hOCR - OCR Workflow and Output embedded in HTML

1. Revision History

hOCR has been originally developed by Thomas Breuel.

See the releases and full commit history for a revision history.

2. Introduction

The purpose of this document is to define an open standard for representing document layout analysis and OCR results as a subset of HTML. The goal is to reuse as much existing technology as possible, and to arrive at a representation that makes it easy to store, share, process and display OCR results.

This specification defines many features that can represent a variety of OCR-related information. However, being built on top of HTML, hOCR is designed to make it easy to start simple and gradually use more complex constructs when necessary.

Consider you have an HTML document that encodes a book: Wrapping page elements in [<div class="ocr_page">](#ocr_page) tags will convey the page boundaries to hOCR-capable agents and turn the HTML document into an hOCR document.

3. Terminology and Representation

This document describes a representation of various aspects of OCR output in an XML-like format. That is, we define as set of tags containing text and other tags, together with attributes of those tags. However, since the content we are representing is formatted text,

However, we are not actually using a new XML for the representation; instead embed the representation in XHTML (or HTML) because [XHTML1] and XHTML processing already define many aspects of OCR output representation that would otherwise need additional, separate and ad-hoc definitions. These aspects include:

standard representations for common logical structuring elements, including section headings, citations, tables, emphasis, line breaks, quotations, citations, and preformatted text
standard representations for fonts, embedded images, embedded vector graphics, tables, languages, writing direction, colors
standard representations for geometric layout and positioning
output files that are understood without any further modification by widely used viewers (browsers), editors, conversion tools, and indexing tools
libraries for parsing and generating the content
support for document metadata

We are embedding this information inside HTML by encoding it within valid tags and attributes inside HTML; We are going to use the terms "elements" and "properties" for referring to embedded markup.

Specify that class must be a single value

Elements are defined by the class= attribute on an arbitrary HTML tag. All elements in this format have a class name of the form ocr..._....

Properties are defined by putting information into the title= attribute of an HTML tag. Properties in title attributes are of the form “name values...”, and multiple properties are separated by semicolons.

<div class="ocr_page" id="page_1">
  <div class="ocr_carea" id="column_2" title="bbox 313 324 733 1922">
    <div class="ocr_par" id="par_7"> ... </div>
    <div class="ocr_par" id="par_19"> ... </div>
  </div>
</div>

3.1. General Properties

The following properties can apply to most elements (where it makes sense):

3.1.1. bbox

bbox x0 y0 x1 y1

The bbox - short for "bounding box" - of an element is a rectangular box around this element, which is defined by the upper-left corner (x0, y0) and the lower-right corner (x1, y1).

the values are with reference to the the top-left corner of the document image and measured in pixels
the order of the values are x0 y0 x1 y1 = "left top right bottom"
use x_bboxes below for character bounding boxes
do not use bbox unless the bounding box of the layout component is, in fact, rectangular
some non-rectangular layout components may have rectangular bounding boxes if the non-rectangularity is caused by floating elements around which text flows

See also the section §5.2.1 bbox (typesetting).

<span class='ocr_line' id='line_1'
    title="bbox 10 20 160 30">...</span>

The bounding box bbox of this line is shown in blue and it is span by the upper-left corner (10, 20) and the lower-right corner (160, 30). All coordinates are measured with reference to the top-left corner of the document image which border is drawn in black.

3.1.2. textangle

textangle alpha

The angle in degrees by which textual content has been rotate relative to the rest of the page (if not present, the angle is assumed to be zero); rotations are counter-clockwise, so an angle of 90 degrees is vertical text running from bottom to top in Latin script; note that this is different from reading order, which should be indicated using standard HTML properties

3.2. Non-recommended general properties

The following properties can apply to most elements but should not be used unless there is no alternative:

3.2.1. poly

poly x0 y0 x1 y1 ...

