如何在Python中将8bpp位图转换为1bpp位图

现有代码可生成灰度（8bpp）图像，但需要为打印机生成1bpp（二进制）图像。已安装Pillow，习惯从PIL导入Image，编写了conv_to_1bpp方法将值为255的像素设为1，但生成的图像仍为8bpp格式。

from PIL import Image
import numpy as np
import os
import sys
import struct


class BmpMaker:

    def __init__(self):
        self.ffp = None
        self.img_width = None
        self.img_height = None
        self.ws_activation = None
        self.bpp = None

    def make_image(self, img_width, img_height, ws_activation):
        """Creates a bitmap."""

        self.ffp = None
        self.img_width = img_width
        self.img_height = img_height
        self.ws_activation = ws_activation
       
        # Create new black image - L mode for b/w
        img = Image.new('L', (img_width, img_height))
        # Convert to Numpy array for easy processing
        na = np.array(img)
        # set the amount of white space activation
        white_rows = ws_activation
        # make white lines
        na[0:white_rows, 0:999] = 255
        # Revert to PIL Image from Numpy array and save
        self.ffp = self.make_img_title(img_width, img_height, ws_activation)
        Image.fromarray(na).save(self.ffp)
        self.bpp = self.get_bitdepth()
        return self.ffp

    def make_img_title(self, img_width, img_height, ws_activation):
        if ws_activation == 0:
            mystr = f"{img_height}x{img_width}"
        elif ws_activation is not None:
            mystr = f"{ws_activation}_{img_height}x{img_width}"
        self.ffp = os.path.join(os.getcwd(), mystr + ".bmp")
        print("img_ffp : ", self.ffp)
        return self.ffp

    def get_bitdepth(self):
        # Read first 100 bytes
        with open(self.ffp, 'rb') as f:
            BMP = f.read(100)

        if BMP[0:2] != b'BM':
            sys.exit('ERROR: Incorrect BMP signature')

        # Get BITMAPINFOHEADER size
        BITMAPINFOHEADERSIZE = struct.unpack('<i', BMP[14:18])[0]
        okSizes = [40, 52, 56, 108, 124]
        if BITMAPINFOHEADERSIZE not in okSizes:
            sys.exit(f'ERROR: BITMAPINFOHEADER size was {BITMAPINFOHEADERSIZE},'
                     f' expected one of {okSizes}')

        # Get bits per pixel
        self.bpp = struct.unpack('<H', BMP[28:30])[0]
        print(f'bbp: {self.bpp}')
        return self.bpp

    def get_img_bitdepth(self, img):
        # Read first 100 bytes
        with open(img, 'rb') as f:
            BMP = f.read(100)

        if BMP[0:2] != b'BM':
            sys.exit('ERROR: Incorrect BMP signature')

        # Get BITMAPINFOHEADER size
        BITMAPINFOHEADERSIZE = struct.unpack('<i', BMP[14:18])[0]
        okSizes = [40, 52, 56, 108, 124]
        if BITMAPINFOHEADERSIZE not in okSizes:
            sys.exit(f'ERROR: BITMAPINFOHEADER size was {BITMAPINFOHEADERSIZE},'
                     f' expected one of {okSizes}')

        # Get bits per pixel
        img_bpp = struct.unpack('<H', BMP[28:30])[0]
        print(f'bbp: {img_bpp}')
        return img_bpp

    def conv_to_1bpp(self):
        img = Image.open(self.ffp)
        arr = np.array(img)
        # Set all values at indices where the array equals 255 to 1.
        arr[arr==255] = 1
        print('\n'*3, arr)
        Image.fromarray(arr).save("my_img.bmp")
        print(self.get_img_bitdepth("my_img.bmp"))


if __name__ == "__main__":
    import PIL
    print('Pillow Version:', PIL.__version__)
    bm = BmpMaker()
    base_image = bm.make_image(1000, 8, 5)

    print("bitmap ffp: ", bm.ffp)
    print("bitmap bitdepth : ", bm.bpp)
    bm.conv_to_1bpp()

内容的提问来源于stack exchange，提问作者Windy71