Extracting Text & Images from PDF Files

Blog 关于用pdfminer解析PDF的文章，测试一下，国内部分学报paper的pdf页因为使用LTImage，而实际又不是，所以识别不出来。
但文章很有价值。
文章来源：http://denis.papathanasiou.org/2010/08/04/extracting-text-images-from-pdf-files/

Update: January 29, 2012

I’ve corrected this code to work with the current version of pdfminer and it’s now available as a github repo: 

---

 is a pdf parsing library written in  by .

In addition to the  and  command line tools, there is a way of.

Since that’s exactly the kind of programmatic parsing I wanted to use PDFMiner for, this is a more complete example, which continues where the default documentation stops.

This example is still a work-in-progress, with room for improvement.

In the next few sections, I describe how I built up each function, resolving problems I encountered along the way. The impatient can just  instead.

Basic Framework

Here are the python imports we need for PDFMiner:

from pdfminer.pdfparser import PDFParser, PDFDocument, PDFNoOutlines
from pdfminer.pdfinterp import PDFResourceManager, PDFPageInterpreter
from pdfminer.converter import PDFPageAggregator
from pdfminer.layout import LAParams, LTTextBox, LTTextLine, LTFigure, LTImage

Since PDFMiner requires a series of initializations for each pdf file, I’ve started with this wrapper () function to take care of the basic preliminary actions (file IO, PDFMminer object creation and connection, etc.).

def with_pdf (pdf_doc, pdf_pwd, fn, *args):
    """Open the pdf document, and apply the function, returning the results"""
    result = None
    try:
        # open the pdf file
        fp = open(pdf_doc, 'rb')
        # create a parser object associated with the file object
        parser = PDFParser(fp)
        # create a PDFDocument object that stores the document structure
        doc = PDFDocument()
        # connect the parser and document objects
        parser.set_document(doc)
        doc.set_parser(parser)
        # supply the password for initialization
        doc.initialize(pdf_pwd)

        if doc.is_extractable:
            # apply the function and return the result
            result = fn(doc, *args)

        # close the pdf file
        fp.close()
    except IOError:
        # the file doesn't exist or similar problem
        pass
    return result

The first two parameters are the name of the pdf file, and its password. The third parameter, fn, is a  which takes the instance of the pdfminer.pdfparser.PDFDocument created, and applies whatever action we want (get the table of contents, walk through the pdf page by page, etc.)

The last part of the signature, *args, is an optional list of parameters that can be passed to the high-order function as needed (I could have gone with keyword arguments here instead, but a simple list is enough for these examples).

As a warm-up, here’s an example of how to use the with_pdf() function to :

def _parse_toc (doc):
    """With an open PDFDocument object, get the table of contents (toc) data
    [this is a higher-order function to be passed to with_pdf()]"""
    toc = []
    try:
        outlines = doc.get_outlines()
        for (level,title,dest,a,se) in outlines:
            toc.append( (level, title) )
    except PDFNoOutlines:
        pass
    return toc

The _parse_toc() function is the higher-order function which gets passed to with_pdf() as the fn parameter. It expects a single parameter, doc, which is the the instance of the pdfminer.pdfparser.PDFDocument created within with_pdf() itself (note that if with_pdf() couldn’t find the file, then _parse_toc() doesn’t get called).

With all the PDFMiner overhead and initialization done by with_pdf(), _parse_toc() can just focus on collecting the table of content data and returning them as a list. The get_outlines() can raise a “PDFNoOutlines” error, so I catch it as an exception, and simply return an empty list in that case.

All that’s left to do is define the function that invokes _parse_toc() for a specific pdf file; this is also the function that any external users of this module would use to get the table of contents list. Note that the pdf password defaults to an empty string (which is what PDFMiner will use for documents that aren’t password-protected), but that can be overriden as needed.

def get_toc (pdf_doc, pdf_pwd=''):
    """Return the table of contents (toc), if any, for this pdf file"""
    return with_pdf(pdf_doc, pdf_pwd, _parse_toc)

Page Parsing

Next, onto layout analysis. Using the with_pdf() wrapper, we can reproduce the example in the documentation with this higher-order function:

def _parse_pages (doc):
    """With an open PDFDocument object, get the pages and parse each one
    [this is a higher-order function to be passed to with_pdf()]"""
    rsrcmgr = PDFResourceManager()
    laparams = LAParams()
    device = PDFPageAggregator(rsrcmgr, laparams=laparams)
    interpreter = PDFPageInterpreter(rsrcmgr, device)

    for page in doc.get_pages():
        interpreter.process_page(page)
        # receive the LTPage object for this page
        layout = device.get_result()
        # layout is an LTPage object which may contain child objects like LTTextBox, LTFigure, LTImage, etc.

