image-table-ocr/pdf_table_extraction_and_ocr.org

# -*- org-image-actual-width: 500; -*-

#+TITLE: PDF Parsing
#+PROPERTY: header-args :session *Python*
#+STARTUP: inlineimages
#+OPTIONS: ^:nil

#+BEGIN_COMMENT
Some notes about the header for those not familiar with Org Mode:

The property `header-args` with ~:session \*Python\*~ will cause all evaluated
source code blocks to be evaluated in the buffer named "\*Python\*", which is the
default buffer name for the buffer connected to an inferior python process. This
is useful for interactive development. It gives you a REPL to work with rather
than having to constantly evaluate source code blocks and view the results
output to try any change.

Another note along those lines is that when source code blocks are evaluated,
some unnecessary output is printed in the ~*Python*~ buffer. Adding ~:results
output~ to a code block will minimize that noise.
#+END_COMMENT

* Preparing our data
** 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 [[https://manpages.debian.org/testing/poppler-utils/pdfimages.1.en.html][pdfimages]] from [[https://poppler.freedesktop.org/][Poppler]].

#+NAME: pdf-to-images
#+BEGIN_SRC python :results none
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
#+END_SRC

** Detecting image orientation and applying rotation.

Tesseract can detect orientation and we can then use [[https://www.imagemagick.org/script/mogrify.php][ImageMagick's mogrify]] to
rotate the image.

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

#+BEGIN_EXAMPLE
➜  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
#+END_EXAMPLE

#+NAME: fix-orientation
#+BEGIN_SRC python :results none
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])
#+END_SRC

* 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.

#+BEGIN_SRC python :noweb-ref detect-table :results none :noweb no-export
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
#+END_SRC

#+HEADER: :post html-image-size(text=*this*, width="500px")
#+BEGIN_SRC python :noweb-ref test-detect-table :noweb no-export :results raw
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"
#+END_SRC

* 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.

*** 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.

#+BEGIN_SRC python :noweb-ref blur :results none
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)
#+END_SRC

#+HEADER: :post html-image-size(text=*this*, width="500px")
#+BEGIN_SRC python :noweb no-export :results raw :exports both
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"
#+END_SRC

#+RESULTS:
#+ATTR_HTML: :width 500px :height 100%
[[file:resources/examples/example-table-blurred.png]]

*** 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.

#+BEGIN_SRC python :noweb-ref threshold :results none
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,
)
#+END_SRC

#+HEADER: :post html-image-size(text=*this*, width="500px")
#+BEGIN_SRC python :noweb no-export :results raw :exports both
<<threshold>>
cv2.imwrite("resources/examples/example-table-thresholded.png", img_bin)
"resources/examples/example-table-thresholded.png"
#+END_SRC

#+RESULTS:
#+ATTR_HTML: :width 500px :height 100%
[[file:resources/examples/example-table-thresholded.png]]

*** 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.

#+BEGIN_SRC python :noweb-ref lines-of-table :results none
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
#+END_SRC

#+HEADER: :post html-image-size(text=*this*, width="500px")
#+BEGIN_SRC python :noweb no-export :results raw :exports both
<<lines-of-table>>
cv2.imwrite("resources/examples/example-table-lines.png", mask)
"resources/examples/example-table-lines.png"
#+END_SRC

#+RESULTS:
#+ATTR_HTML: :width 500px :height 100%
[[file:resources/examples/example-table-lines.png]]

*** 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][https://docs.opencv.org/trunk/d9/d61/tutorial_py_morphological_ops.html]].

#+BEGIN_QUOTE
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.
#+END_QUOTE

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.

