# PDF Parsing
Author: Eric Ihli

## Table of Contents

- 1. Preparing our data
  - 1.1. Converting PDFs to images
  - 1.2. Detecting image orientation and applying rotation.
- 2. Detecting tables
- 3. OCR tables
  - - 3.0.1. Blur
    - 3.0.2. Threshold
    - 3.0.3. Finding the vertical and horizontal lines of the table
    - 3.0.4. Finding the contours
    - 3.0.5. Sorting the bounding rectangles
    - 3.0.6. Cropping each cell to the text
    - 3.0.7. OCR each cell
- 4. Files
  - 4.1. setup.py
  - 4.2. table_image_ocr
    - 4.2.1. table_image_ocr/__init__.py
    - 4.2.2. table_image_ocr/util.py
    - 4.2.3. table_image_ocr/prepare_pdfs.py
    - 4.2.4. table_image_ocr/extract_tables.py
    - 4.2.5. table_image_ocr/extract_cells_from_table.py
- 5. Utils

## 1 Preparing our data

### 1.1 Converting PDFs to images

Not all pdfs need to be sent through OCR to extract the text content. If you can click and drag to highlight text in the pdf, then the tools in this library probably aren’t necessary.

This code calls out to pdfimages from Poppler.

    def pdf_to_images(pdf_filepath):
        """
        Turn a pdf into images
        """
        directory, filename = os.path.split(pdf_filepath)
        with working_dir(directory):
            image_filenames = pdfimages(pdf_filepath)

        # Since pdfimages creates a number of files named each for there page number
        # and doesn't return us the list that it created
        return [os.path.join(directory, f) for f in image_filenames]

    def pdfimages(pdf_filepath):
        """
        Uses the `pdfimages` utility from Poppler
        (https://poppler.freedesktop.org/). Creates images out of each page. Images
        are prefixed by their name sans extension and suffixed by their page number.
        """
        directory, filename = os.path.split(pdf_filepath)
        filename_sans_ext = filename.split(".pdf")[0]
        subprocess.run(["pdfimages", "-png", pdf_filepath, filename.split(".pdf")[0]])
        image_filenames = find_matching_files_in_dir(filename_sans_ext, directory)
        logger.debug("Converted {} into files:\n{}".format(pdf_filepath, "\n".join(image_filenames)))
        return image_filenames

    def find_matching_files_in_dir(file_prefix, directory):
        files = [
            filename
            for filename in os.listdir(directory)
            if re.match(r"{}.*\.png".format(re.escape(file_prefix)), filename)
        ]
        return files

### 1.2 Detecting image orientation and applying rotation.

Tesseract can detect orientation and we can then use ImageMagick’s mogrify to rotate the image.

Here’s an example of the output we get from orientation detection with Tesseract.

    ➜  example/ tesseract --psm 0 example-000.png -
    Page number: 0
    Orientation in degrees: 90
    Rotate: 270
    Orientation confidence: 26.86
    Script: Latin
    Script confidence: 2.44

    def preprocess_img(filepath):
        """
        Processing that involves running shell executables,
        like mogrify to rotate.
        """
        rotate = get_rotate(filepath)
        logger.debug("Rotating {} by {}.".format(filepath, rotate))
        mogrify(filepath, rotate)

    def get_rotate(image_filepath):
        output = (
            subprocess.check_output(["tesseract", "--psm", "0", image_filepath, "-"])
            .decode("utf-8")
            .split("\n")
        )
        output = next(l for l in output if "Rotate: " in l)
        output = output.split(": ")[1]
        return output

    def mogrify(image_filepath, rotate):
        subprocess.run(["mogrify", "-rotate", rotate, image_filepath])

## 2 Detecting tables

This answer from opencv.org was heavily referenced while writing the code around table detection: https://answers.opencv.org/question/63847/how-to-extract-tables-from-an-image/.

It’s much easier to OCR a table when the table is the only thing in the image. This code detects tables in an image and returns a list of images of just the tables, no surrounding text or noise.

