When i am using UIImagePNGRepresentation or UIImageJPEGRepresentation for converting UIImage into NSdata, the image size is too much increased

Question

When i am using UIImagePNGRepresentation or UIImageJPEGRepresentation for converting UIImage into NSdata, the image size is too much increased.

Steps to Reproduce:

1)Open Xcode and select new project as single view based application

2)Open ViewController.xib and add two buttons named as i)Test Online Image ii)Test Local image

3)Add two IBActions

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

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

4)Connect "Test Online Image" to "-(IBAction)ClickOnLineImageTest:(id)sender"

and "Test Local image" to " -(IBAction)ClickLocalImageTest:(id)sender;"

5)impalement "-(IBAction)ClickLocalImageTest:(id)sender" method like as following

- (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) impalement "- (IBAction)ClickOnLineImageTest:(id)sender" method as following

- (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)Please download "hero_ipad_retina.jpg" image from here and save in your resources named as "hero_ipad_retina.jpg"

7)Now run this project on Xcode 4.0 later and IOS3.0 above SDK

**

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******************

My Question :

why it is increasing its size ? and what is the optimized way to convert image to NSData?

Notes: Please download "hero_ipad_retina.jpg" image from here and save in your resources

Answer 1

"hero_ipad_retina.jpg" is a compressed jpg image

This line:

[[NSData dataWithContentsOfFile:path] length]/1024

gives it's compressed file size...

This line:

[UIImagePNGRepresentation(img) length]/1024

uncompresses the image and converts it to PNG which is a lossless file format. It's size is inevitably much larger.

This line:

[UIImageJPEGRepresentation(img, 1.0) length]/1024  

uncompresses the image and recompresses it to a JPG representation. You have set the quality to maximum (1.0) so - in comparison with the original which was no doubt compressed to lower quality - you get a larger file size. If you set quality to 0.5 you will get a small file size (around 42K)

This is a great reminder of why you should treat jpeg images with caution. Every time you access a jpeg imageRep, you are uncompressing. If you then recompress - even at full quality - you are downgrading the quality of the image (as each lossy compress is worse than the previous). Artefacts increase and become particularly noticeable with graphic images (flat colours, straight/contrasting edges). PNG is always safer - it is lossless at 24-bit, and at 8-bit is good at dealing with regions of flat colour.

update

To get the size of an image in memory:

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

From this you can work out the compression ratios for PNG, JPG and the original file (divide by 1024 for kilobytes to get the correct ratios with the above figures).