CGFloat scaledWidth = targetWidth;
  CGFloat scaledHeight = targetHeight;
  CGPoint thumbnailPoint = CGPointMake(0.0,0.0);

  if (CGSizeEqualToSize(imageSize, targetSize) == NO)
  {
    CGFloat widthFactor = targetWidth / width;
    CGFloat heightFactor = targetHeight / height;
    if (widthFactor > heightFactor)
      scaleFactor = widthFactor; // scale to fit height
    else
      scaleFactor = heightFactor; // scale to fit width

    scaledWidth= width * scaleFactor;
    scaledHeight = height * scaleFactor;
    // center the image
    if (widthFactor > heightFactor)
    {
      thumbnailPoint.y = (targetHeight - scaledHeight) * 0.5;
    }
    else if (widthFactor < heightFactor)
    {
      thumbnailPoint.x = (targetWidth - scaledWidth) * 0.5;
    }
  }

  UIGraphicsBeginImageContext(targetSize); // this will crop
  CGRect thumbnailRect = CGRectZero;
  thumbnailRect.origin = thumbnailPoint;
  thumbnailRect.size.width= scaledWidth;
  thumbnailRect.size.height = scaledHeight;
  [sourceImage drawInRect:thumbnailRect];
  newImage = UIGraphicsGetImageFromCurrentImageContext();

  if(newImage == nil)
    NSLog(@"could not scale image");
  //pop the context to get back to the default
  UIGraphicsEndImageContext();

  return newImage;

}