CODEWITHSUNDEEP
iosmemoryuiimageviewuiimage
kwmaeng
kwmaeng

Reputation: 641

UIImage takes up much more memory than its NSData

I'm loading a UIImage from NSData with the following code

var image = UIImage(data: data!)

However, there is a weird behavior.

At first, I used png data, and the NSData was about 80kB each.

When I set UIImage with the data, the UIImage took up 128kb each.

(Checked with Allocation instrument, the size of ImageIO_PNG_Data)

Then I changed to use jpeg instead, and the NSData became about 7kb each.

But still, the UIImage is 128kb each, so when displaying the image I get no memory advantage! (The NSData reduced to 80kb -> 7kb and still the UIImage takes up the same amount of memory)

It is weird, why the UIImage should take up 128kb when the original data is just 7kb?

Can I reduce this memory usage by UIImage without shrinking the size of the UIImage itself??

Note that I'm not dealing with high resolution image so resizing the image is not an option (The NSData is already 7kb!!)

Any help will be appreciated.

Thanks!!

Upvotes: 4

Views: 1128

Answers (2)

Rob
Rob

Reputation: 438232

When you access the NSData, it is often compressed (with either PNG or JPEG). When you use the UIImage, there is an uncompressed pixel buffer which is often 4 bytes per pixel (one byte for red, green, blue, and alpha, respectively). There are other formats, but it illustrates the basic idea, that the JPEG or PNG representations can be compressed, when you start using an image, it is uncompressed.

In your conclusion, you say that resizing not an option and that the NSData is already 7kb. I would suggest that resizing should be considered if the resolution of the image is greater than the resolution (the points of the bounds/frame times the scale of the device) of the UIImageView in which you're using it. The question of whether to resize is not a function of the size of the NSData, but rather the resolution of the view. So, if you have a 1000x1000 pixel image that you're using in a small thumbview in a table view, then regardless of how small the JPEG representation is, you should definitely resize the image.

Upvotes: 4

Aaron Brager
Aaron Brager

Reputation: 66302

This is normal. When the image is stored as NSData, it is compressed (usually using PNG or JPG compression). When it's a UIImage, the image is decompressed, which allows it to be drawn quickly on the screen.

Upvotes: 1

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