How to know the image size after applying aspect fit for the image in an UIImageView

I am loading an image to an imageview with mode as 'Aspect Fit'. I need to know the size to which my image is being scaled to. Please help.

Why not use the OS function AVMakeRectWithAspectRatioInsideRect?

I wanted to use AVMakeRectWithAspectRatioInsideRect() without including the AVFoundation framework.

So I've implemented the following two utility functions:

CGSize CGSizeAspectFit(CGSize aspectRatio, CGSize boundingSize)
{
	float mW = boundingSize.width / aspectRatio.width;
	float mH = boundingSize.height / aspectRatio.height;
	if( mH < mW )
		boundingSize.width = boundingSize.height / aspectRatio.height * aspectRatio.width;
	else if( mW < mH )
		boundingSize.height = boundingSize.width / aspectRatio.width * aspectRatio.height;
	return boundingSize;
}

CGSize CGSizeAspectFill(CGSize aspectRatio, CGSize minimumSize)
{
	float mW = minimumSize.width / aspectRatio.width;
	float mH = minimumSize.height / aspectRatio.height;
	if( mH > mW )
		minimumSize.width = minimumSize.height / aspectRatio.height * aspectRatio.width;
	else if( mW > mH )
		minimumSize.height = minimumSize.width / aspectRatio.width * aspectRatio.height;
	return minimumSize;
}

Edit: Optimized below by removing duplicate divisions.

CGSize CGSizeAspectFit(const CGSize aspectRatio, const CGSize boundingSize)
{
    CGSize aspectFitSize = CGSizeMake(boundingSize.width, boundingSize.height);
	float mW = boundingSize.width / aspectRatio.width;
	float mH = boundingSize.height / aspectRatio.height;
	if( mH < mW )
		aspectFitSize.width = mH * aspectRatio.width;
	else if( mW < mH )
		aspectFitSize.height = mW * aspectRatio.height;
	return aspectFitSize;
}

CGSize CGSizeAspectFill(const CGSize aspectRatio, const CGSize minimumSize)
{
    CGSize aspectFillSize = CGSizeMake(minimumSize.width, minimumSize.height);
	float mW = minimumSize.width / aspectRatio.width;
	float mH = minimumSize.height / aspectRatio.height;
	if( mH > mW )
		aspectFillSize.width = mH * aspectRatio.width;
	else if( mW > mH )
		aspectFillSize.height = mW * aspectRatio.height;
	return aspectFillSize;
}

End of edit

This takes a given size (first parameter) and maintains its aspect ratio. It then fills the given bounds (second parameter) as much as possible without violating the aspect ratio.

Using this to answer the original question:

// Using aspect fit, scale the image (size) to the image view's size.
CGSize sizeBeingScaledTo = CGSizeAspectFit(theImage.size, theImageView.frame.size);

Note how the image determines the aspect ratio, while the image view determines the size to be filled.

Feedback is very welcome.

Please see @Paul-de-Lange's answer instead of this one

---

I couldn't find anything in an easily accessible variable that had this, so here is the brute force way:

- (CGSize) aspectScaledImageSizeForImageView:(UIImageView *)iv image:(UIImage *)im {

float x,y;
float a,b;
x = iv.frame.size.width;
y = iv.frame.size.height;
a = im.size.width;
b = im.size.height;

if ( x == a && y == b ) {           // image fits exactly, no scaling required
    // return iv.frame.size;
}
else if ( x > a && y > b ) {         // image fits completely within the imageview frame
    if ( x-a > y-b ) {              // image height is limiting factor, scale by height
        a = y/b * a;
        b = y;
    } else {
        b = x/a * b;                // image width is limiting factor, scale by width
        a = x;
    }
} 
else if ( x < a && y < b ) {        // image is wider and taller than image view
    if ( a - x > b - y ) {          // height is limiting factor, scale by height
        a = y/b * a;
        b = y;
    } else {                        // width is limiting factor, scale by width
        b = x/a * b;
        a = x;
    }
}
else if ( x < a && y > b ) {        // image is wider than view, scale by width
    b = x/a * b;
    a = x;
}
else if ( x > a && y < b ) {        // image is taller than view, scale by height
    a = y/b * a;
    b = y;
}
else if ( x == a ) {
    a = y/b * a;
    b = y;
} else if ( y == b ) {
    b = x/a * b;
    a = x;
}
return CGSizeMake(a,b);

