hocrtransform.py

#!/usr/bin/env python3
# SPDX-FileCopyrightText: 2010 Jonathan Brinley
# SPDX-FileCopyrightText: 2013-2014 Julien Pfefferkorn
# SPDX-FileCopyrightText: 2022 James R. Barlow
# SPDX-License-Identifier: MIT

"""Transform .hocr and page image to text PDF."""

from __future__ import annotations

import argparse
import os
import re
import warnings
from math import atan, cos, sin
from pathlib import Path
from typing import Any, NamedTuple
from xml.etree import ElementTree

with warnings.catch_warnings():
    # reportlab uses deprecated load_module
    warnings.filterwarnings(
        'ignore', category=DeprecationWarning, message=r".*load_module.*"
    )
    from reportlab.lib.colors import black, cyan, magenta, red
    from reportlab.lib.units import inch
    from reportlab.pdfgen.canvas import Canvas

# According to Wikipedia these languages are supported in the ISO-8859-1 character
# set, meaning reportlab can generate them and they are compatible with hocr,
# assuming Tesseract has the necessary languages installed. Note that there may
# not be language packs for them.
HOCR_OK_LANGS = frozenset(
    [
        # Languages fully covered by Latin-1:
        'afr',  # Afrikaans
        'alb',  # Albanian
        'ast',  # Leonese
        'baq',  # Basque
        'bre',  # Breton
        'cos',  # Corsican
        'eng',  # English
        'eus',  # Basque
        'fao',  # Faoese
        'gla',  # Scottish Gaelic
        'glg',  # Galician
        'glv',  # Manx
        'ice',  # Icelandic
        'ind',  # Indonesian
        'isl',  # Icelandic
        'ita',  # Italian
        'ltz',  # Luxembourgish
        'mal',  # Malay Rumi
        'mga',  # Irish
        'nor',  # Norwegian
        'oci',  # Occitan
        'por',  # Portugeuse
        'roh',  # Romansh
        'sco',  # Scots
        'sma',  # Sami
        'spa',  # Spanish
        'sqi',  # Albanian
        'swa',  # Swahili
        'swe',  # Swedish
        'tgl',  # Tagalog
        'wln',  # Walloon
        # Languages supported by Latin-1 except for a few rare characters that OCR
        # is probably not trained to recognize anyway:
        'cat',  # Catalan
        'cym',  # Welsh
        'dan',  # Danish
        'deu',  # German
        'dut',  # Dutch
        'est',  # Estonian
        'fin',  # Finnish
        'fra',  # French
        'hun',  # Hungarian
        'kur',  # Kurdish
        'nld',  # Dutch
        'wel',  # Welsh
    ]
)


Element = ElementTree.Element


class Rect(NamedTuple):
    """A rectangle for managing PDF coordinates."""

    x1: Any
    y1: Any
    x2: Any
    y2: Any


class HocrTransformError(Exception):
    """Error while applying hOCR transform."""


class HocrTransform:

    """
    A class for converting documents from the hOCR format.
    For details of the hOCR format, see:
    http://kba.cloud/hocr-spec/
    """

    box_pattern = re.compile(r'bbox((\s+\d+){4})')
    baseline_pattern = re.compile(
        r'''
        baseline \s+
        ([\-\+]?\d*\.?\d*) \s+  # +/- decimal float
        ([\-\+]?\d+)            # +/- int''',
        re.VERBOSE,
    )
    ligatures = str.maketrans(
        {'ff': 'ff', 'ffi': 'f‌f‌i', 'ffl': 'f‌f‌l', 'fi': 'fi', 'fl': 'fl'}
    )

    def __init__(self, *, hocr_filename: str | Path, dpi: float):
        self.dpi = dpi
        self.hocr = ElementTree.parse(os.fspath(hocr_filename))

        # if the hOCR file has a namespace, ElementTree requires its use to
        # find elements
        matches = re.match(r'({.*})html', self.hocr.getroot().tag)
        self.xmlns = ''
        if matches:
            self.xmlns = matches.group(1)

