需求：识别图片中的矩形框，标出矩形区域，并可以裁剪矩形区域

实现方式：

1、使用CIDetector类识别矩形区域

ps:也可以使用Vision(仅支持iOS11.0以上版本)框架的的VNDetectRectanglesRequest进行矩形框识别

当前使用的detectorType为CIDetectorTypeRectangle(矩形识别)，可以通过修改其他type进行面部识别、二维码识别、文本识别

// Types to be used for +[CIDetector detectorOfType:context:options:]

/* Specifies a detector type for face recognition. */
CORE_IMAGE_EXPORT NSString* const CIDetectorTypeFace NS_AVAILABLE(10_7, 5_0);

/* Specifies a detector type for rectangle detection. */
CORE_IMAGE_EXPORT NSString* const CIDetectorTypeRectangle NS_AVAILABLE(10_10, 8_0);

/* Specifies a detector type for barcode detection. */
CORE_IMAGE_EXPORT NSString* const CIDetectorTypeQRCode NS_AVAILABLE(10_10, 8_0);

/* Specifies a detector type for text detection. */
#if __OBJC2__
CORE_IMAGE_EXPORT NSString* const CIDetectorTypeText NS_AVAILABLE(10_11, 9_0);
#endif

具体实现代码：

    // 初始化 - type选择CIDetectorTypeRectangle
    CIDetector *detector = [CIDetector detectorOfType:CIDetectorTypeRectangle context:nil options:@{CIDetectorAccuracy : CIDetectorAccuracyHigh}];
    
    // 获取矩形区域数组
    NSArray <CIFeature *>*rectangles = [detector featuresInImage:[CIImage imageWithCGImage:image.CGImage]];
    
    if (rectangles.count == 0) {
        
        return [[CIRectangleFeature alloc] init];
    } else {
        
        // 最大矩形区域
        CIRectangleFeature *rectangle = (CIRectangleFeature *)rectangles.firstObject;
        
        CGFloat rectangleRect = 0;
        
        for (CIRectangleFeature *rect in rectangles) {
            
            CGPoint p1 = rect.topLeft;
            CGPoint p2 = rect.topRight;
            CGFloat width = hypotf(p1.x - p2.x, p1.y - p2.y);
            
            CGPoint p3 = rect.topLeft;
            CGPoint p4 = rect.bottomLeft;
            CGFloat height = hypotf(p3.x - p4.x, p3.y - p4.y);
            
            CGFloat currentRectangleRect = height + width;
            
            // 获取最大矩形rect
            if (rectangleRect < currentRectangleRect) {
                
                rectangleRect = currentRectangleRect;
                rectangle = rect;
            }
        }
        return rectangle;
    }

CIRectangleFeature包含五个属性

    // 矩形区域
    CGRect bounds;
    // 矩形的四个顶点
    CGPoint topLeft;
    CGPoint topRight;
    CGPoint bottomLeft;
    CGPoint bottomRight;

通过CIRectangleFeature的四个顶点可以进行矩形区域的显示和裁剪

2、获取到矩形区域四个顶点使用LImageView显示矩形区域，并可以进行自由裁剪

/// 通过图片中矩形的四个顶点设置选中框的8个点
/// @param points 四个顶点数组 - 顺序为：左上 -> 右上 -> 右下 -> 左下
- (void)setPointWithRectanglePoints:(NSArray *)points {
    
    self.areaView.hidden = NO;
    
    CGPoint topLeft = CGPointFromString(points[0]);
    CGPoint topRight = CGPointFromString(points[1]);
    CGPoint bottomRight = CGPointFromString(points[2]);
    CGPoint bottomLeft = CGPointFromString(points[3]);
    
    // 四个顶点都为0时 - 默认选中整个图片
    if (CGPointEqualToPoint(topLeft, CGPointZero) &&
        CGPointEqualToPoint(topRight, CGPointZero) &&
        CGPointEqualToPoint(bottomLeft, CGPointZero) &&
        CGPointEqualToPoint(bottomRight, CGPointZero)) {
        
        [self resetAreaViewRect];
        
        return;
    }
    
    /// 将图像坐标系转为UIKit坐标系
    
    // 图片与imageView宽高比
    CGFloat scaleW = CGRectGetWidth(self.imageView.frame)/self.originImage.size.width;
    CGFloat scaleH = CGRectGetHeight(self.imageView.frame)/self.originImage.size.height;
    
    CGAffineTransform transform = CGAffineTransformMakeTranslation(0.f, CGRectGetHeight(self.imageView.frame));
    transform = CGAffineTransformScale(transform, 1, -1);
    
    transform = CGAffineTransformScale(transform, scaleW, scaleH);
    