}

This simple function will calculate size of image after aspect fit:

Swift 5.1

extension UIImageView {

    var imageSizeAfterAspectFit: CGSize {
        var newWidth: CGFloat
        var newHeight: CGFloat

        guard let image = image else { return frame.size }

        if image.size.height >= image.size.width {
            newHeight = frame.size.height
            newWidth = ((image.size.width / (image.size.height)) * newHeight)

            if CGFloat(newWidth) > (frame.size.width) {
                let diff = (frame.size.width) - newWidth
                newHeight = newHeight + CGFloat(diff) / newHeight * newHeight
                newWidth = frame.size.width
            }
        } else {
            newWidth = frame.size.width
            newHeight = (image.size.height / image.size.width) * newWidth

            if newHeight > frame.size.height {
                let diff = Float((frame.size.height) - newHeight)
                newWidth = newWidth + CGFloat(diff) / newWidth * newWidth
                newHeight = frame.size.height
            }
        }
        return .init(width: newWidth, height: newHeight)
    }
}

Objective C:

 -(CGSize)imageSizeAfterAspectFit:(UIImageView*)imgview{
    
    
    float newwidth;
    float newheight;
    
    UIImage *image=imgview.image;
    
    if (image.size.height>=image.size.width){
        newheight=imgview.frame.size.height;
        newwidth=(image.size.width/image.size.height)*newheight;
        
        if(newwidth>imgview.frame.size.width){
            float diff=imgview.frame.size.width-newwidth;
            newheight=newheight+diff/newheight*newheight;
            newwidth=imgview.frame.size.width;
        }
        
    }
    else{
        newwidth=imgview.frame.size.width;
        newheight=(image.size.height/image.size.width)*newwidth;
        
        if(newheight>imgview.frame.size.height){
            float diff=imgview.frame.size.height-newheight;
            newwidth=newwidth+diff/newwidth*newwidth;
            newheight=imgview.frame.size.height;
        }
    }
    
    NSLog(@"image after aspect fit: width=%f height=%f",newwidth,newheight);
    
    
    //adapt UIImageView size to image size
    //imgview.frame=CGRectMake(imgview.frame.origin.x+(imgview.frame.size.width-newwidth)/2,imgview.frame.origin.y+(imgview.frame.size.height-newheight)/2,newwidth,newheight);
    
    return CGSizeMake(newwidth, newheight);
    
}

Swift 3 Human Readable Version

extension UIImageView {

    /// Find the size of the image, once the parent imageView has been given a contentMode of .scaleAspectFit
    /// Querying the image.size returns the non-scaled size. This helper property is needed for accurate results.
    var aspectFitSize: CGSize {
        guard let image = image else { return CGSize.zero }

        var aspectFitSize = CGSize(width: frame.size.width, height: frame.size.height)
        let newWidth: CGFloat = frame.size.width / image.size.width
        let newHeight: CGFloat = frame.size.height / image.size.height

        if newHeight < newWidth {
            aspectFitSize.width = newHeight * image.size.width
        } else if newWidth < newHeight {
            aspectFitSize.height = newWidth * image.size.height
        }

        return aspectFitSize
    }

    /// Find the size of the image, once the parent imageView has been given a contentMode of .scaleAspectFill
    /// Querying the image.size returns the non-scaled, vastly too large size. This helper property is needed for accurate results.
    var aspectFillSize: CGSize {
        guard let image = image else { return CGSize.zero }