        # get dimension in pt (not pixel!!!!) of the OCRed image
        self.width, self.height = None, None
        for div in self.hocr.findall(self._child_xpath('div', 'ocr_page')):
            coords = self.element_coordinates(div)
            pt_coords = self.pt_from_pixel(coords)
            self.width = pt_coords.x2 - pt_coords.x1
            self.height = pt_coords.y2 - pt_coords.y1
            # there shouldn't be more than one, and if there is, we don't want
            # it
            break
        if self.width is None or self.height is None:
            raise HocrTransformError("hocr file is missing page dimensions")

    def __str__(self):  # pragma: no cover
        """
        Return the textual content of the HTML body
        """
        if self.hocr is None:
            return ''
        body = self.hocr.find(self._child_xpath('body'))
        if body:
            return self._get_element_text(body)
        else:
            return ''

    def _get_element_text(self, element: Element):
        """
        Return the textual content of the element and its children
        """
        text = ''
        if element.text is not None:
            text += element.text
        for child in element:
            text += self._get_element_text(child)
        if element.tail is not None:
            text += element.tail
        return text

    @classmethod
    def element_coordinates(cls, element: Element) -> Rect:
        """
        Returns a tuple containing the coordinates of the bounding box around
        an element
        """
        out = Rect._make(0 for _ in range(4))
        if 'title' in element.attrib:
            matches = cls.box_pattern.search(element.attrib['title'])
            if matches:
                coords = matches.group(1).split()
                out = Rect._make(int(coords[n]) for n in range(4))
        return out

    @classmethod
    def baseline(cls, element: Element) -> tuple[float, float]:
        """
        Returns a tuple containing the baseline slope and intercept.
        """
        if 'title' in element.attrib:
            matches = cls.baseline_pattern.search(element.attrib['title'])
            if matches:
                return float(matches.group(1)), int(matches.group(2))
        return (0.0, 0.0)

    def pt_from_pixel(self, pxl) -> Rect:
        """
        Returns the quantity in PDF units (pt) given quantity in pixels
        """
        return Rect._make((c / self.dpi * inch) for c in pxl)

    def _child_xpath(self, html_tag: str, html_class: str | None = None) -> str:
        xpath = f".//{self.xmlns}{html_tag}"
        if html_class:
            xpath += f"[@class='{html_class}']"
        return xpath

    @classmethod
    def replace_unsupported_chars(cls, s: str) -> str:
        """
        Given an input string, returns the corresponding string that:
        * is available in the Helvetica facetype
        * does not contain any ligature (to allow easy search in the PDF file)
        """
        return s.translate(cls.ligatures)

    def topdown_position(self, element):
        pxl_line_coords = self.element_coordinates(element)
        line_box = self.pt_from_pixel(pxl_line_coords)
        # Coordinates here are still in the hocr coordinate system, so 0 on the y axis
        # is the top of the page and increasing values of y will move towards the
        # bottom of the page.
        return line_box.y2

    def to_pdf(
        self,
        *,
        out_filename: Path,
        image_filename: Path | None = None,
        show_bounding_boxes: bool = False,
        fontname: str = "Helvetica",
        invisible_text: bool = False,
        interword_spaces: bool = False,
    ) -> None:
        """
        Creates a PDF file with an image superimposed on top of the text.
        Text is positioned according to the bounding box of the lines in
        the hOCR file.
        The image need not be identical to the image used to create the hOCR
        file.
        It can have a lower resolution, different color mode, etc.