A closed polygon for elements with non-rectangular bounds

this property must not be used unless there is no other way of representing the layout of the page using rectangular bounding boxes, since most tools will simply not have the capability of dealing with non-rectangular layouts
note that the natural and correct representation of many non-rectangular layouts is in terms of rectangular content areas and rectangular floats
documents using polygonal borders anywhere must indicate this by adding ocrp_poly to the list of ocr-capabilities (see §12.1 Capabilities)
documents should attempt to provide a reasonable bbox equivalent as well

3.2.2. order

order n

The reading order of the element (an integer)

this property must not be used unless there is no other way of representing the reading order of the page by element ordering within the page, since many tools will not be able to deal with content that is not in reading order

3.2.3. presence

Use of property presence

presence presence must be declared in the document meta data

3.2.4. cflow

cflow s

This property relates the flow between multiple ocr_carea elements, and between ocr_carea and ocr_linear elements.

The content flow on the page that this element is a part of

s must be a unique string for each content flow
must be present on ocr_carea and ocrx_block tags when reading order is attempted and multiple content flows are present
presence must be declared in the document meta data

3.2.5. baseline

baseline pn pn-1 ... p0

This property applies primarily to textlines.

The baseline is described by a polynomial of order n with the coefficients pn ... p0 with n = 1 for a linear (i.e. straight) line.

The polynomial is in the coordinate system of the line, with the bottom left of the bounding box as the origin.

The hOCR output for the first line of eurotext.tif contains the following information:

<span class='ocr_line' id='line_1_1'
    title="bbox 105 66 823 113; baseline 0.015 -18">...</span>

bbox is the bounding box of the line in image coordinates (blue). The two numbers for the baseline are the slope (1st number) and constant term (2nd number) of a linear equation describing the baseline relative to the bottom left corner of the bounding box (red). The baseline crosses the y-axis at -18 and its slope angle is arctan(0.015) = 0.86°.

4. Logical Structuring Elements

Logical Tags/classes

The classes defined in this section for logically structuring a hOCR document have their standard meaning as used in the publishing industry and tools like LaTeX, MS Word, and others.

Elements with these classes should use the recommended HTML tag given below. This is entirely optional, it may not be possible or desirable to actually choose those tags (e.g., when adding hOCR information to an existing HTML output routine).

ocr_document: Recommended HTML Tag: div
ocr_title: Recommended HTML Tag: h1
ocr_author
ocr_abstract
ocr_part: Recommended HTML Tag: h1
ocr_chapter: Recommended HTML Tag: h1
ocr_section: Recommended HTML Tag: h2
ocr_subsection: Recommended HTML Tag: h3
ocr_subsubsection: Recommended HTML Tag: h4
ocr_display
ocr_blockquote: Recommended HTML Tag: blockquote
ocr_par: Recommended HTML Tag: p
ocr_linear
ocr_caption: Image captions may be indicated using the ocr_caption element; such an element refers to the image(s) contained within the same float, or the immediately adjacent image if both the image and the ocr_caption element are in running text.

Tags must be nested as indicated by the following list, but not all tags within the hierarchy need to be present.

ocr_document
- ocr_linear
  - ocr_title
  - ocr_author
  - ocr_abstract
  - ocr_part
    - ocr_chapter
      - ocr_section ▻ ocr_subsection ▻ ocr_subsubsection
        
        ocr_display
        
        ocr_blockquote
        
        ocr_par

For all of these elements except ocr_linear, there exists a natural linear ordering defined by reading order (ocr_linear indicates that the elements contained in it have a linear ordering). At the level of ocr_linear, there may not be a single distinguished order. A common example of ocr_linear is a newspaper, in which a single newspaper may contain many linear, but there is no unique reading order for the different linear. OCR evaluation tools should therefore be sensitive to the order of all elements other than ocr_linear.

Textual information like section numbers and bullets must be represented as text inside the containing element.

Documents whose logical structure does not map naturally onto these logical structuring elements must not use them for other purposes.

