我使用了 GPUImage 库，我的前置摄像头 session 预设是 AVCaptureSessionPresetPhoto，后置摄像头是 AVCaptureSessionPresetHigh，

if (self.isFrontFacingCameraPresent) {
    [self setCaptureSessionPreset: AVCaptureSessionPresetHigh];
} else {
    [self setCaptureSessionPreset:AVCaptureSessionPresetPhoto];
}
[self rotateCamera];

1. 初始状态为使用前置摄像头，分辨率为1280x960；

2. 现在换了后置摄像头，分辨率为1920x1080；

3. 然后换前置摄像头，分辨率是1280x720，很奇怪；

我检查了这个委托(delegate)方法:

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection

获取宽度和高度:

    CVImageBufferRef cameraFrame = CMSampleBufferGetImageBuffer(sampleBuffer);
    int bufferWidth = (int) CVPixelBufferGetWidth(cameraFrame);
    int bufferHeight = (int) CVPixelBufferGetHeight(cameraFrame);

bufferHeight是720，不知道为什么换了后置摄像头，高度从960变成了720!也许这是苹果的错误？

最佳答案

我解决了这个问题，再见更改 rotateCamera 函数，我重写了一个用于前后切换相机的函数:

- (void)switchCameraFrontAndBack {

    NSError *error;
    AVCaptureDeviceInput *newVideoInput;
    AVCaptureDevicePosition currentCameraPosition = self.cameraPosition;

    if (currentCameraPosition == AVCaptureDevicePositionBack)
    {
        currentCameraPosition = AVCaptureDevicePositionFront;
    }
    else
    {
        currentCameraPosition = AVCaptureDevicePositionBack;
    }

    AVCaptureDevice *backFacingCamera = nil;
    NSArray *devices = [AVCaptureDevice devicesWithMediaType:AVMediaTypeVideo];
    for (AVCaptureDevice *device in devices)
    {
        if ([device position] == currentCameraPosition)
        {
            backFacingCamera = device;
        }
    }
    newVideoInput = [[AVCaptureDeviceInput alloc] initWithDevice:backFacingCamera error:&error];

    if (newVideoInput != nil)
    {
        [_captureSession beginConfiguration];
        [_captureSession removeInput:videoInput];
        [self configSessionPreset:currentCameraPosition];
        if ([_captureSession canAddInput:newVideoInput])
        {
            [_captureSession addInput:newVideoInput];
            videoInput = newVideoInput;
        }
        else
        {
            [_captureSession addInput:videoInput];
        }
        [_captureSession commitConfiguration];
    }

    _inputCamera = backFacingCamera;
    [self setOutputImageOrientation:self.outputImageOrientation];
}

- (void)configSessionPreset:(AVCaptureDevicePosition)currentPosition {
    if (currentPosition == AVCaptureDevicePositionBack) {
        if (WIDTH <= Iphone4SWidth) {
            if ([self.captureSession canSetSessionPreset:AVCaptureSessionPreset1280x720]) {
                [self setCaptureSessionPreset:AVCaptureSessionPreset1280x720];
            } else if ([self.captureSession canSetSessionPreset:AVCaptureSessionPreset1920x1080]) {
                [self setCaptureSessionPreset:AVCaptureSessionPreset1920x1080];
            }
        } else {
            if ([self.captureSession canSetSessionPreset:AVCaptureSessionPreset1920x1080]) {
                [self setCaptureSessionPreset:AVCaptureSessionPreset1920x1080];
            } else {
                [self setCaptureSessionPreset: AVCaptureSessionPresetHigh];
            }
        }
    } else {
        [self setCaptureSessionPreset:AVCaptureSessionPresetPhoto];
    }
}

关于ios切换相机前后分辨率改变AVCaptureSessionPresetHigh，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/33728032/