        var aspectFillSize = CGSize(width: frame.size.width, height: frame.size.height)
        let newWidth: CGFloat = frame.size.width / image.size.width
        let newHeight: CGFloat = frame.size.height / image.size.height

        if newHeight > newWidth {
            aspectFillSize.width = newHeight * image.size.width
        } else if newWidth > newHeight {
            aspectFillSize.height = newWidth * image.size.height
        }

        return aspectFillSize
    }

}

I also wanted to calculate height after the aspect ratio is applied to be able to calculate the height of table view's cell. So, I achieved via little math

ratio = width / height

and height would become

height = width / ratio

So code snippet would be

UIImage *img = [UIImage imageNamed:@"anImage"];
float aspectRatio = img.size.width/img.size.height;
float requiredHeight = self.view.bounds.size.width / aspectRatio;

For Swift use below code

func imageSizeAspectFit(imgview: UIImageView) -> CGSize {
        var newwidth: CGFloat
        var newheight: CGFloat
        let image: UIImage = imgFeed.image!
        
        if image.size.height >= image.size.width {
            newheight = imgview.frame.size.height;
            newwidth = (image.size.width / image.size.height) * newheight
            if newwidth > imgview.frame.size.width {
                let diff: CGFloat = imgview.frame.size.width - newwidth
                newheight = newheight + diff / newheight * newheight
                newwidth = imgview.frame.size.width
            }
        }
        else {
            newwidth = imgview.frame.size.width
            newheight = (image.size.height / image.size.width) * newwidth
            if newheight > imgview.frame.size.height {
                let diff: CGFloat = imgview.frame.size.height - newheight
                newwidth = newwidth + diff / newwidth * newwidth
                newheight = imgview.frame.size.height
            }
        }

       print(newwidth, newheight)
        //adapt UIImageView size to image size
        return CGSizeMake(newwidth, newheight)
    }

And Call Function

imgFeed.sd_setImageWithURL(NSURL(string:"Your image URL")))
self.imageSizeAfterAspectFit(imgFeed)

Maybe does not fit your case, but this simple approach solve my problem in a similar case:

	UIImageView *imageView = [[UIImageView alloc] initWithImage:bigSizeImage];
	[imageView sizeToFit];

After image view executes sizeToFit if you query the imageView.frame.size you will get the new image view size that fits the new image size.

Swift 4: Frame for .aspectFit image is -

import AVFoundation

let x: CGRect = AVMakeRect(aspectRatio: myImage.size, insideRect: sampleImageView.frame)

+(UIImage *)CreateAResizeImage:(UIImage *)Img ThumbSize:(CGSize)ThumbSize 
{
    float actualHeight = Img.size.height;
    float actualWidth = Img.size.width;
    
    if(actualWidth==actualHeight) 
    {
        actualWidth = ThumbSize.width; 
        actualHeight = ThumbSize.height;
    }
    
    float imgRatio = actualWidth/actualHeight;
    float maxRatio = ThumbSize.width/ThumbSize.height; //320.0/480.0;
    
    
    if(imgRatio!=maxRatio)
    {
        if(imgRatio < maxRatio)
        {
            imgRatio = ThumbSize.height / actualHeight; //480.0 / actualHeight;
            actualWidth = imgRatio * actualWidth;
            actualHeight = ThumbSize.height; //480.0;
        }
        else
        {
            imgRatio = ThumbSize.width / actualWidth; //320.0 / actualWidth;
            actualHeight = imgRatio * actualHeight;
            actualWidth = ThumbSize.width; //320.0;
        }
    } 
    else 
    {
        actualWidth = ThumbSize.width;
        actualHeight = ThumbSize.height; 
    }
    
    
    CGRect rect = CGRectMake(0, 0, (int)actualWidth, (int)actualHeight);
    UIGraphicsBeginImageContext(rect.size);
    [Img drawInRect:rect];
    UIImage *NewImg = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return NewImg;
}

