我有一些代码来调整图像的大小，这样我就可以得到图像中心的缩放块-我用这个来获取一个UIImage，并返回一个小的，正方形的图像表示，类似于在照片应用程序的相册视图中看到的。(我知道我可以使用UIImageView和调整裁剪模式来实现相同的结果，但这些图像有时显示在UIWebViews中)。
我已经开始注意到这段代码中的一些崩溃，我有点难住了。我有两种不同的理论，不知道哪一种是正确的。
理论1)我通过绘制到目标尺寸的屏幕外图像上下文来实现裁剪。因为我想要图像的中心部分，所以我将传递给drawwinrect的CGRect参数设置为比图像上下文的边界更大的值。我希望这是符合规定的，但我是不是在试图掩盖其他我不应该触及的记忆?
理论2)我在后台线程中做所有这些。我知道UIKit的某些部分被限制在主线程中。我假设/希望绘制到屏幕外的视图不是其中之一。我错了吗?
(哦，我真怀念NSImage的drawwinrect:fromRect:operation:fraction:方法。)
当前回答
- (UIImage *)getSubImage:(CGRect) rect{
CGImageRef subImageRef = CGImageCreateWithImageInRect(self.CGImage, rect);
CGRect smallBounds = CGRectMake(rect.origin.x, rect.origin.y, CGImageGetWidth(subImageRef), CGImageGetHeight(subImageRef));
UIGraphicsBeginImageContext(smallBounds.size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextDrawImage(context, smallBounds, subImageRef);
UIImage* smallImg = [UIImage imageWithCGImage:subImageRef];
UIGraphicsEndImageContext();
return smallImg;
}
其他回答
下面的代码片段可能会有所帮助。
import UIKit
extension UIImage {
func cropImage(toRect rect: CGRect) -> UIImage? {
if let imageRef = self.cgImage?.cropping(to: rect) {
return UIImage(cgImage: imageRef)
}
return nil
}
}
这是我的UIImage作物实现，它遵从imageOrientation属性。所有方向都经过了彻底的测试。
inline double rad(double deg)
{
return deg / 180.0 * M_PI;
}
UIImage* UIImageCrop(UIImage* img, CGRect rect)
{
CGAffineTransform rectTransform;
switch (img.imageOrientation)
{
case UIImageOrientationLeft:
rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(90)), 0, -img.size.height);
break;
case UIImageOrientationRight:
rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-90)), -img.size.width, 0);
break;
case UIImageOrientationDown:
rectTransform = CGAffineTransformTranslate(CGAffineTransformMakeRotation(rad(-180)), -img.size.width, -img.size.height);
break;
default:
rectTransform = CGAffineTransformIdentity;
};
rectTransform = CGAffineTransformScale(rectTransform, img.scale, img.scale);
CGImageRef imageRef = CGImageCreateWithImageInRect([img CGImage], CGRectApplyAffineTransform(rect, rectTransform));
UIImage *result = [UIImage imageWithCGImage:imageRef scale:img.scale orientation:img.imageOrientation];
CGImageRelease(imageRef);
return result;
}
- (UIImage *)getSubImage:(CGRect) rect{
CGImageRef subImageRef = CGImageCreateWithImageInRect(self.CGImage, rect);
CGRect smallBounds = CGRectMake(rect.origin.x, rect.origin.y, CGImageGetWidth(subImageRef), CGImageGetHeight(subImageRef));
UIGraphicsBeginImageContext(smallBounds.size);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextDrawImage(context, smallBounds, subImageRef);
UIImage* smallImg = [UIImage imageWithCGImage:subImageRef];
UIGraphicsEndImageContext();
return smallImg;
}
Swift 3版本
func cropImage(imageToCrop:UIImage, toRect rect:CGRect) -> UIImage{
let imageRef:CGImage = imageToCrop.cgImage!.cropping(to: rect)!
let cropped:UIImage = UIImage(cgImage:imageRef)
return cropped
}
let imageTop:UIImage = UIImage(named:"one.jpg")! // add validation
在这个桥接函数CGRectMake -> CGRect的帮助下(这个答案由@rob mayoff回答):
func CGRectMake(_ x: CGFloat, _ y: CGFloat, _ width: CGFloat, _ height: CGFloat) -> CGRect {
return CGRect(x: x, y: y, width: width, height: height)
}
用法是:
if var image:UIImage = UIImage(named:"one.jpg"){
let croppedImage = cropImage(imageToCrop: image, toRect: CGRectMake(
image.size.width/4,
0,
image.size.width/2,
image.size.height)
)
}
输出:
(UIImage *)squareImageWithImage:(UIImage *)image scaledToSize:(CGSize)newSize {
double ratio;
double delta;
CGPoint offset;
//make a new square size, that is the resized imaged width
CGSize sz = CGSizeMake(newSize.width, newSize.width);
//figure out if the picture is landscape or portrait, then
//calculate scale factor and offset
if (image.size.width > image.size.height) {
ratio = newSize.width / image.size.width;
delta = (ratio*image.size.width - ratio*image.size.height);
offset = CGPointMake(delta/2, 0);
} else {
ratio = newSize.width / image.size.height;
delta = (ratio*image.size.height - ratio*image.size.width);
offset = CGPointMake(0, delta/2);
}
//make the final clipping rect based on the calculated values
CGRect clipRect = CGRectMake(-offset.x, -offset.y,
(ratio * image.size.width) + delta,
(ratio * image.size.height) + delta);
//start a new context, with scale factor 0.0 so retina displays get
//high quality image
if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {
UIGraphicsBeginImageContextWithOptions(sz, YES, 0.0);
} else {
UIGraphicsBeginImageContext(sz);
}
UIRectClip(clipRect);
[image drawInRect:clipRect];
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newImage;
}