The blurring, thresholding, and line detection is used here as well as later on for cell extraction. They are good techniques for cleaning an image up in a way that makes things like shape detection more accurate.

    def find_tables(image):
        <<blur>>
        <<threshold>>
        <<lines-of-table>>
        contours, heirarchy = cv2.findContours(
            mask, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE,
        )

        MIN_TABLE_AREA = 1e5
        contours = [c for c in contours if cv2.contourArea(c) > MIN_TABLE_AREA]
        perimeter_lengths = [cv2.arcLength(c, True) for c in contours]
        epsilons = [0.1 * p for p in perimeter_lengths]
        approx_polys = [cv2.approxPolyDP(c, e, True) for c, e in zip(contours, epsilons)]
        bounding_rects = [cv2.boundingRect(a) for a in approx_polys]

        # The link where a lot of this code was borrowed from recommends an
        # additional step to check the number of "joints" inside this bounding rectangle.
        # A table should have a lot of intersections. We might have a rectangular image
        # here though which would only have 4 intersections, 1 at each corner.
        # Leaving that step as a future TODO if it is ever necessary.
        images = [image[y:y+h, x:x+w] for x, y, w, h in bounding_rects]
        return images

    import cv2

    <<detect-table>>

    image_filename = "resources/examples/example-page.png"
    image = cv2.imread(image_filename, cv2.IMREAD_GRAYSCALE)
    image = find_tables(image)[0]
    cv2.imwrite("resources/examples/example-table.png", image)
    "resources/examples/example-table.png"

## 3 OCR tables

Find the bounding box of each cell in the table. Run tesseract on each cell. Print a comma seperated output.

We’ll start with an image shown at the end of the previous section.

#### 3.0.1 Blur

Blurring helps to make noise less noisy so that the overall structure of an image is more detectable.

That gray row at the bottom is kind of noisy. If we don’t somehow clean it up, OpenCV will detect all sorts of odd shapes in there and it will throw off our cell detection.

Cleanup can be accomplished with a blur followed by some thresholding.

    BLUR_KERNEL_SIZE = (17, 17)
    STD_DEV_X_DIRECTION = 0
    STD_DEV_Y_DIRECTION = 0
    blurred = cv2.GaussianBlur(image, BLUR_KERNEL_SIZE, STD_DEV_X_DIRECTION, STD_DEV_Y_DIRECTION)

    image = ~cv2.imread("resources/examples/example-table.png", cv2.IMREAD_GRAYSCALE)
    <<blur>>
    cv2.imwrite("resources/examples/example-table-blurred.png", blurred)
    "resources/examples/example-table-blurred.png"

#### 3.0.2 Threshold

We’ve got a bunch of pixels that are gray. Thresholding will turn them all either black or white. Having all black or white pixels lets us do morphological transformations like erosion and dilation.

    MAX_COLOR_VAL = 255
    BLOCK_SIZE = 15
    SUBTRACT_FROM_MEAN = -2

    img_bin = cv2.adaptiveThreshold(
        ~blurred,
        MAX_COLOR_VAL,
        cv2.ADAPTIVE_THRESH_MEAN_C,
        cv2.THRESH_BINARY,
        BLOCK_SIZE,
        SUBTRACT_FROM_MEAN,
    )

    <<threshold>>
    cv2.imwrite("resources/examples/example-table-thresholded.png", img_bin)
    "resources/examples/example-table-thresholded.png"

#### 3.0.3 Finding the vertical and horizontal lines of the table

Note: There’s a wierd issue with the results of the example below when it’s evaluated as part of an export or a full-buffer evaluation. If you evaluate the example by itself, it looks the way it’s intended. If you evaluate it as part of an entire buffer evaluation, it’s distorted.

    vertical = horizontal = img_bin.copy()
    SCALE = 5
    image_width, image_height = horizontal.shape
    horizontal_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (int(image_width / SCALE), 1))
    horizontally_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, horizontal_kernel)
    vertical_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (1, int(image_height / SCALE)))
    vertically_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, vertical_kernel)

    horizontally_dilated = cv2.dilate(horizontally_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (40, 1)))
    vertically_dilated = cv2.dilate(vertically_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (1, 60)))

    mask = horizontally_dilated + vertically_dilated

    <<lines-of-table>>
    cv2.imwrite("resources/examples/example-table-lines.png", mask)
    "resources/examples/example-table-lines.png"

#### 3.0.4 Finding the contours

Blurring and thresholding allow us to find the lines. Opening the lines allows us to find the contours.

An “Opening” is an erosion followed by a dilation. Great examples and descriptions of each morphological operation can be found at https://docs.opencv.org/trunk/d9/d61/tutorial_py_morphological_ops.html.

