How to get corners using GPUImageHarrisCornerDetectionFilter

I am trying to get the corner points from a still image using GPUImageHarrisCornerDetectionFilter.

I have looked at the example code from the project, I have looked at the documentation, and I have looked at this post that is about the same thing: GPUImage Harris Corner Detection on an existing UIImage gives a black screen output

But I can't make it work - and I have a hard time understanding how this is supposed to work with still images.

What I have at this point is this:

func harrisCorners() -> [CGPoint] {
    var points = [CGPoint]()

    let stillImageSource: GPUImagePicture = GPUImagePicture(image: self.image)
    let filter = GPUImageHarrisCornerDetectionFilter()

    filter.cornersDetectedBlock = { (cornerArray:UnsafeMutablePointer<GLfloat>, cornersDetected:UInt, frameTime:CMTime) in
        for index in 0..<Int(cornersDetected) {
            points.append(CGPoint(x:CGFloat(cornerArray[index * 2]), y:CGFloat(cornerArray[(index * 2) + 1])))
        }
    }

    filter.forceProcessingAtSize(self.image.size)
    stillImageSource.addTarget(filter)
    stillImageSource.processImage()

    return points
}

This function always returns [] so it's obviously not working.

An interesting detail - I compiled the FilterShowcaseSwift project from GPUImage examples, and the filter fails to find very clear corners, like on a sheet of paper on a black background.