        Arguments:
            out_filename: Path of PDF to write.
            image_filename: Image to use for this file. If omitted, the OCR text
                is shown.
            show_bounding_boxes: Show bounding boxes around various text regions,
                for debugging.
            fontname: Name of font to use.
            invisible_text: If True, text is rendered invisible so that is
                selectable but never drawn. If False, text is visible and may
                be seen if the image is skipped or deleted in Acrobat.
            interword_spaces: If True, insert spaces between words rather than
                drawing each word without spaces. Generally this improves text
                extraction.
        """
        # create the PDF file
        # page size in points (1/72 in.)
        pdf = Canvas(
            os.fspath(out_filename),
            pagesize=(self.width, self.height),
            pageCompression=1,
        )

        # draw bounding box for each paragraph
        # light blue for bounding box of paragraph
        pdf.setStrokeColor(cyan)
        # light blue for bounding box of paragraph
        pdf.setFillColor(cyan)
        pdf.setLineWidth(0)  # no line for bounding box
        for elem in self.hocr.iterfind(self._child_xpath('p', 'ocr_par')):
            elemtxt = self._get_element_text(elem).rstrip()
            if len(elemtxt) == 0:
                continue

            pxl_coords = self.element_coordinates(elem)
            pt = self.pt_from_pixel(pxl_coords)  # pylint: disable=invalid-name

            # draw the bbox border
            if show_bounding_boxes:  # pragma: no cover
                pdf.rect(
                    pt.x1, self.height - pt.y2, pt.x2 - pt.x1, pt.y2 - pt.y1, fill=1
                )

        found_lines = False
        for line in (
            element
            for element in self.hocr.iterfind(self._child_xpath('span'))
            if 'class' in element.attrib
            and element.attrib['class'] in {'ocr_header', 'ocr_line', 'ocr_textfloat'}
        ):
            found_lines = True
            self._do_line(
                pdf,
                line,
                "ocrx_word",
                fontname,
                invisible_text,
                interword_spaces,
                show_bounding_boxes,
            )

        if not found_lines:
            # Tesseract did not report any lines (just words)
            root = self.hocr.find(self._child_xpath('div', 'ocr_page'))
            self._do_line(
                pdf,
                root,
                "ocrx_word",
                fontname,
                invisible_text,
                interword_spaces,
                show_bounding_boxes,
            )
        # put the image on the page, scaled to fill the page
        if image_filename is not None:
            pdf.drawImage(
                os.fspath(image_filename), 0, 0, width=self.width, height=self.height
            )

        # finish up the page and save it
        pdf.showPage()
        pdf.save()

    @classmethod
    def polyval(cls, poly, x):  # pragma: no cover
        return x * poly[0] + poly[1]

    def _do_line(
        self,
        pdf: Canvas,
        line: Element | None,
        elemclass: str,
        fontname: str,
        invisible_text: bool,
        interword_spaces: bool,
        show_bounding_boxes: bool,
    ):
        if line is None:
            return
        pxl_line_coords = self.element_coordinates(line)
        line_box = self.pt_from_pixel(pxl_line_coords)
        line_height = line_box.y2 - line_box.y1

        slope, pxl_intercept = self.baseline(line)
        if abs(slope) < 0.005:
            slope = 0.0
        angle = atan(slope)
        cos_a, sin_a = cos(angle), sin(angle)

        text = pdf.beginText()
        intercept = pxl_intercept / self.dpi * inch

        # Don't allow the font to break out of the bounding box. Division by
        # cos_a accounts for extra clearance between the glyph's vertical axis
        # on a sloped baseline and the edge of the bounding box.
        fontsize = (line_height - abs(intercept)) / cos_a
        text.setFont(fontname, fontsize)
        if invisible_text:
            text.setTextRenderMode(3)  # Invisible (indicates OCR text)

