PDF rendering with Ghostscript (via subprocess)

ghostscript_shell.py

# SPDX-FileCopyrightText: 2024 geisserml <[email protected]>
# SPDX-FileCopyrightText: 2024 James R. Barlow <[email protected]>
# SPDX-License-Identifier: MPL-2.0

# Initial code derived from ocrmypdf/_exec/ghostscript.py
# Note that Ghostscript is AGPL-licensed. However, we are calling it via subprocess here, so not sure whether copyleft would actually apply.
# See also https://www.gnu.org/licenses/gpl-faq.en.html#MereAggregation

import io
import os
import re
import sys
import shutil
import logging
import subprocess
import PIL.Image
logger = logging.getLogger(__name__)


def get_ghostscript():
    
    # TODO consider searching the windows registry, as python-ghostscript and ocrmypdf do
    # https://gitlab.com/pdftools/python-ghostscript/-/blob/9f84bf0e02f04eaad4bd998b9c5bef2be55e6389/ghostscript/_gsprint.py#L501
    # https://github.com/jbarlow83/OCRmyPDF/blob/master/src/ocrmypdf/subprocess/_windows.py
    
    if sys.platform.startswith('win32'):
        gs = shutil.which('gswin64c')
        if not gs:
            gs = shutil.which('gswin32c')
    else:
        gs = shutil.which('gs')
    
    return gs


def _gs_error_reported(stream):
    return bool( re.search('error', stream, flags=re.IGNORECASE) )

def _gs_rasterise_pdf(
        input_file,
        *,
        pageno,
        raster_dpi,
        password = None,
        raster_device = 'png16m',
    ):
    """
    Rasterize one page of a PDF at resolution *raster_dpi*.
    *pageno* is the visual (1-based) page number.
    Note that Ghostscript takes /UserUnit into account on its own.
    """
    
    raster_dpi = round(raster_dpi, 6)
    gs = get_ghostscript()
    if not gs:
        raise RuntimeError("Ghostscript could not be found. Make sure it is installed and added to $PATH.")
    
    args_gs = []
    args_gs.extend(
        [
            gs,
            '-dQUIET',
            '-dSAFER',
            '-dBATCH',
            '-dNOPAUSE',
            '-dNOPROMPT',
            f'-sDEVICE={raster_device}',
            f'-dFirstPage={pageno}',
            f'-dLastPage={pageno}',
            f'-r{raster_dpi:f}x{raster_dpi:f}',
            '-dTextAlphaBits=4',
            '-dGraphicsAlphaBits=4',
            '-dInterpolateControl=-1',
        ]
    )
    
    if password is not None:
        args_gs.append(f'-sPDFPassword={password}')
    
    args_gs.extend(
        [
            '-o',
            '-',
            '-sstdout=%stderr',
            '-dAutoRotatePages=/None',
            '-f',
            os.fspath(input_file),
        ]
    )
    
    #logger.debug(args_gs)
    
    try:
        pipe = subprocess.run(
            args_gs,
            stdout = subprocess.PIPE,
            stderr = subprocess.PIPE,
            check = True,
        )
    
    except subprocess.CalledProcessError as error_msg:
        logger.error(error_msg.stderr.decode(errors='replace'))
        raise RuntimeError('Ghostscript rasterizing failed')
    
    else:
        stderr = pipe.stderr.decode(errors='replace')
        if _gs_error_reported(stderr):
            logger.error(stderr)
    
    return PIL.Image.open( io.BytesIO(pipe.stdout) )


def invoke_ghostscript_shell(filepath, index, scale=4, password=None):
    # Note, this does not handle rotation yet -> TODO
    return _gs_rasterise_pdf(
        filepath,
        password   = password,
        pageno     = index + 1,
        raster_dpi = scale * 72,
    )

TODO: see if we can call ghostscript through the ABI-level bindings from https://gitlab.com/pdftools/python-ghostscript

I seem to remember having already tried this at some point, a long time ago, but don't recall what exactly was the problem back then. I think it might have been that I didn't manage to get the in-memory data, and had to use file output anyway.

Yet, even that would already be preferable to avoid subprocess overhead, especially with repeated calls.

Also, multi-page rendering with -sPageList=pageranges would be preferable to avoid launching a new subprocess for each page.

I stared working on this: https://gist.github.com/mara004/428a9aad5d553d4631ab0b5119eb74b2