AVFoundation：如何更改帧速率

Question

我想降低AVCaptureSession的帧速率，因为我的视频处理目前无法应对30fps。我已经找到了类似的问题，但到目前为止，提出的答案对我来说并不奏效。顺便说一句，我正在使用内置的macBook相机。

我试过了：

    if ([_camera lockForConfiguration:nil])
    {
        _camera.activeVideoMinFrameDuration = CMTimeMake(1, 20);
        _camera.activeVideoMaxFrameDuration = CMTimeMake(1, 20);
        [_camera unlockForConfiguration];
    }

但是这不会编译并在第三行引发Thread1：signal SIGABRT错误。我怀疑这是因为该设备可能不支持20 fps。

论坛的另一个尝试是：

- (void)attemptToConfigureFPS
{

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

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

    int desiredFrameRate = 10;

    for (AVFrameRateRange *range in format.videoSupportedFrameRateRanges) {

        NSLog(@"Pre Minimum frame rate: %f  Max = %f", range.minFrameRate, range.maxFrameRate);


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

            NSLog(@"Setting Frame Rate.");

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

            self.activeVideoMinFrameDuration = self.activeVideoMaxFrameDuration;

            NSLog(@"Post Minimum frame rate: %f  Max = %f", range.minFrameRate, range.maxFrameRate);

            break;
        }
    }

    [_camera unlockForConfiguration];


    // Audit the changes
    for (AVFrameRateRange *range in format.videoSupportedFrameRateRanges) {

        NSLog(@"Post Minimum frame rate: %f  Max = %f", range.minFrameRate, range.maxFrameRate);

    }
}

这次代码编译，但是调试器的检查显示，

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

评估为NO，因此永远不会执行应调整帧速率的其余代码。这对我来说似乎很奇怪，非常感谢你的帮助。非常感谢你提前！