首先通过#import <AVFoundation/AVFoundation.h> 捕捉手机屏幕图像。
主要代码：

@interface ViewController ()<AVCaptureVideoDataOutputSampleBufferDelegate>
//硬件设备
@property (nonatomic, strong) AVCaptureDevice *device;
//输入流
@property (nonatomic, strong) AVCaptureDeviceInput *input;
//协调输入输出流的数据
@property (nonatomic, strong) AVCaptureSession *session;
//预览层
@property (nonatomic, strong) AVCaptureVideoPreviewLayer *previewLayer;
//输出流
@property (nonatomic, strong) AVCaptureVideoDataOutput *videoDataOutput;

@end

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view.layer addSublayer:self.previewLayer];
    [self.session startRunning];
}
#pragma mark - AVCaptureVideoDataOutputSampleBufferDelegate
//AVCaptureVideoDataOutput获取实时图像，这个代理方法的回调频率很快，几乎与手机屏幕的刷新频率一样快
- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection{
    //设置图像方向，否则largeImage取出来是反的
    [connection setVideoOrientation:AVCaptureVideoOrientationPortrait];
    //获取图像图片（使用下面人脸捕捉代码即可）   
 largeImage = [self imageFromSampleBuffer:sampleBuffer];
}

//CMSampleBufferRef转NSImage
-(UIImage *)imageFromSampleBuffer:(CMSampleBufferRef)sampleBuffer{
    // 为媒体数据设置一个CMSampleBuffer的Core Video图像缓存对象
    CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
    // 锁定pixel buffer的基地址
    CVPixelBufferLockBaseAddress(imageBuffer, 0);
    // 得到pixel buffer的基地址
    void *baseAddress = CVPixelBufferGetBaseAddress(imageBuffer);
    // 得到pixel buffer的行字节数
    size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
    // 得到pixel buffer的宽和高
    size_t width = CVPixelBufferGetWidth(imageBuffer);
    size_t height = CVPixelBufferGetHeight(imageBuffer);
    // 创建一个依赖于设备的RGB颜色空间
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    // 用抽样缓存的数据创建一个位图格式的图形上下文（graphics context）对象
    CGContextRef context = CGBitmapContextCreate(baseAddress, width, height, 8, bytesPerRow, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst);
    // 根据这个位图context中的像素数据创建一个Quartz image对象
    CGImageRef quartzImage = CGBitmapContextCreateImage(context);
    // 解锁pixel buffer
    CVPixelBufferUnlockBaseAddress(imageBuffer,0);
    // 释放context和颜色空间
    CGContextRelease(context); CGColorSpaceRelease(colorSpace);
    // 用Quartz image创建一个UIImage对象image
    UIImage *image = [UIImage imageWithCGImage:quartzImage];
    // 释放Quartz image对象
    CGImageRelease(quartzImage);
    return (image);
}


-(AVCaptureDeviceInput *)input{
    if (_input == nil) {
        _input = [[AVCaptureDeviceInput alloc] initWithDevice:self.device error:nil];
    }
    return _input;
}


-(AVCaptureVideoDataOutput *)videoDataOutput{
    if (_videoDataOutput == nil) {
        _videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];
        [_videoDataOutput setSampleBufferDelegate:self queue:dispatch_get_main_queue()];
        //设置像素格式，否则CMSampleBufferRef转换NSImage的时候CGContextRef初始化会出问题
        [_videoDataOutput setVideoSettings:[NSDictionary dictionaryWithObject:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA] forKey:(id)kCVPixelBufferPixelFormatTypeKey]];
    }
    return _videoDataOutput;
}

-(AVCaptureSession *)session{
    if (_session == nil) {
        _session = [[AVCaptureSession alloc] init];
        if ([_session canAddInput:self.input]) {
            [_session addInput:self.input];
        }
        if ([_session canAddOutput:self.videoDataOutput]) {
            [_session addOutput:self.videoDataOutput];
        }
    }
    return _session;
}

-(AVCaptureVideoPreviewLayer *)previewLayer{
    if (_previewLayer == nil) {
        _previewLayer = [[AVCaptureVideoPreviewLayer alloc] initWithSession:self.session];
        _previewLayer.frame = self.view.layer.bounds;
    }
    return _previewLayer;
}

主要使用#import <CoreImage/CoreImage.h>框架捕捉，简单代码如下

// 图像识别能力：可以在CIDetectorAccuracyHigh(较强的处理能力)与CIDetectorAccuracyLow(较弱的处理能力)中选择，因为想让准确度高一些在这里选择CIDetectorAccuracyHigh
    NSDictionary *opts = [NSDictionary dictionaryWithObject:
                          CIDetectorAccuracyHigh forKey:CIDetectorAccuracy];
    // 将图像转换为CIImage
    CIImage *faceImage = [CIImage imageWithCGImage:image.CGImage];
    CIDetector *faceDetector=[CIDetector detectorOfType:CIDetectorTypeFace context:nil options:opts];
    // 识别出人脸数组
    NSArray *features = [faceDetector featuresInImage:faceImage options: [NSDictionary dictionaryWithObject:@(5) forKey:CIDetectorImageOrientation]];

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