> Contours can be explained simply as a curve joining all the continuous points (along the boundary), having same color or intensity. The contours are a useful tool for shape analysis and object detection and recognition.

We can search those contours to find rectangles of certain size.

To do that, we can use OpenCV’s `approxPolyEP` function. It takes as arguments the contour (list of contiguous points), and a number representing how different the polygon perimeter length can be from the true perimeter length of the contour. `0.1` (10%) seems to be a good value. The difference in perimeter length between a 4-sided polygon and a 3-sided polygon is greater than 10% and the difference between a 5+ sided polygon and a 4-sided polygon is less than 10%. So a 4-sided polygon is the polygon with the fewest sides that leaves the difference in perimeter length within our 10% threshold.

Then we just get the bounding rectangle of that polygon and we have our cells!

We might need to do a little more filtering of those rectangles though. We might have accidentally found some noise such as another image on the page or a title header bar or something. If we know our cells are all within a certain size (by area of pixels) then we can filter out the junk cells by removing cells above/below certain sizes.

    contours, heirarchy = cv2.findContours(
        mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE,
    )

    perimeter_lengths = [cv2.arcLength(c, True) for c in contours]
    epsilons = [0.05 * p for p in perimeter_lengths]
    approx_polys = [cv2.approxPolyDP(c, e, True) for c, e in zip(contours, epsilons)]

    # Filter out contours that aren't rectangular. Those that aren't rectangular
    # are probably noise.
    approx_rects = [p for p in approx_polys if len(p) == 4]
    bounding_rects = [cv2.boundingRect(a) for a in approx_polys]

    # Filter out rectangles that are too narrow or too short.
    MIN_RECT_WIDTH = 40
    MIN_RECT_HEIGHT = 10
    bounding_rects = [
        r for r in bounding_rects if MIN_RECT_WIDTH < r[2] and MIN_RECT_HEIGHT < r[3]
    ]

    # The largest bounding rectangle is assumed to be the entire table.
    # Remove it from the list. We don't want to accidentally try to OCR
    # the entire table.
    largest_rect = max(bounding_rects, key=lambda r: r[2] * r[3])
    bounding_rects = [b for b in bounding_rects if b is not largest_rect]

    cells = [c for c in bounding_rects]

#### 3.0.5 Sorting the bounding rectangles

We want to process these from left-to-right, top-to-bottom.

I’ve thought of a straightforward algorithm for it, but it could probably be made more efficient.

We’ll find the most rectangle with the most top-left corner. Then we’ll find all of the rectangles that have a center that is within the top-y and bottom-y values of that top-left rectangle. Then we’ll sort those rectangles by the x value of their center. We’ll remove those rectangles from the list and repeat.

    def cell_in_same_row(c1, c2):
        c1_center = c1[1] + c1[3] - c1[3] / 2
        c2_bottom = c2[1] + c2[3]
        c2_top = c2[1]
        return c2_top < c1_center < c2_bottom

    orig_cells = [c for c in cells]
    rows = []
    while cells:
        first = cells[0]
        rest = cells[1:]
        cells_in_same_row = sorted(
            [
                c for c in rest
                if cell_in_same_row(c, first)
            ],
            key=lambda c: c[0]
        )

        row_cells = sorted([first] + cells_in_same_row, key=lambda c: c[0])
        rows.append(row_cells)
        cells = [
            c for c in rest
            if not cell_in_same_row(c, first)
        ]

    # Sort rows by average height of their center.
    def avg_height_of_center(row):
        centers = [y + h - h / 2 for x, y, w, h in row]
        return sum(centers) / len(centers)

    rows.sort(key=avg_height_of_center)

To test if this code works, let’s try sorting the bounding rectangles and numbering them from right to left, top to bottom.

    import cv2
    image = cv2.imread("resources/examples/example-table.png", cv2.IMREAD_GRAYSCALE)
    <<blur>>
    <<threshold>>
    <<lines-of-table>>
    <<bounding-rects>>
    <<sort-contours>>

