Python Wand将PDF转换为PNG禁用透明（alpha_channel）

Posted 2021-04-09

我正在尝试将PDF转换为PNG - 这一切都很好，但是，即使我相信我已禁用它，输出图像仍然是透明的：

with Image(filename='sample.pdf', resolution=300) as img:
    img.background_color = Color("white")
    img.alpha_channel = False
    img.save(filename='image.png')

以上产生图像但透明，我也试过以下：

with Image(filename='sample.pdf', resolution=300, background=Color('white')) as img:
    img.alpha_channel = False
    img.save(filename='image.png')

产生此错误：

Traceback (most recent call last):
  File "file_convert.py", line 20, in <module>
    with Image(filename='sample.pdf', resolution=300, background=Color('white')) as img:
  File "/Users/Frank/.virtualenvs/wand/lib/python2.7/site-packages/wand/image.py", line 1943, in __init__
    raise TypeError("blank image parameters can't be used with image "
TypeError: blank image parameters can't be used with image opening parameters

答案

我也有一些PDF转换为PNG。这对我有用，看起来比合成图像更简单，如上图所示：

all_pages = Image(blob=self.pdf)        # PDF will have several pages.
single_image = all_pages.sequence[0]    # Just work on first page
with Image(single_image) as i:
    i.format = 'png'
    i.background_color = Color('white') # Set white background.
    i.alpha_channel = 'remove'          # Remove transparency and replace with bg.

参考：wand.image

另一答案

从previous answer，尝试创建一个背景颜色的空图像，然后合成。

from wand.image import Image
from wand.color import Color

with Image(filename="sample.pdf", resolution=300) as img:
  with Image(width=img.width, height=img.height, background=Color("white")) as bg:
    bg.composite(img,0,0)
    bg.save(filename="image.png")

另一答案

编译其他答案，这是我用来将PDF转换为页面的函数：

import os
from wand.image import Image
from wand.color import Color


def convert_pdf(filename, output_path, resolution=150):
    """ Convert a PDF into images.

        All the pages will give a single png file with format:
        {pdf_filename}-{page_number}.png

        The function removes the alpha channel from the image and
        replace it with a white background.
    """
    all_pages = Image(filename=filename, resolution=resolution)
    for i, page in enumerate(all_pages.sequence):
        with Image(page) as img:
            img.format = 'png'
            img.background_color = Color('white')
            img.alpha_channel = 'remove'

            image_filename = os.path.splitext(os.path.basename(filename))[0]
            image_filename = '{}-{}.png'.format(image_filename, i)
            image_filename = os.path.join(output_path, image_filename)

            img.save(filename=image_filename)

另一答案

另一个答案（与白色图像合成）有效，但仅限于最后一页，直接设置Alpha通道也是如此。以下是关于wand 0.4.2的工作：

im = wand_image(filename='/tmp/foo.pdf', resolution=200)
for i, page in enumerate(im.sequence):
    with wand_image(page) as page_image:
        page_image.alpha_channel = False
        page_image.save(filename='/tmp/foo.pdf.images/page-%s.png' % i)

我想这可能是魔杖中的一个错误。似乎设置PDF的alpha通道应该影响所有页面，但事实并非如此。

另一答案

对于那些仍然有问题的人，我找到了解决方案（它适用于0.4.1及更高版本，我不确定早期版本）。所以你应该使用这样的东西：

with Image(filename='sample.pdf', resolution=300) as img:
img.background_color = Color("white")
img.alpha_channel = 'remove'
img.save(filename='image.png')