And this external function, which defines the specific pdf file to analyze:

def get_pages (pdf_doc, pdf_pwd=''):
    """Process each of the pages in this pdf file"""
    with_pdf(pdf_doc, pdf_pwd, _parse_pages)

So far, this code doesn’t do anything exciting: it just loads each page into a pdfminer.layout.LTPage object, closes the pdf file, and exits.

Within each pdfminer.layout.LTPage instance, though, is an objs attribute, which defines the tree of pdfminer.layout.LT* child objects as in the documentation:

In this example, I’m going to collect all the text from each page in a top-down, left-to-right sequence, merging any multiple columns into a single stream of consecutive text.

The results are not always perfect, but I’m using a fuzzy logic based on physical position and column width, which is very good in most cases.

I’m also going to save any images found to a separate folder, and mark their position in the text with tags.

Right now, I’m only able to extract jpeg images, whereas xpdf’s pdfimages tool is capable of getting to non-jpeg images and saving them as ppm format.

I’m not sure if the problem is within PDFMiner or how I’m using it, but since , I suspect it’s the former.

This requires a few updates to the _parse_pages() function, as follows:

def _parse_pages (doc, images_folder):
    """With an open PDFDocument object, get the pages, parse each one, and return the entire text
    [this is a higher-order function to be passed to with_pdf()]"""
    rsrcmgr = PDFResourceManager()
    laparams = LAParams()
    device = PDFPageAggregator(rsrcmgr, laparams=laparams)
    interpreter = PDFPageInterpreter(rsrcmgr, device)

    text_content = [] # a list of strings, each representing text collected from each page of the doc
    for i, page in enumerate(doc.get_pages()):
        interpreter.process_page(page)
        # receive the LTPage object for this page
        layout = device.get_result()
        # layout is an LTPage object which may contain child objects like LTTextBox, LTFigure, LTImage, etc.
        text_content.append(parse_lt_objs(layout.objs, (i+1), images_folder))

    return text_content

and the updated get_pages() function becomes:

def get_pages (pdf_doc, pdf_pwd='', images_folder='/tmp'):
    """Process each of the pages in this pdf file and print the entire text to stdout"""
    print '\n\n'.join(with_pdf(pdf_doc, pdf_pwd, _parse_pages, *tuple([images_folder])))

New in both functional signatures is images_folder, which is a parameter that refers to the place on the local filesystem where any extracted images will be be saved (this is also an example of why defining with_pdf() with an optional *args list comes in handy).

Aggregating Text

Within the _parse_pages() function, text_content is a new variable of type list, which collects the text of each page, and I’ve added an enumeration structure around doc.get_pages(), to keep track of which page we’re accessing at any given time. This is useful for saving images correctly, since some pdf files use the same image name in multiple places to refer to different images (this creates problems for , for example).

The new critical line in _parse_pages() is this one:

text_content.append(parse_lt_objs(layout.objs, (i+1), images_folder))

Since the tree of page objects is recursive in nature (e.g., a pdfminer.layout.LTFigure object may have multiple child objects), it’s better to handle the actual text parsing and image collection in a separate function. That function, parse_lt_objs(), looks like this:

def parse_lt_objs (lt_objs, page_number, images_folder, text=[]):
    """Iterate through the list of LT* objects and capture the text or image data contained in each"""
    text_content = [] 

    for lt_obj in lt_objs:
        if isinstance(lt_obj, LTTextBox) or isinstance(lt_obj, LTTextLine):
            # text
            text_content.append(lt_obj.get_text())
        elif isinstance(lt_obj, LTImage):
            # an image, so save it to the designated folder, and note it's place in the text 
            saved_file = save_image(lt_obj, page_number, images_folder)
            if saved_file:
                # use html style  tag to mark the position of the image within the text
                text_content.append('')
            else:
                print >> sys.stderr, "Error saving image on page", page_number, lt_obj.__repr__
        elif isinstance(lt_obj, LTFigure):
            # LTFigure objects are containers for other LT* objects, so recurse through the children
            text_content.append(parse_lt_objs(lt_obj.objs, page_number, images_folder, text_content))

    return '\n'.join(text_content)

In this example, I’m concerned with just four objects which may appear within a pdfminer.layout.LTPage object:

1. LTTextBox and LLTextLine (which, because the text extraction is exactly the same, I treat as one case)
2. LTImage (which we’ll try to save on to the local filesystem in the designated folder)
3. LTFigure (which we’ll treat as a simple container for other objects, hence the recursive call in that case)

For the simple text and image extraction I’m doing here, this is enough. There is room for improvement, though, since I’m ignoring several types of pdfminer.layout.LT* objects which do appear in pdf pages.