    FONT_SCALE = 0.7
    FONT_COLOR = (127, 127, 127)
    for i, row in enumerate(rows):
        for j, cell in enumerate(row):
            x, y, w, h = cell
            cv2.putText(
                image,
                "{},{}".format(i, j),
                (int(x + w - w / 2), int(y + h - h / 2)),
                cv2.FONT_HERSHEY_SIMPLEX,
                FONT_SCALE,
                FONT_COLOR,
                2,
            )
    cv2.imwrite("resources/examples/example-table-cells-numbered.png", image)
    "resources/examples/example-table-cells-numbered.png"

    def extract_cell_images_from_table(image):
        BLUR_KERNEL_SIZE = (17, 17)
        STD_DEV_X_DIRECTION = 0
        STD_DEV_Y_DIRECTION = 0
        blurred = cv2.GaussianBlur(image, BLUR_KERNEL_SIZE, STD_DEV_X_DIRECTION, STD_DEV_Y_DIRECTION)
        MAX_COLOR_VAL = 255
        BLOCK_SIZE = 15
        SUBTRACT_FROM_MEAN = -2

        img_bin = cv2.adaptiveThreshold(
            ~blurred,
            MAX_COLOR_VAL,
            cv2.ADAPTIVE_THRESH_MEAN_C,
            cv2.THRESH_BINARY,
            BLOCK_SIZE,
            SUBTRACT_FROM_MEAN,
        )
        vertical = horizontal = img_bin.copy()
        SCALE = 5
        image_width, image_height = horizontal.shape
        horizontal_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (int(image_width / SCALE), 1))
        horizontally_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, horizontal_kernel)
        vertical_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (1, int(image_height / SCALE)))
        vertically_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, vertical_kernel)

        horizontally_dilated = cv2.dilate(horizontally_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (40, 1)))
        vertically_dilated = cv2.dilate(vertically_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (1, 60)))

        mask = horizontally_dilated + vertically_dilated
        contours, heirarchy = cv2.findContours(
            mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE,
        )

        perimeter_lengths = [cv2.arcLength(c, True) for c in contours]
        epsilons = [0.05 * p for p in perimeter_lengths]
        approx_polys = [cv2.approxPolyDP(c, e, True) for c, e in zip(contours, epsilons)]

        # Filter out contours that aren't rectangular. Those that aren't rectangular
        # are probably noise.
        approx_rects = [p for p in approx_polys if len(p) == 4]
        bounding_rects = [cv2.boundingRect(a) for a in approx_polys]

        # Filter out rectangles that are too narrow or too short.
        MIN_RECT_WIDTH = 40
        MIN_RECT_HEIGHT = 10
        bounding_rects = [
            r for r in bounding_rects if MIN_RECT_WIDTH < r[2] and MIN_RECT_HEIGHT < r[3]
        ]

        # The largest bounding rectangle is assumed to be the entire table.
        # Remove it from the list. We don't want to accidentally try to OCR
        # the entire table.
        largest_rect = max(bounding_rects, key=lambda r: r[2] * r[3])
        bounding_rects = [b for b in bounding_rects if b is not largest_rect]

        cells = [c for c in bounding_rects]
        def cell_in_same_row(c1, c2):
            c1_center = c1[1] + c1[3] - c1[3] / 2
            c2_bottom = c2[1] + c2[3]
            c2_top = c2[1]
            return c2_top < c1_center < c2_bottom

        orig_cells = [c for c in cells]
        rows = []
        while cells:
            first = cells[0]
            rest = cells[1:]
            cells_in_same_row = sorted(
                [
                    c for c in rest
                    if cell_in_same_row(c, first)
                ],
                key=lambda c: c[0]
            )

            row_cells = sorted([first] + cells_in_same_row, key=lambda c: c[0])
            rows.append(row_cells)
            cells = [
                c for c in rest
                if not cell_in_same_row(c, first)
            ]

        # Sort rows by average height of their center.
        def avg_height_of_center(row):
            centers = [y + h - h / 2 for x, y, w, h in row]
            return sum(centers) / len(centers)

        rows.sort(key=avg_height_of_center)
        cell_images_rows = []
        for row in rows:
            cell_images_row = []
            for x, y, w, h in row:
                cell_images_row.append(image[y:y+h, x:x+w])
            cell_images_rows.append(cell_images_row)
        return cell_images_rows

    <<extract-cells-from-table>>
    image = cv2.imread("resources/examples/example-table.png", cv2.IMREAD_GRAYSCALE)
    cell_images_rows = extract_cell_images_from_table(image)
    cv2.imwrite("resources/examples/example-table-cell-1-1.png", cell_images_rows[1][1])
    "resources/examples/example-table-cell-1-1.png"

#### 3.0.6 Cropping each cell to the text

OCR with Tesseract works best when there is about 10 pixels of white border around the text.

