IOS: Get displayed image size in pixels

cocoaiosswift

In my app, I'm displaying an image of a rectangle from the assets library. The image is 100×100 pixels. I'm only using the 1x slot for this asset.

I want to display this image at 300×300 pixels. Doing this using points is quite simple but I can't figure out how to get UIImageView to set the size in pixels.

Alternatively, if I can't set the size in pixels to display, I'd like to get the size in pixels that the image is being displayed.

I have tried using .scale on the UIImageView and UIImage instances, but it's always 1. Even though I have set constraints to 150 and 300.

Best Answer

To get size in pixels of UIImageView:

let widthInPixels = imageView.frame.width * UIScreen.mainScreen().scale
let heightInPixels = imageView.frame.height * UIScreen.mainScreen().scale

To get size in pixels of UIImage:

let widthInPixels = image.size.width * image.scale
let heightInPixels = image.size.height * image.scale

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A couple of suggestions are provided as answers to this question. I had suggested the technique described in this post, with the relevant code:

+ (UIImage*)imageWithImage:(UIImage*)image 
               scaledToSize:(CGSize)newSize;
{
   UIGraphicsBeginImageContext( newSize );
   [image drawInRect:CGRectMake(0,0,newSize.width,newSize.height)];
   UIImage* newImage = UIGraphicsGetImageFromCurrentImageContext();
   UIGraphicsEndImageContext();

   return newImage;
}

As far as storage of the image, the fastest image format to use with the iPhone is PNG, because it has optimizations for that format. However, if you want to store these images as JPEGs, you can take your UIImage and do the following:

NSData *dataForJPEGFile = UIImageJPEGRepresentation(theImage, 0.6);

This creates an NSData instance containing the raw bytes for a JPEG image at a 60% quality setting. The contents of that NSData instance can then be written to disk or cached in memory.

Best Answer

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