This single line can do this job

CGSize sizeInView = AVMakeRectWithAspectRatioInsideRect(imgViewFake.image.size, imgViewFake.bounds).size;

Swift 3 UIImageView extension:

import AVFoundation

extension UIImageView {
  var imageSize: CGSize {
    if let image = image {
      return AVMakeRect(aspectRatio: image.size, insideRect: bounds).size
    }
    return CGSize.zero
  }  
}

The accepted answer is incredibly complicated and fails for some edge cases. I think this solution is much more elegant:

- (CGSize) sizeOfImage:(UIImage*)image inAspectFitImageView:(UIImageView*)imageView
{
    UKAssert(imageView.contentMode == UIViewContentModeScaleAspectFit, @"Image View must use contentMode = UIViewContentModeScaleAspectFit");

    CGFloat imageViewWidth = imageView.bounds.size.width;
    CGFloat imageViewHeight = imageView.bounds.size.height;

    CGFloat imageWidth = image.size.width;
    CGFloat imageHeight = image.size.height;

    CGFloat scaleFactor = MIN(imageViewWidth / imageWidth, imageViewHeight / imageHeight);

    return CGSizeMake(image.size.width*scaleFactor, image.size.height*scaleFactor);
}

Here is my solution for same problem: https://github.com/alexgarbarev/UIImageView-ImageFrame

Advantages:

• UIViewContentMode modes supported
• Can query for scales and for rect separately
• Can ask about image frame right from UIImageView

Here's my AVFoundation-less solution.

First here's a CGSize extension for calculating a size that would fit another size:

extension CGSize
{
    func sizeThatFitsSize(_ aSize: CGSize) -> CGSize
    {
        let width = min(self.width * aSize.height / self.height, aSize.width)
        return CGSize(width: width, height: self.height * width / self.width)
    }
}

So the solution to OP's problem gets down to:

let resultSize = image.size.sizeThatFitsSize(imageView.bounds.size)

Also here's another extension for fitting a rect within another rect (it utilizes the above CGSize extension):

extension CGRect
{
    func rectThatFitsRect(_ aRect:CGRect) -> CGRect
    {
        let sizeThatFits = self.size.sizeThatFitsSize(aRect.size)

        let xPos = (aRect.size.width - sizeThatFits.width) / 2
        let yPos = (aRect.size.height - sizeThatFits.height) / 2

        let ret = CGRect(x: xPos, y: yPos, width: sizeThatFits.width, height: sizeThatFits.height)
        return ret
    }
}

Swift 5 Extension

extension CGSize {
    func aspectFit(to size: CGSize) -> CGSize {
        let mW = size.width / self.width;
        let mH = size.height / self.height;
        
        var result = size
        if( mH < mW ) {
            result.width = size.height / self.height * self.width;
        }
        else if( mW < mH ) {
            result.height = size.width / self.width * self.height;
        }
        
        return result;
    }
    
    func aspectFill(to size: CGSize) -> CGSize {
        let mW = size.width / self.width;
        let mH = size.height / self.height;
        
        var result = size
        if( mH > mW ) {
            result.width = size.height / self.height * self.width;
        }
        else if( mW > mH ) {
            result.height = size.width / self.width * self.height;
        }
        return result;
    }
}

I'm using the following in Swift:

private func CGSizeAspectFit(aspectRatio:CGSize,boundingSize:CGSize) -> CGSize
{
    var aspectFitSize = boundingSize
    let mW = boundingSize.width / aspectRatio.width
    let mH = boundingSize.height / aspectRatio.height
    if( mH < mW )
    {
        aspectFitSize.width = mH * aspectRatio.width
    }
    else if( mW < mH )
    {
        aspectFitSize.height = mW * aspectRatio.height
    }
    return aspectFitSize
}

private func CGSizeAspectFill(aspectRatio:CGSize,minimumSize:CGSize) -> CGSize
{
    var aspectFillSize = minimumSize
    let mW = minimumSize.width / aspectRatio.width
    let mH = minimumSize.height / aspectRatio.height
    if( mH > mW )
    {
        aspectFillSize.width = mH * aspectRatio.width
    }
    else if( mW > mH )
    {
        aspectFillSize.height = mW * aspectRatio.height
    }
    return aspectFillSize
}

