前言

此文的目的是介绍通过iOS设备摄像头获得视频数据的一些API及基本设置。

主要API

• AVCaptureDevice //输入设备，摄像头或者麦克风
• AVCaptureInput //用于配置输入端口
• AVCaptureOutput //用于管理输出视频或者静态图片
• AVCaptureSession //用于协调从输入到输出的数据流
• AVCaptureVideoPreviewLayer //实时预览摄像头采集的图片数据

AVCaptureSession初始化 & 设置分辨率

AVCaptureSession的初始化仅仅是初始化一个实例对象，分辨率一般测试用直播使用AVCaptureSessionPreset640x480已经足够，后期可以根据需求设置

//初始化session
self.session = [[AVCaptureSession alloc] init];

//设置摄像头的分辨率640*480
if ([self.session canSetSessionPreset:AVCaptureSessionPreset640x480]) {
    self.session.sessionPreset = AVCaptureSessionPreset640x480;
}

• AVCaptureSessionPresetHigh //Highest recording quality. This varies per device.
• AVCaptureSessionPresetMedium //Suitable for Wi-Fi sharing. The actual values may change.
• AVCaptureSessionPresetLow //Suitable for 3G sharing. The actual values may change.
• AVCaptureSessionPreset640x480 //VGA.
• AVCaptureSessionPreset1280x720 //720p HD.
• AVCaptureSessionPresetPhoto //Full photo resolution. This is not supported for video output.

AVCaptureDevice初始化 & 自动变焦

使用默认方式获取到的摄像头是后置，如果需要获取前置要遍历所有摄像头设备，找到[device position] == AVCaptureDevicePositionFront。

//默认获取后置摄像头
self.videoDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

or 

//遍历所有视频采集设备
NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
for (AVCaptureDevice *device in devices) {
    //找到前置摄像头
    if ([device position] == AVCaptureDevicePositionFront) {
        self.videoDevice = device;
        break;
    }
}

拍摄动态视频都是要自动变焦的，我们用到支持自动变焦AVCaptureFocusModeContinuousAutoFocus

注意：要在设备上设置捕获属性，必须首先使用lockForConfiguration:获取设备上的锁。这避免了做出可能与其他应用程序中的设置不兼容的更改。

//自动变焦
if([self.videoDevice isFocusModeSupported:AVCaptureFocusModeContinuousAutoFocus]){
    NSError *error;
    BOOL isLooked = [self.videoDevice lockForConfiguration:&error];
    if(isLooked && error == nil){
        self.videoDevice.focusMode = AVCaptureFocusModeContinuousAutoFocus;
        [self.videoDevice unlockForConfiguration];
    }
}

设置帧率

帧率可以动态去设置，self.videoDevice.activeFormat.videoSupportedFrameRateRanges可以获取到摄像头帧率的支持范围，设置时同样需要对设备上锁。

//在设置帧率之前先获取帧率支持范围
NSLog(@"支持的帧速范围是: %@",[self.videoDevice.activeFormat.videoSupportedFrameRateRanges objectAtIndex:0]);

//设置帧速
NSError *error;
BOOL isLocked = [self.videoDevice lockForConfiguration:&error];

if (isLocked && error == nil) {
    
    if (self.videoDevice.activeFormat.videoSupportedFrameRateRanges){
        [self.videoDevice setActiveVideoMinFrameDuration:CMTimeMake(1, fps)];
        [self.videoDevice setActiveVideoMaxFrameDuration:CMTimeMake(1, fps)];
    }

    [self.videoDevice unlockForConfiguration];
}

其他设置

//输出设置
self.videoDataOutput = [[AVCaptureVideoDataOutput alloc] init];

//kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange 表示原始数据的格式为YUV420
NSDictionary *settings = [[NSDictionary alloc] initWithObjectsAndKeys:[NSNumber numberWithUnsignedInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange], kCVPixelBufferPixelFormatTypeKey, nil];
self.videoDataOutput.videoSettings = settings;

//保证不会在结束录制时卡顿，丢弃最后的视频帧
self.videoDataOutput.alwaysDiscardsLateVideoFrames = YES;

//创建输出处理队列
_videoQueue = dispatch_queue_create("VideoQueue", DISPATCH_QUEUE_SERIAL);

//指定输出监听代理及队列
[self.videoDataOutput setSampleBufferDelegate:self queue:_videoQueue];

if([self.session canAddOutput:self.videoDataOutput]){
    [self.session addOutput:self.videoDataOutput];
}

//获取输出连接
self.videoConnection = [self.videoDataOutput connectionWithMediaType:AVMediaTypeVideo];

//设置输出图像的方向
self.videoConnection.videoOrientation = AVCaptureVideoOrientationPortrait;

代理方法处理采集到的画面

在设置好所有的参数便可以调用session来控制摄像头开始、暂停以及结束，通过AVCaptureVideoDataOutputSampleBufferDelegate的方法监听采集到的数据。

//停止
- (void)stopVideoCapture{
    [self.session stopRunning];
    [[UIApplication sharedApplication] setIdleTimerDisabled:NO];
}

//进入后台暂停
- (void)pauseCameraCapture{
    [self.session stopRunning];
}

//进入前台开始
- (void)resumeCameraCapture{
    [self.session startRunning];
}

#pragma mark - AVCaptureVideoDataOutputSampleBufferDelegate 
/*
 CMSampleBufferRef: 帧缓存数据，描述当前帧信息
 CMSampleBufferGetXXX : 获取帧缓存信息
 CMSampleBufferGetDuration : 获取当前帧播放时间
 CMSampleBufferGetImageBuffer : 获取当前帧图片信息
 */ 
 // 获取帧数据 
 - (void)captureOutput:(AVCaptureOutput *)output didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection { 

     // captureSession 会话如果没有强引用，这里不会得到执行 
     NSLog(@"----- sampleBuffer ----- %@", sampleBuffer); 
}

参考

《iOS - 视频采集详解》
《Still and Video Media Capture》