If you try to run get_pages() now, you might get this error, in the text_content.append(lt_obj.get_text()) line (it will depend on the content of the pdf file you’re trying to parse, as well as how your instance of Python is configured, and whether or not you installed PDFMiner with ).

UnicodeEncodeError: 'ascii' codec can't encode character u'\u2014' in position 61: ordinal not in range(128)

As Eliot explains, "This error occurs when you pass a Unicode string containing non-English characters (Unicode characters beyond 128) to something that expects an ASCII bytestring. The default encoding for a Python bytestring is ASCII."

This function, which I wrote after reading , solves the problem:

def to_bytestring (s, enc='utf-8'):
    """Convert the given unicode string to a bytestring, using the standard encoding,
    unless it's already a bytestring"""
    if s:
        if isinstance(s, str):
            return s
        else:
            return s.encode(enc)

So the updated version of parse_lt_objs() becomes:

def parse_lt_objs (lt_objs, page_number, images_folder, text=[]):
    """Iterate through the list of LT* objects and capture the text or image data contained in each"""
    text_content = [] 

    for lt_obj in lt_objs:
        if isinstance(lt_obj, LTTextBox) or isinstance(lt_obj, LTTextLine):
            # text
            text_content.append(lt_obj.get_text())
        elif isinstance(lt_obj, LTImage):
            # an image, so save it to the designated folder, and note it's place in the text 
            saved_file = save_image(lt_obj, page_number, images_folder)
            if saved_file:
                # use html style  tag to mark the position of the image within the text
                text_content.append('')
            else:
                print >> sys.stderr, "Error saving image on page", page_number, lt_obj.__repr__
        elif isinstance(lt_obj, LTFigure):
            # LTFigure objects are containers for other LT* objects, so recurse through the children
            text_content.append(parse_lt_objs(lt_obj.objs, page_number, images_folder, text_content))

    return '\n'.join(text_content)

Running this version gives reasonable results on pdf files where the text is single-column, and without many sidebars, abstracts, summary quotes, or other fancy typesetting layouts.

It really breaks down, though, in the case of multi-column pages: the resulting text_content jumps from one paragraph to the next, in no coherent order.

PDFMiner does provide two grouping functions, group_textbox_lr_tb and group_textbox_tb_rl [lr=left-to-right, tb=top-to-bottom], but they do the grouping literally, without considering the likelihood that the content of one textbox logically belongs after another’s.

Fortunately, though, each object also provides a bbox (bounding box) attribute, which is a four-part tuple of the object’s page position: (x0, y0, x1, y1).

Using the bbox data, we can group the text according to its position and width, making it more likely the columns we join together this way represent the correct logical flow of the text.

To aggregate the text this way, I added the following Python dictionary variable to the parse_lt_objs() code, just before iterating through the list of lt_objs: page_text={}.

The key for each entry is a tuple of the bbox’s (x0, x1) points, and the corresponding value is a list of text strings found within that bbox. The x0 value tells me the left offset for a given piece of text and the x1 value tells me how wide it is.

So by grouping text which starts at the same horizontal plane and has the same width, I can aggregate all paragraphs belonging to the same column, regardless of their vertical position or length.

Conceptually, each entry in the page_text dictionary represents all the text associated with each physical column.

When I tried this the first time, I was surprised (though in retrospect, I shouldn’t have been, since nothing about parsing pdfs is neat or clean), that two textboxes which look perfectly aligned visually have slightly different x0 and x1 values (at least according to PDFMiner).

For example, one paragraph may have x0 and x1 values of 28.16 and 153.32 respectively, and the paragraph right underneath it had an x0 value of 29.04 and an x1 value of 152.09.

