为AVCaptureSession设置自定义AVFrameRateRange

Question

我试图用AVCaptureSession每秒拍摄5张照片,我不确定我理解AVFrameRange的含义.目前我有一些设置设备的代码:

AVCaptureDevice *device = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeVideo];

并尝试设置activeVideoMinFrameDuration和activeVideoMaxFrameDuration自定义值CMTimeMake(1, 5).Apple告诉我,我只能使用他们提供的AVFrameRanges之一.

当我对它们进行NSLog时,我得到(2,30),(2,60)和(2,24).我首先想知道这意味着什么？这是相机运行的帧速率还是捕获帧的间隔(即我正在尝试做什么)？

如果不是,我该怎样做才能在sampleBufferDelegate方法上每秒保存5帧？目前它给了我每一个帧,因为每次有一个帧时都会调用该方法,所以我只需要一些指针来说明我每秒只能抓5个.

Answer 1

这是我们使用的工作代码,它将帧速率设置为每秒5帧.

如果您在使用此代码时测量对CaptureOutput的调用,您可以看到相机帧每200毫秒调用一次(即每秒5帧).(我们只是对此进行了测试以确认.)

更改desiredFrameRate以获取其他相机帧速率.

- (void)attemptToConfigure5FPS
{
    NSError *error;
    if (![self lockForConfiguration:&error]) {
        NSLog(@"Could not lock device %@ for configuration: %@", self, error);
        return;
    }

    AVCaptureDeviceFormat *format = self.activeFormat;
    double epsilon = 0.00000001;

    int desiredFrameRate = 5;

    for (AVFrameRateRange *range in format.videoSupportedFrameRateRanges) {

            if (range.minFrameRate <= (desiredFrameRate + epsilon) &&
            range.maxFrameRate >= (desiredFrameRate - epsilon)) {

            self.activeVideoMaxFrameDuration = (CMTime){
                .value = 1,
                .timescale = desiredFrameRate,
                .flags = kCMTimeFlags_Valid,
                .epoch = 0,
            };
            self.activeVideoMinFrameDuration = (CMTime){
                .value = 1,
                .timescale = desiredFrameRate,
                .flags = kCMTimeFlags_Valid,
                .epoch = 0,
            };           
            break;
        }
    }

    [self unlockForConfiguration];
}

Answer 2

选择自定义帧速率的代码如下 - 添加了对Apple RosyWriter 的检查以验证当前格式是否支持所选的 FPS

- (void)configureCamera:(AVCaptureDevice *)videoDevice withFrameRate:(int)desiredFrameRate
{
    BOOL isFPSSupported = NO;
    AVCaptureDeviceFormat *currentFormat = [videoDevice activeFormat];
    for ( AVFrameRateRange *range in currentFormat.videoSupportedFrameRateRanges ) {
        if ( range.maxFrameRate >= desiredFrameRate && range.minFrameRate <= desiredFrameRate )        {
            isFPSSupported = YES;
            break;
        }
    }

    if( isFPSSupported ) {
        if ( [videoDevice lockForConfiguration:NULL] ) {
            videoDevice.activeVideoMaxFrameDuration = CMTimeMake( 1, desiredFrameRate );
            videoDevice.activeVideoMinFrameDuration = CMTimeMake( 1, desiredFrameRate );
            [videoDevice unlockForConfiguration];
        }
    }
}

如果当前格式 ( activeFormat) 不支持您选择的 FPS，请使用以下代码进行更改activeFormat，然后选择 FPS。不过，需要获得格式尺寸来满足您的需求。

- (void)configureCamera:(AVCaptureDevice *)device withFrameRate:(int)desiredFrameRate
{
    AVCaptureDeviceFormat *desiredFormat = nil;
    for ( AVCaptureDeviceFormat *format in [device formats] ) {
        for ( AVFrameRateRange *range in format.videoSupportedFrameRateRanges ) {
            if ( range.maxFrameRate >= desiredFrameRate && range.minFrameRate <= desiredFrameRate ) {
                desiredFormat = format;
                goto desiredFormatFound;
            }
        }
    }

    desiredFormatFound:
    if ( desiredFormat ) {
        if ( [device lockForConfiguration:NULL] == YES ) {
            device.activeFormat = desiredFormat ;
            device.activeVideoMinFrameDuration = CMTimeMake ( 1, desiredFrameRate );
            device.activeVideoMaxFrameDuration = CMTimeMake ( 1, desiredFrameRate );
            [device unlockForConfiguration];
        }
    }
}

注意：AVCaptureConnection videoMinFrameDuration不推荐使用 设置 FPS。