Our bounding rectangles may have picked up some stray pixels from the horizontal and vertical lines of the cells in the table. It’s probobly just a few pixels, much fewer than the width of the text. If that’s the case, then we can remove that noise with a simple open morph.

Once the stray border pixels have been removed, we can expand our border using `openMakeBorder`.

    def crop_to_text(image):
        kernel = cv2.getStructuringElement(cv2.MORPH_CROSS, (4, 4))
        opened = cv2.morphologyEx(~image, cv2.MORPH_OPEN, kernel)

        contours, hierarchy = cv2.findContours(opened, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
        bounding_rects = [cv2.boundingRect(c) for c in contours]
        # The largest contour is certainly the text that we're looking for.
        largest_rect = max(bounding_rects, key=lambda r: r[2] * r[3])
        x, y, w, h = largest_rect
        cropped = image[y:y+h, x:x+w]
        bordered = cv2.copyMakeBorder(cropped, 5, 5, 5, 5, cv2.BORDER_CONSTANT, None, 255)
        return bordered

    import cv2
    <<crop-to-text>>
    image = cv2.imread("resources/examples/example-table-cell-1-1.png", cv2.IMREAD_GRAYSCALE)
    image = crop_to_text(image)
    cv2.imwrite("resources/examples/example-table-cell-1-1-cropped.png", image)
    "resources/examples/example-table-cell-1-1-cropped.png"

#### 3.0.7 OCR each cell

If we cleaned up the images well enough, we might get some accurate OCR!

There is plenty that could have gone wrong along the way.

The first step to troubleshooting is to view the intermediate images and see if there’s something about your image that is obviously abnormal, like some really thick noise or a wrongly detected table.

If everything looks reasonable but the OCR is doing something like turning a period into a comma, then you might need to do some custom Tesseract training.

    def crop_to_text(image):
        kernel = cv2.getStructuringElement(cv2.MORPH_CROSS, (4, 4))
        opened = cv2.morphologyEx(~image, cv2.MORPH_OPEN, kernel)

        contours, hierarchy = cv2.findContours(opened, cv2.RETR_LIST, cv2.CHAIN_APPROX_SIMPLE)
        bounding_rects = [cv2.boundingRect(c) for c in contours]
        # The largest contour is certainly the text that we're looking for.
        largest_rect = max(bounding_rects, key=lambda r: r[2] * r[3])
        x, y, w, h = largest_rect
        cropped = image[y:y+h, x:x+w]
        bordered = cv2.copyMakeBorder(cropped, 5, 5, 5, 5, cv2.BORDER_CONSTANT, None, 255)
        return bordered

    def ocr_image(image, config):
        cropped = crop_to_text(image)
        return pytesseract.image_to_string(
            ~cropped,
            config=config
        )

    import pytesseract
    import cv2
    image = cv2.imread("resources/examples/example-table-cell-1-1.png", cv2.IMREAD_GRAYSCALE)
    <<ocr-image>>
    ocr_image(image, "--psm 7")

    9.09

## 4 Files

### 4.1 setup.py

    import setuptools

    with open("README.md", "r") as fh:
        long_description = fh.read()

    setuptools.setup(
        name="example-pkg-YOUR-USERNAME-HERE", # Replace with your own username
        version="0.0.1",
        author="Example Author",
        author_email="author@example.com",
        description="A small example package",
        long_description=long_description,
        long_description_content_type="text/markdown",
        url="https://github.com/pypa/sampleproject",
        packages=setuptools.find_packages(),
        classifiers=[
            "Programming Language :: Python :: 3",
            "License :: OSI Approved :: MIT License",
            "Operating System :: OS Independent",
        ],
        python_requires='>=3.6',
    )

### 4.2 table_image_ocr

#### 4.2.1 table_image_ocr/__init__.py

#### 4.2.2 table_image_ocr/util.py

    from contextlib import contextmanager
    import functools
    import logging
    import os
    import tempfile

    from bs4 import BeautifulSoup as bs
    import requests

    logger = get_logger()

    @contextmanager
    def working_dir(directory):
        original_working_dir = os.getcwd()
        try:
            os.chdir(directory)
            yield directory
        finally:
            os.chdir(original_working_dir)

    def download(url, filepath):
        response = request_get(url)
        data = response.content
        with open(filepath, "wb") as f:
            f.write(data)

    def make_tempdir(identifier):
        return tempfile.mkdtemp(prefix="{}_".format(identifier))

#### 4.2.3 table_image_ocr/prepare_pdfs.py

Takes a variable number of pdf files and creates images out of each page of the file using `pdfimages` from Poppler. Images are created in the same directory that contains the pdf.

