activeArea

    private func activeArea(pixelBuffer: CVPixelBuffer) -> CGRect {
        let width = CVPixelBufferGetWidth(pixelBuffer)
        let height = CVPixelBufferGetHeight(pixelBuffer)
        let verticalInset = Double(height) * self.activeInsetProcent
        let horizontalInset = Double(width) * self.activeInsetProcent
        return CGRect(x: horizontalInset,
                      y: verticalInset,
                      width: Double(width) - 2*horizontalInset,
                      height: Double(height) - 2*verticalInset)
    }