5. Typesetting Related Elements

The following typesetting related elements are based on a typesetting model as found in most typesetting systems, including XSL:FO, (La)TeX, LibreOffice, and Microsoft Word.

In those systems, each page is divided into a number of areas. Each area can either be a part of the body text (or multiple body texts, in the case of newspaper layouts). The content of the areas derives from a linear stream of textual content, which flows into the areas, filling them linewise in their preferred directions.

Overlayed onto the page is a set of floating elements; floating elements exist outside the normal reading order. Floating elements may be introduced by the textual content, or they may be related to the page itself (anchoring is a logical property). In typesetting systems, floating elements may be anchored to the page, to paragraphs, or to the content stream. Floating elements can overlap content areas and render on top of or under content, or they can force content to flow around them. The default for floating elements in this spec is that their anchor is undefined (it is a logical property, not a typesetting property), and that text flows around them. Note that with rectangular content areas and rectangular floats, already a wide variety of non-rectangular text shapes can be realized.

There is currently no way of indicating anchoring or flow-around properties for floating elements; properties need to be defined for this.

5.1. Classes for typesetting elements

The following classes, as well as floats are used for type-setting elements.

5.1.1. ocr_page

The ocr_page element must be present in all hOCR documents.

5.1.2. ocr_column

OBSOLETE

Please use ocr_carea instead

5.1.3. ocr_carea

"ocr content area" or "body area"

Used to be called ~~ocr_column~~

The ocr_carea elements should appear in reading order unless this is impossible because of some other structuring requirement. If the document contains multiple ocr_linear streams, then each ocr_carea must indicate which stream it belongs to.

Note that for many documents, the actual ground truth careas are well-defined by the document style of the original document before printing and scanning. From a single page, the careas of the original document style cannot be recovered exactly. However, the partition of a document by ocr_carea for an individual page shall be considered correct relative to ground truth if

all the text contained in a ground truth carea is fully contained within a single ocr_carea,
no text outside a ground truth carea is contained within an ocr_carea, and
the ocr_carea appear in the same order as the text flow relationships between the ground truth careas.

5.1.4. ocr_line

In typesetting systems, content areas are filled with “blocks”, but most of those blocks are not recoverable or semantically meaningful. However, one type of block is visible and very important for OCR engines: the line. Lines are typesetting blocks that only contain glyphs (“inlines” in XSL terminology). They are represented by the ocr_line area.

ocr_line should be in a 

5.1.5. ocr_separator

Any separator or similar element

5.1.6. ocr_noise

Any noise element that isn’t part of typesetting

5.2. Recommended Properties for typesetting elements

The following properties should be present:

5.2.1. `bbox (typesetting)`

The bounding box of the page; for pages, the top left corner must be at (0,0), so a typical page bounding box will look like bbox 0 0 2300 3200

5.2.2. image

image imagefile

image file name used as input
syntactically, must be a UNIX-like pathname or http URL (no Windows pathnames)
may be relative
cannot be resolved to the actual file in general (e.g., if the hOCR file becomes separated from the image file)
if the hOCR file is present in a directory hierarchy or file archive, should resolve to the corresponding image file

5.2.3. imagemd5

imagemd5 checksum

MD5 fingerprint of the image file that this page was derived from
allows re-associating pages with source images

5.2.4. ppageno

ppageno n

the physical page number
the front cover is page number 0
should be unique
must not be present unless the pages in the document have a physical ordering
must not be present unless it is well defined and unique

5.2.5. lpageno

lpageno string

the logical page number expressed on the page
may not be numerical (e.g., Roman numerals)
usually is unique
must not be present unless it has been recognized from the page and is unambiguous

5.3. Optional Properties for typesetting elements

The following properties MAY be present:

5.3.1. scan_res

scan_res x_res y_res

scanning resolution in DPI

5.3.2. x_scanner

x_scanner string

a representation of the scanner

5.3.3. x_source

x_source string

an implementation-dependent representation of the document source
could be a URL or a /gfs/ path
offsets within a multipage format (e.g., TIFF) may be represented using additional strings or using URL parameters or fragments
examples
- x_source /gfs/cc/clean/012345678911 17
- x_source http://pageserver/012345678911&page=17

In addition to the standard properties, the ocr_line area supports the following additional properties:

5.3.4. hardbreak

hardbreak n

a zero (default) indicates that the end of the line is not a hard (explicit) line break, but a break due to text flow
a one indicates that the line is a hard (explicit) line break

Any special characters representing the desired end-of-line processing must be present inside the ocr_line element. Examples of such special characters are a soft hyphen ("", U+00AD), a hard line break ( ), or whitespace () for soft line breaks.

5.4. Classes for floats

Floats should not be nested.

The following floats are defined:

5.4.1. ocr_float

ocr_float

5.4.2. ocr_separator

ocr_separator in the context of float classes.

5.4.3. ocr_textfloat

ocr_textfloat

5.4.4. ocr_textimage

ocr_textimage

5.4.5. ocr_image

ocr_image

5.4.6. ocr_linedrawing

Something that could be represented well and naturally in a vector graphics format like SVG (even if it is actually represented as PNG)

5.4.7. ocr_photo

Something that requires JPEG or PNG to be represented well

5.4.8. ocr_header

ocr_header

5.4.9. ocr_footer

ocr_footer

5.4.10. ocr_pageno

ocr_pageno

5.4.11. ocr_table

ocr_table

6. Inline Representations

There is some content that should behave and flow like text

6.1. Classes for Inline Representation

6.1.1. ocr_glyph

An individual glyph represented as an image (e.g., an unrecognized character)

Must contain a single <img> tag, or be present on one

6.1.2. ocr_glyphs

Multiple glyphs represented as an image (e.g., an unrecognized word)

Must contain a single <img> tag, or be present on one

6.1.3. ocr_dropcap

An individual glyph representing a dropcap

May contain text or an <img> tag; the alt of the image tag should contain the corresponding text

6.1.4. ocr_chem

A chemical formula

Must contain either a single <img> tag or [CML] markup, or be present on one

6.1.5. ocr_math

A mathematical formula

Must contain either a single <img> tag or [MathML] markup, or be present on one

Mathematical and chemical formulas that float must be put into an ocr_float section.

Mathematical and chemical formulas that are “display” mode should be put into an ocr_display section.

6.1.6. Superscript and Subscript

Superscripts and subscripts, when not in ocr_math or ocr_chem formulas, must be represented using the HTML  and  tags, even if special Unicode characters are available.

6.1.7. Non-breaking space

Non-breaking spaces must be represented using the HTML   entity.

6.1.8. Non-default spaces

Different space widths should be indicated using HTML and &ensp;, &emsp,  , &zwnj;, &zwj;.

6.1.9. Hyphenation

Soft hyphens must be represented using the HTML  entity.

The HTML &lrm; and &rlm; entities (indicating writing direction) must not be used; all writing direction changes must be indicated with tags.

6.1.10. Ruby characters

Furigana and similar constructs must be represented using their correct Unicode encoding.

7. Character Information

7.1. Classes for Character Information

Character-level information may be put on any element that contains only a single "line" of text.

7.1.1. ocr_cinfo

If no other layout element applies, the ocr_cinfo element may be used.

7.2. Properties for Character Information

7.2.1. cuts

cuts c1 c2 c3 ...

character segmentation cuts (see below)
there must be a bbox property relative to which the cuts can be interpreted

7.2.2. nlp

nlp c1 c2 c3 ...

estimate of the negative log probabilities of each character by the recognizer

For left-to-write writing directions, cuts are sequences of deltas in the x and y direction; the first delta in each path is an offset in the x direction relative to the last x position of the previous path. The subsequent deltas alternate between up and right moves.