To get around this, I wrote the following update function, which assigns key tuples based on how close an (x0, x1) pair lies within an existing entry’s key. The 20 percent value was arrived at by trial-and-error, and seems to be acceptable for most pdf files I tried.

def update_page_text_hash (h, lt_obj, pct=0.2):
    """Use the bbox x0,x1 values within pct% to produce lists of associated text within the hash"""
    x0 = lt_obj.bbox[0]
    x1 = lt_obj.bbox[2]
    key_found = False
    for k, v in h.items():
        hash_x0 = k[0]
        if x0 >= (hash_x0 * (1.0-pct)) and (hash_x0 * (1.0+pct)) >= x0:
            hash_x1 = k[1]
            if x1 >= (hash_x1 * (1.0-pct)) and (hash_x1 * (1.0+pct)) >= x1:
                # the text inside this LT* object was positioned at the same
                # width as a prior series of text, so it belongs together
                key_found = True
                v.append(to_bytestring(lt_obj.get_text()))
                h[k] = v
    if not key_found:
        # the text, based on width, is a new series,
        # so it gets its own series (entry in the hash)
        h[(x0,x1)] = [to_bytestring(lt_obj.get_text())]
    return h

With this in place, I could update the parse_lt_objs() to use it.

def parse_lt_objs (lt_objs, page_number, images_folder, text=[]):
    """Iterate through the list of LT* objects and capture the text or image data contained in each"""
    text_content = [] 

    page_text = {} # k=(x0, x1) of the bbox, v=list of text strings within that bbox width (physical column)
    for lt_obj in lt_objs:
        if isinstance(lt_obj, LTTextBox) or isinstance(lt_obj, LTTextLine):
            # text, so arrange is logically based on its column width
            page_text = update_page_text_hash(page_text, lt_obj)
        elif isinstance(lt_obj, LTImage):
            # an image, so save it to the designated folder, and note it's place in the text 
            saved_file = save_image(lt_obj, page_number, images_folder)
            if saved_file:
                # use html style  tag to mark the position of the image within the text
                text_content.append('')
            else:
                print >> sys.stderr, "error saving image on page", page_number, lt_obj.__repr__
        elif isinstance(lt_obj, LTFigure):
            # LTFigure objects are containers for other LT* objects, so recurse through the children
            text_content.append(parse_lt_objs(lt_obj.objs, page_number, images_folder, text_content))

    for k, v in sorted([(key,value) for (key,value) in page_text.items()]):
        # sort the page_text hash by the keys (x0,x1 values of the bbox),
        # which produces a top-down, left-to-right sequence of related columns
        text_content.append('\n'.join(v))

    return '\n'.join(text_content)

The last block before the return statement sorts the page_text (x0, x1) keys so that the resulting text is returned in a top-down, left-to-right sequence, based on where the text appeared visually on the page.

Extracting Images

The last thing to discuss in this example is the extraction of images.

As I mentioned above, this area needs improvement, since it seems that I can only extract jpeg images using PDFMiner (though to be fair to Yusuke, he does describe it as a tool that “focuses entirely on getting and analyzing text data“, so perhaps doing more than jpeg is out-of-scope for this library).

Within parse_lt_objs(), the following function is called if an LTImage is found; it was based on studying the  and how it handled image extraction requests:

def save_image (lt_image, page_number, images_folder):
    """Try to save the image data from this LTImage object, and return the file name, if successful"""
    result = None
    if lt_image.stream:
        file_stream = lt_image.stream.get_rawdata()
        file_ext = determine_image_type(file_stream[0:4])
        if file_ext:
            file_name = ''.join([str(page_number), '_', lt_image.name, file_ext])
            if write_file(images_folder, file_name, lt_image.stream.get_rawdata(), flags='wb'):
                result = file_name
    return result

The save_image() function needs the following two supporting functions defined:

def determine_image_type (stream_first_4_bytes):
    """Find out the image file type based on the magic number comparison of the first 4 (or 2) bytes"""
    file_type = None
    bytes_as_hex = b2a_hex(stream_first_4_bytes)
    if bytes_as_hex.startswith('ffd8'):
        file_type = '.jpeg'
    elif bytes_as_hex == '89504e47':
        file_type = ',png'
    elif bytes_as_hex == '47494638':
        file_type = '.gif'
    elif bytes_as_hex.startswith('424d'):
        file_type = '.bmp'
    return file_type

The determine_image_type() function is based on the concept of , where it’s (sometimes) possible to tell what a binary stream means by exmaing the first two or fours bytes.

In theory, a pdf file can have any of these image types, but in practice, the only one PDFMiner can seem to find as an LTImage object are jpegs.

def write_file (folder, filename, filedata, flags='w'):
    """Write the file data to the folder and filename combination
    (flags: 'w' for write text, 'wb' for write binary, use 'a' instead of 'w' for append)"""
    result = False
    if os.path.isdir(folder):
        try:
            file_obj = open(os.path.join(folder, filename), flags)
            file_obj.write(filedata)
            file_obj.close()
            result = True
        except IOError:
            pass
    return result

The write_file() function is just basic file IO, but it does some convenient things around checking that the designated folder exists, too.

