private function _getFormat(){
        $format = '';
        switch(strtoupper($this->format)){
            case 'PNG':
                $format = CINDrawing::IMG_FORMAT_PNG;
                break;
            case 'JPEG':
                $format = CINDrawing::IMG_FORMAT_JPEG;
                break;
            case 'GIF':
                $format = CINDrawing::IMG_FORMAT_GIF;
                break;
            case 'WBMP':
                $format = CINDrawing::IMG_FORMAT_WBMP;
                break;
            default:
                $format = CINDrawing::IMG_FORMAT_PNG;
                break;
        }
        return $format;
    }