        # Intercept is normally negative, so this places it above the bottom
        # of the line box
        baseline_y2 = self.height - (line_box.y2 + intercept)

        if show_bounding_boxes:  # pragma: no cover
            # draw the baseline in magenta, dashed
            pdf.setDash()
            pdf.setStrokeColor(magenta)
            pdf.setLineWidth(0.5)
            # negate slope because it is defined as a rise/run in pixel
            # coordinates and page coordinates have the y axis flipped
            pdf.line(
                line_box.x1,
                baseline_y2,
                line_box.x2,
                self.polyval((-slope, baseline_y2), line_box.x2 - line_box.x1),
            )
            # light green for bounding box of word/line
            pdf.setDash(6, 3)
            pdf.setStrokeColor(red)

        text.setTextTransform(cos_a, -sin_a, sin_a, cos_a, line_box.x1, baseline_y2)
        pdf.setFillColor(black)  # text in black

        elements = line.findall(self._child_xpath('span', elemclass))
        for elem in elements:
            elemtxt = self._get_element_text(elem).strip()
            elemtxt = self.replace_unsupported_chars(elemtxt)
            if elemtxt == '':
                continue

            pxl_coords = self.element_coordinates(elem)
            box = self.pt_from_pixel(pxl_coords)
            if interword_spaces:
                # if  `--interword-spaces` is true, append a space
                # to the end of each text element to allow simpler PDF viewers
                # such as PDF.js to better recognize words in search and copy
                # and paste. Do not remove space from last word in line, even
                # though it would look better, because it will interfere with
                # naive text extraction. \n does not work either.
                elemtxt += ' '
                box = Rect._make(
                    (
                        box.x1,
                        line_box.y1,
                        box.x2 + pdf.stringWidth(' ', fontname, line_height),
                        line_box.y2,
                    )
                )
            box_width = box.x2 - box.x1
            font_width = pdf.stringWidth(elemtxt, fontname, fontsize)

            # draw the bbox border
            if show_bounding_boxes:  # pragma: no cover
                pdf.rect(
                    box.x1, self.height - line_box.y2, box_width, line_height, fill=0
                )

            # Adjust relative position of cursor
            # This is equivalent to:
            #   text.setTextOrigin(pt.x1, self.height - line_box.y2)
            # but the former generates a full text reposition matrix (Tm) in the
            # content stream while this issues a "offset" (Td) command.
            # .moveCursor() is relative to start of the text line, where the
            # "text line" means whatever reportlab defines it as. Do not use
            # use .getCursor(), since moveCursor() rather unintuitively plans
            # its moves relative to .getStartOfLine().
            # For skewed lines, in the text transform we set up a rotated
            # coordinate system, so we don't have to account for the
            # incremental offset. Surprisingly most PDF viewers can handle this.
            cursor = text.getStartOfLine()
            dx = box.x1 - cursor[0]
            dy = baseline_y2 - cursor[1]
            text.moveCursor(dx, dy)

            # If reportlab tells us this word is 0 units wide, our best seems
            # to be to suppress this text
            if font_width > 0:
                text.setHorizScale(100 * box_width / font_width)
                text.textOut(elemtxt)
        pdf.drawText(text)


if __name__ == "__main__":
    parser = argparse.ArgumentParser(description='Convert hocr file to PDF')
    parser.add_argument(
        '-b',
        '--boundingboxes',
        action="store_true",
        default=False,
        help='Show bounding boxes borders',
    )
    parser.add_argument(
        '-r',
        '--resolution',
        type=int,
        default=300,
        help='Resolution of the image that was OCRed',
    )
    parser.add_argument(
        '-i',
        '--image',
        default=None,
        help='Path to the image to be placed above the text',
    )
    parser.add_argument(
        '--interword-spaces',
        action='store_true',
        default=False,
        help='Add spaces between words',
    )
    parser.add_argument('hocrfile', help='Path to the hocr file to be parsed')
    parser.add_argument('outputfile', help='Path to the PDF file to be generated')
    args = parser.parse_args()

    hocr = HocrTransform(hocr_filename=args.hocrfile, dpi=args.resolution)
    hocr.to_pdf(
        out_filename=args.outputfile,
        image_filename=args.image,
        show_bounding_boxes=args.boundingboxes,
        interword_spaces=args.interword_spaces,
    )