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

NSDateFormatter *formatter = [[NSDateFormatter alloc] init];  

 [formatter setDateFormat:@"YYYY-MM-DD-hh-mm-ss"];  

[cpp] view
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if (UIImagePNGRepresentation(image)==nil) {  

    data = UIImageJPEGRepresentation(image, 1.0);  

}else{  

    data = UIImagePNGRepresentation(image);  

}  

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：