Move UIImage inside UIImageView

问题:

I have an UIImageView (red squares) that will display a UIImage that must be scaled (I can receive images greater or smaller that the UIImageView). After scaling it, the showed part of the UIImage is the center of it.

What I need is to show the part of the image in the blue squares, how can I archive it?

I'm only able to get the image size (height and width), but it display the original size, when it's supposed to be the scaled one.

self.viewIm = [[UIImageView alloc] initWithFrame:CGRectMake(100, 100, 120, 80)];
self.viewIm.backgroundColor = [UIColor greenColor];
self.viewIm.layer.borderColor = [UIColor redColor].CGColor;
self.viewIm.layer.borderWidth = 5.0;
UIImage *im = [UIImage imageNamed:@"benjen"];
self.viewIm.image = im;
self.viewIm.contentMode = UIViewContentModeScaleAspectFill;
//    self.viewim.clipsToBounds = YES;
[self.view addSubview:self.viewIm];

回答1:

To do what you're trying to do, I'd recommend looking into CALayer's contentsRect property.

Since seeing your answer, I've been trying to work out the proper solution for a while, but the mathematics escapes me because contentsRect:'s x and y parameters seem sort of mysterious... But here's some code that may point you in the right direction...

float imageAspect = self.imageView.image.size.width/self.imageView.image.size.height;
float imageViewAspect = self.imageView.frame.size.width/self.imageView.frame.size.height;

if (imageAspect > imageViewAspect) {
    float scaledImageWidth = self.imageView.frame.size.height * imageAspect;
    float offsetWidth = -((scaledImageWidth-self.imageView.frame.size.width)/2);
    self.imageView.layer.contentsRect = CGRectMake(offsetWidth/self.imageView.frame.size.width, 0.0, 1.0, 1.0);
} else if (imageAspect < imageViewAspect) {
    float scaledImageHeight = self.imageView.frame.size.width * imageAspect;
    float offsetHeight = ((scaledImageHeight-self.imageView.frame.size.height)/2);
    self.imageView.layer.contentsRect = CGRectMake(0.0, offsetHeight/self.imageView.frame.size.height, 1.0, 1.0);
}

回答2:

Try something like this:

CGRect cropRect = CGRectMake(0,0,200,200);
CGImageRef imageRef = CGImageCreateWithImageInRect([ImageToCrop CGImage],cropRect);
UIImage *image = [UIImage imageWithCGImage:imageRef]; 
CGImageRelease(imageRef);

回答3:

I found a very good approximation on this answer. In that, the category resize the image, and use the center point to crop after that. I adapt it to crop using (0,0) as origin point. As I don't really need a category, I use it as a single method.

- (UIImage *)imageByScalingAndCropping:(UIImage *)image forSize:(CGSize)targetSize {
    UIImage *sourceImage = image;
    UIImage *newImage = nil;
    CGFloat scaleFactor = 0.0;
    CGFloat scaledWidth = targetSize.width;
    CGFloat scaledHeight = targetSize.height;

    if (CGSizeEqualToSize(image.size, targetSize) == NO) {
        if ((targetSize.width / image.size.width) > (targetSize.height / image.size.height)) {
            scaleFactor = targetSize.width / image.size.width; // scale to fit height
        } else {
            scaleFactor = targetSize.height / image.size.height; // scale to fit width
        }

        scaledWidth  = image.size.width * scaleFactor;
        scaledHeight = image.size.height * scaleFactor;
    }

    UIGraphicsBeginImageContext(targetSize); // this will crop

    CGRect thumbnailRect = CGRectZero;
    thumbnailRect.origin = CGPointZero;
    thumbnailRect.size.width  = scaledWidth;
    thumbnailRect.size.height = scaledHeight;

    [sourceImage drawInRect:thumbnailRect];

    newImage = UIGraphicsGetImageFromCurrentImageContext();

    if(newImage == nil) {
        NSLog(@"could not scale image");
    }

    //pop the context to get back to the default
    UIGraphicsEndImageContext();

    return newImage;
}

And my call is something like this:

self.viewIm = [[UIImageView alloc] initWithFrame:CGRectMake(100, 100, 120, 80)];
self.viewIm.image = [self imageByScalingAndCropping:[UIImage imageNamed:@"benjen"] forSize:CGSizeMake(120, 80)];
[self.view addSubview:self.viewIm];

回答4:

I've spent some time on this and finally created a Swift 3.2 solution (based on one of my answers on another thread, as well as one of the answers above). This code only allows for Y translation of the image, but with some tweaks anyone should be able to add horizontal translation as well ;)

let yOffset: CGFloat = 20
myImageView.contentMode = .scaleAspectFill

//scale image to fit the imageView's width (maintaining aspect ratio), but allow control over the image's Y position
UIGraphicsBeginImageContextWithOptions(myImageView.frame.size, myImageView.isOpaque, 0.0)
let ratio = myImage.size.width / myImage.size.height
let newHeight = myImageView.frame.width / ratio
let rect = CGRect(x: 0, y: -yOffset, width: myImageView.frame.width, height: newHeight)
myImage.draw(in: rect)
let newImage = UIGraphicsGetImageFromCurrentImageContext() ?? myImage
UIGraphicsEndImageContext()

//set the new image
myImageView.image = newImage

Now you can adjust how far down or up you need the image to be by changing the yOffset.