UIImageJPEGRepresentation和UIImagePNGRepresentation

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iOS Develop（205） 

UIImageJPEGRepresentation方法在耗时上比较少 而UIImagePNGRepresentation耗时操作时间比较长

-(void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info

使用UIImagePNGRepresentation取得照片时候可能会造成卡顿的现象

在Iphone上有两种读取图片数据的简单方法: UIImageJPEGRepresentation和UIImagePNGRepresentation. 

UIImageJPEGRepresentation函数需要两个参数:图片的引用和压缩系数.而UIImagePNGRepresentation只需要图片引用作为参数.通过在实际使用过程中,比较发现: UIImagePNGRepresentation(UIImage* image) 要比UIImageJPEGRepresentation(UIImage* image, 1.0) 返回的图片数据量大很多.譬如,同样是读取摄像头拍摄的同样景色的照片, UIImagePNGRepresentation()返回的数据量大小为199K ,而 UIImageJPEGRepresentation(UIImage* image, 1.0)返回的数据量大小只为140KB,比前者少了50多KB.如果对图片的清晰度要求不高,还可以通过设置 UIImageJPEGRepresentation函数的第二个参数,大幅度降低图片数据量.譬如,刚才拍摄的图片, 通过调用UIImageJPEGRepresentation(UIImage* image, 1.0)读取数据时,返回的数据大小为140KB,但更改压缩系数后,通过调用UIImageJPEGRepresentation(UIImage* image, 0.5)读取数据时,返回的数据大小只有11KB多,大大压缩了图片的数据量 ,而且从视角角度看,图片的质量并没有明显的降低.因此,在读取图片数据内容时,建议优先使用UIImageJPEGRepresentation,并可根据自己的实际使用场景,设置压缩系数,进一步降低图片数据量大小.

[cpp] view plaincopy

1. NSDateFormatter *formatter = [[NSDateFormatter alloc] init];  
2.  [formatter setDateFormat:@"YYYY-MM-DD-hh-mm-ss"];  

[cpp] view plaincopy

1. if (UIImagePNGRepresentation(image)==nil) {  
2.     data = UIImageJPEGRepresentation(image, 1.0);  
3. }else{  
4.     data = UIImagePNGRepresentation(image);  
5. }  

利用UIImageJPEGRepresentation与UIGraphicsBeginImageContext进行图片压缩的简单比较

UIImage *image01 = [UIImage imageNamed:@"002.png"];//原图

02. 

03.NSData *dataOf02 =UIImageJPEGRepresentation(image01, 0.5);//压缩图片内容，不影响图片的size，得到一个原大小，但更模糊的图片。

04.UIImage *image02 = [UIImage imageWithData:dataOf02];//压缩图02

05. 

06.UIImage *image03 = [self makeThumbnailFromImage:image01 scale:0.5];//压缩图03

07.NSLog(@"001===%@",NSStringFromCGSize(image01.size));

08.NSLog(@"002===%@",NSStringFromCGSize(image02.size));

09.NSLog(@"003===%@",NSStringFromCGSize(image03.size));

10. 

11.NSData *data001 = UIImageJPEGRepresentation(image01, 1.0);

12.NSData *data002 = UIImageJPEGRepresentation(image02, 1.0);

13.NSData *data003 = UIImageJPEGRepresentation(image03, 1.0);

14. 

15.NSLog(@"002.lenght===%d",data001.length);

16.NSLog(@"002.lenght===%d",data002.length);

17.NSLog(@"003.lenght===%d",data003.length);

18. 

19.//看一下效果

20.UIImageView *imageView01 = [[UIImageView alloc] initWithFrame:CGRectMake(0, 0, image01.size.width, image01.size.height)];

21.imageView01.image = image01;

22.[self.view addSubview:imageView01];

23. 

24.UIImageView *imageView02 = [[UIImageView alloc] initWithFrame:CGRectMake(0, 100, image02.size.width, image02.size.height)];

25.imageView02.image = image02;

26.[self.view addSubview:imageView02];

27. 

28.UIImageView *imageView03 = [[UIImageView alloc] initWithFrame:CGRectMake(0, 200, image03.size.width, image03.size.height)];

29.imageView03.image = image03;

30.[self.view addSubview:imageView03];

view sourceprint?

01.//手动实现图片压缩，可以写到分类里，封装成常用方法。按照大小进行比例压缩，改变了图片的size。

02.- (UIImage *)makeThumbnailFromImage:(UIImage *)srcImage scale:(double)imageScale {

03.UIImage *thumbnail = nil;

04.CGSize imageSize = CGSizeMake(srcImage.size.width * imageScale, srcImage.size.height * imageScale);

05.if (srcImage.size.width != imageSize.width || srcImage.size.height != imageSize.height)

06.{

07.UIGraphicsBeginImageContext(imageSize);

08.CGRect imageRect = CGRectMake(0.0, 0.0, imageSize.width, imageSize.height);

09.[srcImage drawInRect:imageRect];

10.thumbnail = UIGraphicsGetImageFromCurrentImageContext();

11.UIGraphicsEndImageContext();

12.}

13.else

14.{

15.thumbnail = srcImage;

16.}

17.return thumbnail;

18.}

效果图：