如何从GPUImage相机获取UIImage?

我知道我可以用静态相机 capturePhotoAsImageProcessedUpToFilter 方法,但它会发出快门咔嚓声,我还有一些处理要做,所以我不希望它发出声音。

我试过使用过滤器 imageFromCurrentFramebuffer 方法,但这总是变成零。

CGRect frame = CGRectMake(0, 0, self.view.frame.size.height, self.view.frame.size.width);
_backgroundImageView = [[GPUImageView alloc] initWithFrame:frame];
[self.view insertSubview:_backgroundImageView atIndex:0];

_camera = [[GPUImageStillCamera alloc] initWithSessionPreset:AVCaptureSessionPresetHigh cameraPosition:AVCaptureDevicePositionBack];
_camera.outputImageOrientation = UIInterfaceOrientationLandscapeRight;
filter = [[GPUImageBrightnessFilter alloc] init];
filter.brightness = 0;

[_camera addTarget: filter];

[filter addTarget:_backgroundImageView];
[_camera startCameraCapture];
// processTimer is to periodically do processing on what's being shown on screen.
processTimer =[NSTimer timerWithTimeInterval:10 target:self selector:@selector(processImage) userInfo:nil repeats:NO];
[[NSRunLoop mainRunLoop] addTimer:processTimer forMode:NSRunLoopCommonModes];


-(void) processImage {
    [processTimer invalidate];

    UIImage *testImage = [filter imageFromCurrentFramebuffer]; // always nil

    [_camera capturePhotoAsImageProcessedUpToFilter:filter withCompletionHandler:^(UIImage *processedImage, NSError *error) {

         // additional processing here ... don't want shutter sound yet.

    }];
}