问pdfminer3不是从彩色pdf页面中提取文本，如何将pdf页面转换成灰度？

EN

from pdfminer3.layout import LAParams, LTTextBox
from pdfminer3.pdfpage import PDFPage
from pdfminer3.pdfinterp import PDFResourceManager
from pdfminer3.pdfinterp import PDFPageInterpreter
from pdfminer3.converter import PDFPageAggregator
from pdfminer3.converter import TextConverter
import io

resource_manager = PDFResourceManager()
fake_file_handle = io.StringIO()
converter = TextConverter(resource_manager, fake_file_handle, laparams=LAParams())
page_interpreter = PDFPageInterpreter(resource_manager, converter)

with open("pdffile.pdf", 'rb') as fh:
    for page in PDFPage.get_pages(fh,caching=True,check_extractable=True):
        page_interpreter.process_page(page)
    text = fake_file_handle.getvalue()
f1=open("pdftext.txt","w+",encoding="utf8")
for i in text:
    f1.write(str(i))

# close open handles
converter.close()
fake_file_handle.close()

print(text)

(cid:7)(cid:1)(cid:4)(cid:1)(cid:9)(cid:16)(cid:20)(cid:17)

(cid:10)(cid:15)(cid:14)(cid:1)(cid:16)(cid:20)(cid:17)

(cid:1)(cid:14)(cid:18)(cid:1)(cid:12) (cid:18)(cid:1)(cid:19)(cid:2)(cid:1)(cid:17)(cid:9)

(cid:18)(cid:20)(cid:12)(cid:19)(cid:1)(cid:14)(cid:16)(cid:20)(cid:17)