#+BEGIN_SRC python :noweb-ref bounding-rects :results none
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]
#+END_SRC

*** 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.

#+BEGIN_SRC python :noweb-ref sort-contours :results none
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)
#+END_SRC

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

#+HEADER: :post html-image-size(text=*this*, width="500px")
#+BEGIN_SRC python :noweb no-export :results raw :exports both
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"
#+END_SRC

#+RESULTS:
#+ATTR_HTML: :width 500px :height 100%
[[file:resources/examples/example-table-cells-numbered.png]]

#+BEGIN_SRC python :noweb-ref extract-cells-from-table :noweb yes :eval no
def extract_cell_images_from_table(image):
    <<blur>>
    <<threshold>>
    <<lines-of-table>>
    <<bounding-rects>>
    <<sort-contours>>
    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
#+END_SRC

#+HEADER: :post html-image-size(text=*this*, width="200px")
#+BEGIN_SRC python :noweb no-export :results raw :exports both
<<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"
#+END_SRC

*** 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~.

#+BEGIN_SRC python :eval no :noweb-ref crop-to-text
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
#+END_SRC

#+HEADER: :post html-image-size(text=*this*, width="200px")
#+BEGIN_SRC python :noweb no-export :results raw :exports both
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"
#+END_SRC

#+RESULTS:
#+ATTR_HTML: :width 200px :height 100%
[[file:resources/examples/example-table-cell-1-1-cropped.png]]

*** 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.

#+BEGIN_SRC python :noweb-ref ocr-image :eval no :noweb yes
<<crop-to-text>>

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

#+BEGIN_SRC python :noweb no-export :exports both
import pytesseract
image = cv2.imread("resources/examples/example-table-cell-1-1.png", cv2.IMREAD_GRAYSCALE)
<<ocr-image>>
ocr_image(image, "--psm 7")
#+END_SRC

#+RESULTS:
: 9.09

* Files
:PROPERTIES:
:header-args: :mkdirp yes :noweb yes
:END:

#+BEGIN_SRC python :tangle pdf/__init__.py :mkdirp yes :results none

#+END_SRC

#+RESULTS:

** setup.py
#+BEGIN_SRC python :tangle setup.py :results none
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',
)
#+END_SRC

** table_image_ocr
*** table_image_ocr/__init__.py
#+BEGIN_SRC python :tangle table_image_ocr/__init__.py :mkdirp yes :results none

#+END_SRC

*** table_image_ocr/util.py

#+BEGIN_SRC python :tangle table_image_ocr/util.py :mkdirp yes :results none
from contextlib import contextmanager
import functools
import logging
import os
import tempfile

from bs4 import BeautifulSoup as bs
import requests


<<get_logger>>

logger = get_logger()


<<request_cacheing>>

@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))
#+END_SRC

*** 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.

#+BEGIN_SRC shell :eval no :exports code
python -m pdf.prepare_pdfs /tmp/file1/file1.pdf /tmp/file2/file2.pdf ...
#+END_SRC


#+BEGIN_SRC python :tangle pdf/prepare_pdfs.py :noweb yes
import argparse
import logging
import os
import re
import subprocess
import sys

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


<<get-logger>>

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)))


<<pdf-to-images>>
<<fix-orientation>>

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

#+RESULTS:

*** table_image_ocr/extract_tables.py

#+BEGIN_SRC shell
. ~/.virtualenvs/lotto_odds/bin/activate
python -m pdf.extract_tables "resources/examples/example-page.png"
#+END_SRC

#+RESULTS:
| resources/examples/example-page.png           |
| resources/examples/example-page-table-000.png |

#+BEGIN_SRC python :noweb yes :tangle pdf/extract_tables.py :results none
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)))

<<detect-table>>

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

*** table_image_ocr/extract_cells_from_table.py

#+BEGIN_SRC shell :results none
. ~/.virtualenvs/lotto_odds/bin/activate
python -m pdf.extract_cells_from_table "resources/examples/example-table.png"
#+END_SRC

#+BEGIN_SRC python :noweb yes :tangle pdf/extract_cells_from_table.py :results none
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)


<<extract-cells-from-table>>

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

* 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~.

#+NAME: html-image-size
#+BEGIN_SRC emacs-lisp :var text="" :var width="100%" :var height="100%" :results none
(concat "#+ATTR_HTML: :width " width " :height " height "\n[[file:" text "]]")
#+END_SRC

#+BEGIN_SRC emacs-lisp :results none
(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)
#+END_SRC