public function dvi2png()
    {
        // XXX background: -bg 'rgb 0.5 0.5 0.5'
        $command = static::DVIPNG . ' -q -T tight -D ' . $this->density . ' -o ' . $this->actualFile . ' ' . $this->tmpDir . '/' . $this->hash . '.dvi 2>&1';

        if (shell_exec($command) === null) {
            throw new \Exception('Unable to convert the DVI file to PNG (is dvipng installed?)');
        }
    }