Prints each pdf followed by the images extracted from that pdf followed by a blank line.

    python -m pdf.prepare_pdfs /tmp/file1/file1.pdf /tmp/file2/file2.pdf ...

    import argparse
    import logging
    import os
    import re
    import subprocess
    import sys

    from pdf.util import request_get, working_dir, download, make_tempdir

    logger = get_logger()

    parser = argparse.ArgumentParser()
    parser.add_argument("files", nargs="+")

    def main(files):
        pdf_images = []
        for f in files:
            pdf_images.append((f, pdf_to_images(f)))

        for pdf, images in pdf_images:
            for image in images:
                preprocess_img(image)

        for pdf, images in pdf_images:
            print("{}\n{}\n".format(pdf, "\n".join(images)))

    def pdf_to_images(pdf_filepath):
        """
        Turn a pdf into images
        """
        directory, filename = os.path.split(pdf_filepath)
        with working_dir(directory):
            image_filenames = pdfimages(pdf_filepath)

        # Since pdfimages creates a number of files named each for there page number
        # and doesn't return us the list that it created
        return [os.path.join(directory, f) for f in image_filenames]

    def pdfimages(pdf_filepath):
        """
        Uses the `pdfimages` utility from Poppler
        (https://poppler.freedesktop.org/). Creates images out of each page. Images
        are prefixed by their name sans extension and suffixed by their page number.
        """
        directory, filename = os.path.split(pdf_filepath)
        filename_sans_ext = filename.split(".pdf")[0]
        subprocess.run(["pdfimages", "-png", pdf_filepath, filename.split(".pdf")[0]])
        image_filenames = find_matching_files_in_dir(filename_sans_ext, directory)
        logger.debug("Converted {} into files:\n{}".format(pdf_filepath, "\n".join(image_filenames)))
        return image_filenames

    def find_matching_files_in_dir(file_prefix, directory):
        files = [
            filename
            for filename in os.listdir(directory)
            if re.match(r"{}.*\.png".format(re.escape(file_prefix)), filename)
        ]
        return files
    def preprocess_img(filepath):
        """
        Processing that involves running shell executables,
        like mogrify to rotate.
        """
        rotate = get_rotate(filepath)
        logger.debug("Rotating {} by {}.".format(filepath, rotate))
        mogrify(filepath, rotate)

    def get_rotate(image_filepath):
        output = (
            subprocess.check_output(["tesseract", "--psm", "0", image_filepath, "-"])
            .decode("utf-8")
            .split("\n")
        )
        output = next(l for l in output if "Rotate: " in l)
        output = output.split(": ")[1]
        return output

    def mogrify(image_filepath, rotate):
        subprocess.run(["mogrify", "-rotate", rotate, image_filepath])

    if __name__ == "__main__":
        args = parser.parse_args()
        main(args.files)

#### 4.2.4 table_image_ocr/extract_tables.py

    . ~/.virtualenvs/lotto_odds/bin/activate
    python -m pdf.extract_tables "resources/examples/example-page.png"

    import argparse
    import os

    import cv2

    parser = argparse.ArgumentParser()
    parser.add_argument("files", nargs="+")

    def main(files):
        results = []
        for f in files:
            directory, filename = os.path.split(f)
            image = cv2.imread(f, cv2.IMREAD_GRAYSCALE)
            tables = find_tables(image)
            files = []
            for i, table in enumerate(tables):
                filename_sans_extension = os.path.splitext(filename)[0]
                table_filename = "{}-table-{:03d}.png".format(filename_sans_extension, i)
                table_filepath = os.path.join(directory, table_filename)
                files.append(table_filepath)
                cv2.imwrite(table_filepath, table)
            results.append((f, files))

        for image_filename, table_filenames in results:
            print("{}\n{}\n".format(image_filename, "\n".join(table_filenames)))

