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;

}