Finally, to support all three image saving functions, we need the following python imports:

import sys
import os
from binascii import b2a_hex

Sample Results

So, how well does it work? It’s surprisingly good, as it turns out.

Here’s an example from using the above code to process the  (this was part of the process I used to convert this file to e-book format for inclusion in the ).

Page 5, which looks like this visually:

came out like this:

“Leave the ad revenue and crazy 
business model revenue streams 
to the startups with venture 
funding.” 

on the company. But the advantage 
here is that after a few months off 
the ground you'll have a clear sense 
of how soon that day can come. 
Another advantage of a bootstrapped 
company on the SaaS model is that 
it's really easy to calculate your cash 
flow.
It goes without saying that the 
people you work with should have 
complementary skills to your own, 
but the bootstrapper's "slow but 
steady" mindset is just as important 
to the health of your company. 
you'll find a lot of people may not 
be comfortable with this approach. 
Weed those people out as co-found-
ers when you're bootstrapping a 
company. A one and done approach 
won't work here.

off Hours
Almost every bootstrapped company 
begins as an off-hours tinkering 
project. That's true of Carbonmade, 
which Dave built for himself first; 
that's true of TypeFrag, which I built 
over the course of a week during my 

sophomore year in college; that's 
true of 37signals' Basecamp, true of 
Anthony's Hype Machine and lots of 
other companies.

The good thing about bootstrap-
ping is that you don't need to spend 
a single penny outside of server 
costs and you can even do most 
things locally before having to pay 
any money on a server. your biggest 
expense is time, and that's why off 
hours are so important.

Consult on the Side
The way we started Carbonmade, 
the way 37signals started, the way 
Harvest started, and many other 
startups too, was by first running a 
consulting shop. We ran a design con-
sulting company called nterface that 
Carbonmade grew out of. It's great, 
because the money you're bringing 
in through client work tides you over 
while you're waiting for your startup 
to grow.
Carbonmade was live for nearly 18 
months before we started working 

on it full-time. During those first 
18 months, we were taking on lots 
of client work to pay our bills. The 
great thing about consulting through 
the early months is that you can 
take on fewer and fewer jobs as your 
revenue builds up. For example, you 
may need a dozen large projects 
during the first year and only two or 
three during the second year. That 
was the case for us.
I know of other successful 
bootstrapped companies that during 
the first year would take on a single 
client project for a month or two, 
charging an appropriate amount, and 
that would give them just enough 
leeway to work on their startup for 
two or three months. Then they'd 
rinse and repeat. They did this 
for the first year and a half before 
making enough money to work on 
their startup full-time.

there's no need to Rush
When you're bootstrapping there's 
no rush to get things out the door, 
even though that's all you hear these 

  5

While there were some small problems around capitalization and spacing, the conversion did recognize and save the background image, it distinguished the summary quote as being separate from the rest of the text, and the columns were merged correctly, flowing in the same manner the author wrote them.

Room for Improvement

There are several things I’d like to be able to do better; some probably require changes to PDFMiner itself, while others are things in my code which I should improve.

• Column Merging — while the fuzzy heuristic I described works well for the pdf files I’ve parsed so far, I can imagine more complex documents where it would break-down (perhaps this is where the analysis should be more sophisticated, and not ignore so many types of pdfminer.layout.LT* objects).
• Image Extraction — I’d like to be able to be at least as good as pdftoimages, and save every file in ppm or pnm default format, but I’m not sure what I could be doing differently
• Title and Heading Capitalization — this seems to be an issue with PDFMiner, since I get similar results in using the command line tools, but it is annoying to have to go back and fix all the mis-capitalizations manually, particularly for larger documents.
• Title and Heading Fonts and Spacing — a related issue, though probably something in my own code, is that those same title and paragraph headings aren’t distinguished from the rest of the text. In many cases, I have to go back and add vertical spacing and font attributes for those manually.
• Page Number Removal — originally, I thought I could just use a regex for an all-numeric value on a single physical line, but each document does page numbering slightly differently, and it’s very difficult to get rid of these without manually proofreading each page.
• Footnotes — handling these where the note and the reference both appear on the same page is hard enough, but doing it when they span different (even consecutive) pages is worse.