    def find_tables(image):
        BLUR_KERNEL_SIZE = (17, 17)
        STD_DEV_X_DIRECTION = 0
        STD_DEV_Y_DIRECTION = 0
        blurred = cv2.GaussianBlur(image, BLUR_KERNEL_SIZE, STD_DEV_X_DIRECTION, STD_DEV_Y_DIRECTION)
        MAX_COLOR_VAL = 255
        BLOCK_SIZE = 15
        SUBTRACT_FROM_MEAN = -2

        img_bin = cv2.adaptiveThreshold(
            ~blurred,
            MAX_COLOR_VAL,
            cv2.ADAPTIVE_THRESH_MEAN_C,
            cv2.THRESH_BINARY,
            BLOCK_SIZE,
            SUBTRACT_FROM_MEAN,
        )
        vertical = horizontal = img_bin.copy()
        SCALE = 5
        image_width, image_height = horizontal.shape
        horizontal_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (int(image_width / SCALE), 1))
        horizontally_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, horizontal_kernel)
        vertical_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (1, int(image_height / SCALE)))
        vertically_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, vertical_kernel)

        horizontally_dilated = cv2.dilate(horizontally_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (40, 1)))
        vertically_dilated = cv2.dilate(vertically_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (1, 60)))

        mask = horizontally_dilated + vertically_dilated
        contours, heirarchy = cv2.findContours(
            mask, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE,
        )

        MIN_TABLE_AREA = 1e5
        contours = [c for c in contours if cv2.contourArea(c) > MIN_TABLE_AREA]
        perimeter_lengths = [cv2.arcLength(c, True) for c in contours]
        epsilons = [0.1 * p for p in perimeter_lengths]
        approx_polys = [cv2.approxPolyDP(c, e, True) for c, e in zip(contours, epsilons)]
        bounding_rects = [cv2.boundingRect(a) for a in approx_polys]

        # The link where a lot of this code was borrowed from recommends an
        # additional step to check the number of "joints" inside this bounding rectangle.
        # A table should have a lot of intersections. We might have a rectangular image
        # here though which would only have 4 intersections, 1 at each corner.
        # Leaving that step as a future TODO if it is ever necessary.
        images = [image[y:y+h, x:x+w] for x, y, w, h in bounding_rects]
        return images

    if __name__ == "__main__":
        args = parser.parse_args()
        files = args.files
        main(files)

#### 4.2.5 table_image_ocr/extract_cells_from_table.py

    . ~/.virtualenvs/lotto_odds/bin/activate
    python -m pdf.extract_cells_from_table "resources/examples/example-table.png"

    import os
    import sys

    import cv2
    import pytesseract

    def main(f):
        results = []
        directory, filename = os.path.split(f)
        table = cv2.imread(f, cv2.IMREAD_GRAYSCALE)
        rows = extract_cell_images_from_table(table)
        cell_img_dir = os.path.join(directory, "cells")
        os.makedirs(cell_img_dir, exist_ok=True)
        for i, row in enumerate(rows):
            for j, cell in enumerate(row):
                cell_filename = "{:03d}-{:03d}.png".format(i, j)
                path = os.path.join(cell_img_dir, cell_filename)
                cv2.imwrite(path, cell)
                print(cell_filename)

    def extract_cell_images_from_table(image):
        BLUR_KERNEL_SIZE = (17, 17)
        STD_DEV_X_DIRECTION = 0
        STD_DEV_Y_DIRECTION = 0
        blurred = cv2.GaussianBlur(image, BLUR_KERNEL_SIZE, STD_DEV_X_DIRECTION, STD_DEV_Y_DIRECTION)
        MAX_COLOR_VAL = 255
        BLOCK_SIZE = 15
        SUBTRACT_FROM_MEAN = -2

        img_bin = cv2.adaptiveThreshold(
            ~blurred,
            MAX_COLOR_VAL,
            cv2.ADAPTIVE_THRESH_MEAN_C,
            cv2.THRESH_BINARY,
            BLOCK_SIZE,
            SUBTRACT_FROM_MEAN,
        )
        vertical = horizontal = img_bin.copy()
        SCALE = 5
        image_width, image_height = horizontal.shape
        horizontal_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (int(image_width / SCALE), 1))
        horizontally_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, horizontal_kernel)
        vertical_kernel = cv2.getStructuringElement(cv2.MORPH_RECT, (1, int(image_height / SCALE)))
        vertically_opened = cv2.morphologyEx(img_bin, cv2.MORPH_OPEN, vertical_kernel)

