.. _topic_pdf_to_text:

Converting a PDF file to text
*****************************

Most PDF files look like they contain well-structured text. But the reality is
that a PDF file does not contain anything that resembles paragraphs,
sentences or even words. When it comes to text, a PDF file is only aware of
the characters and their placement.

This makes extracting meaningful pieces of text from PDF files difficult.
The characters that compose a paragraph are no different from those that
compose the table, the page footer or the description of a figure. Unlike
other document formats, like a `.txt` file or a word document, the PDF format
does not contain a stream of text.

A PDF document consists of a collection of objects that together describe
the appearance of one or more pages, possibly accompanied by additional
interactive elements and higher-level application data. A PDF file contains
the objects making up a PDF document along with associated structural
information, all represented as a single self-contained sequence of bytes. [1]_

.. _topic_pdf_to_text_layout:

Layout analysis algorithm
=========================

PDFMiner attempts to reconstruct some of those structures by using heuristics
on the positioning of characters. This works well for sentences and
paragraphs because meaningful groups of nearby characters can be made.

The layout analysis consists of three different stages: it groups characters
into words and lines, then it groups lines into boxes and finally it groups
textboxes hierarchically. These stages are discussed in the following
sections. The resulting output of the layout analysis is an ordered hierarchy
of layout objects on a PDF page.

.. figure:: ../_static/layout_analysis_output.png
    :align: center

    The output of the layout analysis is a hierarchy of layout objects.

The output of the layout analysis heavily depends on a couple of parameters.
All these parameters are part of the :ref:`api_laparams` class.

Grouping characters into words and lines
----------------------------------------

The first step in going from characters to text is to group characters in a
meaningful way. Each character has an x-coordinate and a y-coordinate for its
bottom-left corner and upper-right corner, i.e. its bounding box. Pdfminer.six 
uses these bounding boxes to decide which characters belong together.

Characters that are both horizontally and vertically close are grouped onto
one line. How close they should be is determined by the `char_margin`
(M in the figure) and the `line_overlap` (not in figure) parameter. The horizontal
*distance* between the bounding boxes of two characters should be smaller than
the `char_margin` and the vertical *overlap* between the bounding boxes should
be smaller than the `line_overlap`.

.. raw:: html
    :file: ../_static/layout_analysis.html

The values of `char_margin` and `line_overlap` are relative to the size of
the bounding boxes of the characters. The `char_margin` is relative to the
maximum width of either one of the bounding boxes, and the `line_overlap` is
relative to the minimum height of either one of the bounding boxes.

Spaces need to be inserted between characters because the PDF format has no
notion of the space character. A space is inserted if the characters are
further apart than the `word_margin` (W in the figure). The `word_margin` is
relative to the maximum width or height of the new character. Having a smaller
`word_margin` creates smaller words. Note that the `word_margin` should at
least be smaller than the `char_margin` otherwise none of the characters will
be separated by a space.

The result of this stage is a list of lines. Each line consists of a list of
characters. These characters are either original `LTChar` characters that
originate from the PDF file or inserted `LTAnno` characters that
represent spaces between words or newlines at the end of each line.

Grouping lines into boxes
-------------------------

The second step is grouping lines in a meaningful way. Each line has a
bounding box that is determined by the bounding boxes of the characters that
it contains. Like grouping characters, pdfminer.six uses the bounding boxes
to group the lines.

Lines that are both horizontally overlapping and vertically close are grouped.
How vertically close the lines should be is determined by the `line_margin`.
This margin is specified relative to the height of the bounding box. Lines
are close if the gap between the tops (see L :sub:`1` in the figure) and bottoms
(see L :sub:`2`) in the figure) of the bounding boxes are closer together
than the absolute line margin, i.e. the `line_margin` multiplied by the
height of the bounding box.

.. raw:: html
    :file: ../_static/layout_analysis_group_lines.html

The result of this stage is a list of text boxes. Each box consists of a list
of lines.

Grouping textboxes hierarchically
---------------------------------

The last step is to group the text boxes in a meaningful way. This step
repeatedly merges the two text boxes that are closest to each other.

The closeness of bounding boxes is computed as the area that is between the
two text boxes (the blue area in the figure). In other words, it is the area of
the bounding box that surrounds both lines, minus the area of the bounding
boxes of the individual lines.

.. raw:: html
    :file: ../_static/layout_analysis_group_boxes.html

Working with rotated characters
===============================

The algorithm described above assumes that all characters have the same
orientation. However, any writing direction is possible in a PDF. To
accommodate for this, pdfminer.six allows detecting vertical writing with the
`detect_vertical` parameter. This will apply all the grouping steps as if the
pdf was rotated 90 (or 270) degrees

References
==========

.. [1] Adobe System Inc. (2007). *Pdf reference: Adobe portable document
  format, version 1.7.*
