public void processFiles(ActionEvent actionEvent) {
        if (outputDirectory != null) {
            ForkJoinPool pool = new ForkJoinPool(threadsNum.getValue());
            pool.submit(() -> backgroundJob(parallel.isSelected()));
        }
    }

    private void backgroundJob(boolean parallel) {
        double start = System.currentTimeMillis();
        Stream<ImageProcessingJob> stream;

        if (parallel) {
            stream = filesView.getItems().parallelStream();
        } else {
            stream = filesView.getItems().stream();
        }

        stream.forEach(temp -> {
            if (!"zakończone".equals(temp.getStatus())) {
                convertToGrayscale(temp.getFile(), outputDirectory, temp);
            }
        });
        Platform.runLater(() -> time.setText(String.valueOf((System.currentTimeMillis() - start) / 1000)));
        Platform.runLater(() -> time.setVisible(true));
        Platform.runLater(() -> timetext.setVisible(true));
        Platform.runLater(() -> timeunit.setVisible(true));
    }