        horizontally_dilated = cv2.dilate(horizontally_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (40, 1)))
        vertically_dilated = cv2.dilate(vertically_opened, cv2.getStructuringElement(cv2.MORPH_RECT, (1, 60)))

        mask = horizontally_dilated + vertically_dilated
        contours, heirarchy = cv2.findContours(
            mask, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE,
        )

        perimeter_lengths = [cv2.arcLength(c, True) for c in contours]
        epsilons = [0.05 * p for p in perimeter_lengths]
        approx_polys = [cv2.approxPolyDP(c, e, True) for c, e in zip(contours, epsilons)]

        # Filter out contours that aren't rectangular. Those that aren't rectangular
        # are probably noise.
        approx_rects = [p for p in approx_polys if len(p) == 4]
        bounding_rects = [cv2.boundingRect(a) for a in approx_polys]

        # Filter out rectangles that are too narrow or too short.
        MIN_RECT_WIDTH = 40
        MIN_RECT_HEIGHT = 10
        bounding_rects = [
            r for r in bounding_rects if MIN_RECT_WIDTH < r[2] and MIN_RECT_HEIGHT < r[3]
        ]

        # The largest bounding rectangle is assumed to be the entire table.
        # Remove it from the list. We don't want to accidentally try to OCR
        # the entire table.
        largest_rect = max(bounding_rects, key=lambda r: r[2] * r[3])
        bounding_rects = [b for b in bounding_rects if b is not largest_rect]

        cells = [c for c in bounding_rects]
        def cell_in_same_row(c1, c2):
            c1_center = c1[1] + c1[3] - c1[3] / 2
            c2_bottom = c2[1] + c2[3]
            c2_top = c2[1]
            return c2_top < c1_center < c2_bottom

        orig_cells = [c for c in cells]
        rows = []
        while cells:
            first = cells[0]
            rest = cells[1:]
            cells_in_same_row = sorted(
                [
                    c for c in rest
                    if cell_in_same_row(c, first)
                ],
                key=lambda c: c[0]
            )

            row_cells = sorted([first] + cells_in_same_row, key=lambda c: c[0])
            rows.append(row_cells)
            cells = [
                c for c in rest
                if not cell_in_same_row(c, first)
            ]

        # Sort rows by average height of their center.
        def avg_height_of_center(row):
            centers = [y + h - h / 2 for x, y, w, h in row]
            return sum(centers) / len(centers)

        rows.sort(key=avg_height_of_center)
        cell_images_rows = []
        for row in rows:
            cell_images_row = []
            for x, y, w, h in row:
                cell_images_row.append(image[y:y+h, x:x+w])
            cell_images_rows.append(cell_images_row)
        return cell_images_rows

    if __name__ == "__main__":
        main(sys.argv[1])

## 5 Utils

The following code lets us specify a size for images when they are exported to html.

Org supports specifying an export size for an image by putting the `#+ATTR_HTML: :width 100px` before the image. But since our images are in a results drawer, we need a way for our results drawer to do that for us automatically.

Adding `#+ATTR_HTML` after the beginning of the result block introduces a new problem. Org-babel no longer recognizes the result as a result block and doesn’t remove it when a src block is re-evaluated, so we end up just appending new results on each evaluation.

There is nothing configurable that will tell org-babel to remove our line. But we can define a function to do some cleanup and then add it as a before hook with `advice-add`.

    (concat "#+ATTR_HTML: :width " width " :height " height "\n[[file:" text "]]")

    (defun remove-attributes-from-src-block-result (&rest args)
      (let ((location (org-babel-where-is-src-block-result))
            (attr-regexp "[   ]*#\\+ATTR.*$"))
        (when location
          (save-excursion
            (goto-char location)
            (when (looking-at (concat org-babel-result-regexp ".*$"))
              (next-line)
              (while (looking-at attr-regexp)
                (kill-whole-line)))))))

    (advice-add 'org-babel-remove-result :before #'remove-attributes-from-src-block-result)
    (advice-add 'org-babel-execute-src-block :before #'remove-attributes-from-src-block-result)

Author: Eric Ihli

Created: 2020-04-10 Fri 14:10