I'm using it like this:

let aspectSize  = contentMode == .ScaleAspectFill ? CGSizeAspectFill(oldSize,minimumSize: newSize) : CGSizeAspectFit(oldSize, boundingSize: newSize)

let newRect = CGRect( x: (newSize.width - aspectSize.width)/2, y: (newSize.height - aspectSize.height)/2, width: aspectSize.width, height: aspectSize.height)

CGContextSetFillColorWithColor(context,IOSXColor.whiteColor().CGColor)
CGContextFillRect(context, CGRect(origin: CGPointZero,size: newSize))
CGContextDrawImage(context, newRect, cgImage)

If you know only the width of the imageview and when the height of the image is dynamic then you need to scale the image's height according to the given width to remove the white spaces above and below your image. Use the following method from here to scale the height of the image according to the standard width of your screen.

-(UIImage*)imageWithImage: (UIImage*) sourceImage scaledToWidth: (float) i_width
{
    float oldWidth = sourceImage.size.width;
    float scaleFactor = i_width / oldWidth;

    float newHeight = sourceImage.size.height * scaleFactor;
    float newWidth = oldWidth * scaleFactor;

    UIGraphicsBeginImageContext(CGSizeMake(newWidth, newHeight));
    [sourceImage drawInRect:CGRectMake(0, 0, newWidth, newHeight)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();    
    UIGraphicsEndImageContext();
    return newImage;
}

And call it from your cellForRowAtIndexPath: method like this:

UIImage *img = [dictImages objectForKey:yourImageKey]; // loaded the image
cell.imgView.image = [self imageWithImage:img scaledToWidth:self.view.frame.size.width];

Swift 4 version

extension CGSize {
   enum AspectMode {
       case fit
       case fill
   }

   enum Orientation {
       case portrait
       case landscape
   }

   func aspectCorrectSizeToFit(targetSize: CGSize, aspectMode: AspectMode = .fill) -> CGSize {
        switch aspectMode {
        case .fill: return aspectFill(targetSize: targetSize)
        case .fit: return aspectFit(targetSize: targetSize)
        }
    }

    var orientation: Orientation {
        if height >= width { return .portrait }
        else { return .landscape }
    }

    func aspectFit(targetSize: CGSize) -> CGSize {
        let wRatio = targetSize.width / width
        let hRatio = targetSize.height / height
        let scale = min(wRatio, hRatio)
        return applying(CGAffineTransform(scaleX: scale, y: scale))
    }

    func aspectFill(targetSize: CGSize) -> CGSize {
        let wRatio = targetSize.width / width
        let hRatio = targetSize.height / height
        let scale = max(wRatio, hRatio)
        return applying(CGAffineTransform(scaleX: scale, y: scale))
    }
}

The above mentioned methods never give the required values.As aspect fit maintains the same aspect ratio we just need simple maths to calculate the values

Detect the aspect ratio

CGFloat imageViewAspectRatio = backgroundImageView.bounds.size.width / backgroundImageView.bounds.size.height;
CGFloat imageAspectRatio =  backgroundImageView.image.size.width / backgroundImageView.image.size.height;
CGFloat mulFactor = imageViewAspectRatio/imageAspectRatio;

Get the new values

CGFloat newImageWidth = mulFactor * backgroundImageView.bounds.size.width;
CGFloat newImageHeight = mulFactor * backgroundImageView.bounds.size.height;