当我使用UIImagePNGRepresentation或UIImageJPEGRepresentation将UIImage转换为NSdata时,图像大小增加太多

Question

当我使用UIImagePNGRepresentation或UIImageJPEGRepresentation将UIImage转换为NSdata时,图像大小增加太多.

重现步骤:

1)打开Xcode并选择新项目作为基于单一视图的应用程序

2)打开ViewController.xib并添加两个按钮,命名为i)测试在线图像ii)测试本地图像

3)添加两个IBActions

  i)  -(IBAction)ClickLocalImageTest:(id)sender;

  ii) -(IBAction)ClickOnLineImageTest:(id)sender;

4)将"测试在线图像"连接到" -(IBAction)ClickOnLineImageTest:(id)sender"

和"测试本地图像"到" -(IBAction)ClickLocalImageTest:(id)sender;"

5)impalement" -(IBAction)ClickLocalImageTest:(id)sender"方法如下

- (IBAction)ClickLocalImageTest:(id)sender {
    NSLog(@"*************Test Local Image****************\n");
    NSString *path=[[NSBundle mainBundle] pathForResource:@"hero_ipad_retina" ofType:@"jpg"];
    NSLog(@"Before testing image size is :<---- %u kb",[[NSData dataWithContentsOfFile:path] length]/1024);
    UIImage *img  = [UIImage imageNamed:@"hero_ipad_retina.jpg"];
     NSLog(@"UIImagePNGRepresentation: image size is---->: %u kb",[UIImagePNGRepresentation(img) length]/1024);
    NSLog(@"UIImageJPEGRepresentation with scale 1.0: image size is---->: %u kb \n",[UIImageJPEGRepresentation(img, 1.0) length]/1024);
    NSLog(@"*************Completed test****************\n\n\n\n");
} 

6)刺穿" - (IBAction)ClickOnLineImageTest:(id)sender"方法如下

- (IBAction)ClickOnLineImageTest:(id)sender {
     NSLog(@"*************Test Online Image****************\n");
NSLog(@"Before testing image size is :<---- %u kb",[[NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://images.apple.com/home/images/hero_ipad_retina.jpg"]] length]/1024);
UIImage *img  = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:@"http://images.apple.com/home/images/hero_ipad_retina.jpg"]]];
NSLog(@"UIImagePNGRepresentation: image size is---->: %u kb",[UIImagePNGRepresentation(img) length]/1024);
NSLog(@"UIImageJPEGRepresentation with scale 1.0: image size is---->: %u kb \n",[UIImageJPEGRepresentation(img, 1.0) length]/1024);
NSLog(@"*************Completed test****************\n\n\n\n");
}

7)请从这里下载"hero_ipad_retina.jpg"图像并保存在名为"hero_ipad_retina.jpg"的资源中

7)现在在Xcode 4.0上运行这个项目,在SDK上面运行IOS3.0

**

Expected Results:
1)Click on "Test Online Image" button result should be as following 
*************Test Online Image****************
Before testing image size is :<---- 78 kb
UIImagePNGRepresentation: image size is---->: 78 kb
UIImageJPEGRepresentation with scale 1.0: image size is---->: 78 kb
*************Completed test****************
2)1)Click on "Test Local image" button result should be as following
*************Test Local Image****************
Before testing image size is :<---- 78 kb
UIImagePNGRepresentation: image size is---->: 78 kb
UIImageJPEGRepresentation with scale 1.0: image size is---->: 78 kb 
*************Completed test****************
Actual Results:
1)Click on "Test Online Image" button result should be as following 
*************Test Online Image****************
Before testing image size is :<---- 78 kb
UIImagePNGRepresentation: image size is---->: 480 kb
UIImageJPEGRepresentation with scale 1.0: image size is---->: 180 kb
*************Completed test****************
2)1)Click on "Test Local image" button result should be as following
*************Test Local Image****************
Before testing image size is :<---- 78 kb
UIImagePNGRepresentation: image size is---->: 480 kb
UIImageJPEGRepresentation with scale 1.0: image size is---->: 180 kb 
*************Completed test******************

我的问题 :

为什么增加它的大小？什么是将图像转换为NSData的优化方法？

注意:请从此处下载"hero_ipad_retina.jpg"图片并保存在您的资源中

Answer 1

"hero_ipad_retina.jpg"是压缩的jpg图像

这一行:

[[NSData dataWithContentsOfFile:path] length]/1024

给它压缩文件大小......

这一行:

[UIImagePNGRepresentation(img) length]/1024

解压缩图像并将其转换为PNG,这是一种无损文件格式.它的大小不可避免地要大得多.

这一行:

[UIImageJPEGRepresentation(img, 1.0) length]/1024  

解压缩图像并将其重新压缩为JPG表示.您已将质量设置为最大值(1.0),因此 - 与原始版本相比,无疑会将其压缩到较低的质量 - 您可以获得更大的文件大小.如果您将质量设置为0.5,您将获得一个小文件大小(约42K)

这是一个很好的提醒,为什么你应该谨慎对待jpeg图像.每次访问jpeg imageRep时,都会解压缩.如果你再重新压缩 - 即使是全质量 - 你正在降低图像的质量(因为每个有损压缩都比前一个更差).随着图形图像(平面颜色,直线/对比边缘),人工制品增加并变得特别明显.PNG总是更安全 - 它在24位时无损,在8位时非常适合处理平面颜色的区域.

更新

要在内存中获取图像的大小:

NSUInteger sizeInBytes  = 
  CGImageGetHeight(image.CGImage) * CGImageGetBytesPerRow(image.CGImage);

从这里你可以计算PNG,JPG和原始文件的压缩比(除以1024表示千字节,以获得与上述数字的正确比率).