    /// 设置选中区域
    // 设置四个顶点位置
    self.topLeftAreaView.center = CGPointApplyAffineTransform(topLeft, transform);
    self.topRightAreaView.center = CGPointApplyAffineTransform(topRight, transform);
    self.bottomLeftAreaView.center = CGPointApplyAffineTransform(bottomLeft, transform);
    self.bottomRightAreaView.center = CGPointApplyAffineTransform(bottomRight, transform);
    
    // 设置四个中心点位置
    for (UIView *view in self.areaView.subviews) {
        
        if (view == self.topCenterAreaView ||
            view == self.leftCenterAreaView ||
            view == self.bottomCenterAreaView ||
            view == self.rightCenterAreaView) {
            
            AreaView *centerView = (AreaView *)view;
            
            centerView.center = CGPointMake(fabs(centerView.relationView1.center.x - centerView.relationView2.center.x)/2 + MIN(centerView.relationView1.center.x, centerView.relationView2.center.x), fabs(centerView.relationView1.center.y - centerView.relationView2.center.y)/2 + MIN(centerView.relationView1.center.y, centerView.relationView2.center.y));
        }
    }
    
    // 设置蒙层路径
    [self resetMaskPath];
}

LImageView中需要自己定义并实现该方法

3、使用CIImage的- (CIImage *)imageByApplyingFilter:(NSString *)filterName withInputParameters:(nullable NSDictionary<NSString *,id> *)params方法对图片进行裁剪（可自动矫正图片）

/// 通过图片矩形的四个顶点截取图片 - 带矫正
/// @param image 图片
/// @param rectangleFeature 顶点
+ (UIImage *)clipRectangleImage:(UIImage *)image rectanglePoint:(CIRectangleFeature *)rectangleFeature {
    
    UIImage *img;

    /// 判断是否存在矩形框 - 防止截取到空白图片
    if (CGPointEqualToPoint(rectangleFeature.topLeft, CGPointZero) &&
        CGPointEqualToPoint(rectangleFeature.topRight, CGPointZero) &&
        CGPointEqualToPoint(rectangleFeature.bottomLeft, CGPointZero) &&
        CGPointEqualToPoint(rectangleFeature.bottomRight, CGPointZero)) {
        
        img = image;
    } else {
        
        NSMutableDictionary *rectangleCoordinates = [NSMutableDictionary dictionary];
        rectangleCoordinates[@"inputTopLeft"] = [CIVector vectorWithCGPoint:rectangleFeature.topLeft];
        rectangleCoordinates[@"inputTopRight"] = [CIVector vectorWithCGPoint:rectangleFeature.topRight];
        rectangleCoordinates[@"inputBottomLeft"] = [CIVector vectorWithCGPoint:rectangleFeature.bottomLeft];
        rectangleCoordinates[@"inputBottomRight"] = [CIVector vectorWithCGPoint:rectangleFeature.bottomRight];

        CIImage * ciimage = [[CIImage imageWithCGImage:image.CGImage] imageByApplyingFilter:@"CIPerspectiveCorrection" withInputParameters:rectangleCoordinates];
        
        img = [UIImage imageWithCIImage:ciimage];
    }
    
    return img;
}

若使用图片裁剪工具进行图片裁剪需要先将坐标系转为图片坐标系，并将上边图片裁剪方法中的参数CIRectangleFeature改为自己定义的结构体或其他类型参数即可，方法为：

   // 将UIKit坐标系转换为图片坐标系
    
    CGFloat scaleW = self.originImage.size.width/CGRectGetWidth(self.imageView.frame);
    CGFloat scaleH = self.originImage.size.height/CGRectGetHeight(self.imageView.frame);
    
    CGAffineTransform transform = CGAffineTransformMakeTranslation(0.f, self.originImage.size.height);
    
    transform = CGAffineTransformScale(transform, scaleW, scaleH);
    
    transform = CGAffineTransformScale(transform, 1, -1);
    
    
    RectanglePoints points;
    
    points.topLeft = CGPointApplyAffineTransform(self.topLeftAreaView.center, transform);
    points.topRight = CGPointApplyAffineTransform(self.topRightAreaView.center, transform);
    points.bottomRight = CGPointApplyAffineTransform(self.bottomRightAreaView.center, transform);
    points.bottomLeft = CGPointApplyAffineTransform(self.bottomLeftAreaView.center, transform);

RectanglePoints为自己定义的结构体，包含四个顶点坐标

文笔有限，可能有说的不太清楚的地方，敬请谅解。如有错误，请多指教！

Just do IT!