Ios – UIImage to CIImage – size is halved

core-imageiosobjective cuiimagexcode

I'm creating a CIImage from a UIImage as follows:

CIImage* someCIImage = [CIImage imageWithCGImage:someUIImage.CGImage];

Comparing someCIImage.extent.size.width/height with someUIImage.size.width/height, I find that sometimes the CIImage is double size in dimensions, and sometimes its the same size as the UIImage.

It seems like if the UIImage is slightly larger, the CIImage is double the size, whereas if the UIImage is slightly smaller this isn't the case.

Has anyone seen this before/know why this is? This is causing me real trouble as I'm trying to draw a CIImage from a loaded UIImage.

Best Answer

size has to do with the distance between points on the display, as so the number of points on a retina display are doubled. size does not represent the number of pixels as you assumed.

Related Solutions

Ios – Cropping an UIImage

Update 2014-05-28: I wrote this when iOS 3 or so was the hot new thing, I'm certain there are better ways to do this by now, possibly built-in. As many people have mentioned, this method doesn't take rotation into account; read some additional answers and spread some upvote love around to keep the responses to this question helpful for everyone.

Original response:

I'm going to copy/paste my response to the same question elsewhere:

There isn't a simple class method to do this, but there is a function that you can use to get the desired results: CGImageCreateWithImageInRect(CGImageRef, CGRect) will help you out.

Here's a short example using it:

CGImageRef imageRef = CGImageCreateWithImageInRect([largeImage CGImage], cropRect);
// or use the UIImage wherever you like
[UIImageView setImage:[UIImage imageWithCGImage:imageRef]]; 
CGImageRelease(imageRef);

Ios – How to get the size of a scaled UIImage in UIImageView

Objective-C:

-(CGRect)frameForImage:(UIImage*)image inImageViewAspectFit:(UIImageView*)imageView
{
    float imageRatio = image.size.width / image.size.height;
    float viewRatio = imageView.frame.size.width / imageView.frame.size.height;
    if(imageRatio < viewRatio)
    {
        float scale = imageView.frame.size.height / image.size.height;
        float width = scale * image.size.width;
        float topLeftX = (imageView.frame.size.width - width) * 0.5;
        return CGRectMake(topLeftX, 0, width, imageView.frame.size.height);
    }
    else
    {
        float scale = imageView.frame.size.width / image.size.width;
        float height = scale * image.size.height;
        float topLeftY = (imageView.frame.size.height - height) * 0.5;

        return CGRectMake(0, topLeftY, imageView.frame.size.width, height);
    }
}

Swift 4:

func frame(for image: UIImage, inImageViewAspectFit imageView: UIImageView) -> CGRect {
  let imageRatio = (image.size.width / image.size.height)
  let viewRatio = imageView.frame.size.width / imageView.frame.size.height
  if imageRatio < viewRatio {
    let scale = imageView.frame.size.height / image.size.height
    let width = scale * image.size.width
    let topLeftX = (imageView.frame.size.width - width) * 0.5
    return CGRect(x: topLeftX, y: 0, width: width, height: imageView.frame.size.height)
  } else {
    let scale = imageView.frame.size.width / image.size.width
    let height = scale * image.size.height
    let topLeftY = (imageView.frame.size.height - height) * 0.5
    return CGRect(x: 0.0, y: topLeftY, width: imageView.frame.size.width, height: height)
  }
}

Best Answer

Related Solutions

Ios – Cropping an UIImage

Ios – How to get the size of a scaled UIImage in UIImageView

Related Topic