Assume a bounding box of (0,0,300,100); then

cuts("10 11 7 19") =
    [ [(10,0),(10,100)], [(21,0),(21,100)], [(28,0),(28,100)], [(47,0),(47,100)] ]
cuts("10,50,3 11,30,-3") =
    [ [(10,0),(10,50),(13,50),(13,100)], [(21,0),(21,30),(18,30),(18,100)] ]

<span class="ocr_cinfo" title="bbox 0 0 300 100; nlp 1.7 2.3 3.9 2.7; cuts 9 11 7,8,-2 15 3">hello</span>

Cuts are between all codepoints contained within the element, including any whitespace and control characters. Simply use a delta of 0 (zero) for invisible codepoints.

Writing directions other than left-to-right specify cuts as if the bounding box for the element had been rotated by a multiple of 90 degrees such that the writing direction is left to right, then rotated back.

It is undefined what happens when cut paths intersect, with the exception that a delta of 0 always corresponds to an invisible codepoint.

8. OCR Engine-Specific Markup

A few abstractions are used as intermediate abstractions in OCR engines, although they do not have a meaning that can be defined either in terms of typesetting or logical function. Representing them may be useful to represent existing OCR output, say for workflow abstractions.

Common suggested engine-specific markup are:

8.1. Classes for engine specific markup

8.1.1. ocrx_block

ocr_carea vs ocrx_block

any kind of "block" returned by an OCR system
engine-specific because the definition of a "block" depends on the engine

8.1.2. ocrx_line

ocr_line vs ocrx_line

any kind of "line" returned by an OCR system that differs from the standard ocr_line above
might be some kind of "logical" line

8.1.3. ocrx_word

any kind of "word" returned by an OCR system
engine specific because the definition of a "word" depends on the engine

The meaning of these tags is OCR engine specific. However, generators should attempt to ensure the following properties:

An ocrx_block should not contain content from multiple ocr_carea.
The union of all ocrx_blocks should approximately cover all ocr_carea.
an ocrx_block should contain either a float or body text, but not both
an ocrx_block should contain either an image or text, but not both
an ocrx_line should correspond as closely as possible to an ocr_line
ocrx_cinfo should nest inside ocrx_line
ocrx_cinfo should contain only x_confs, x_bboxes, and cuts attributes

ocrx_cinfo?

8.2. Properties for engine-specific markup

The following properties are defined:

8.2.1. x_font

x_font s

OCR-engine specific font names

8.2.2. x_fsize

x_fsize n

OCR-engine specific font size

8.2.3. x_bboxes

x_bboxes b1x0 b1y0 b1x1 b1y1 b2x0 b2y0 b2x1 b2y1 ...

OCR-engine specific boxes associated with each codepoint contained in the element
note that the bbox property is a property for the bounding box of a layout element, not of individual characters
in particular, use , not

8.2.4. x_confs

x_confs c1 c2 c3 ...

OCR-engine specific character confidences
c1 etc. must be numbers
higher values should express higher confidences
if possible, convert character confidences to values between 0 and 100 and have them approximate posterior probabilities (expressed in %)

8.2.5. x_wconf

x_wconf n

OCR-engine specific confidence for the entire contained substring
n must be a number
higher values should express higher confidences
if possible, convert word confidences to values between 0 and 100 and have them approximate posterior probabilities (expressed in %)

9. Font, Text Color, Language, Direction

OCR-generated font and text color information is encoded using standard HTML and CSS attributes on elements with a class of ocr_... or ocrx_.... Language and writing direction should be indicated using the HTML standard attributes lang= and dir=, or alternatively can be indicated as properties on elements.

OCR information and presentation information can be separated by putting the CSS info related to the CSS in an outer element with an ocr_ or ocrx_ class, and then overriding it for the presentation by nesting another  with the actual presentation information inside that:

<span class="ocr_cinfo" style="ocr style"><span style="presentation style"> ... </span></span>

The CSS3 text layout attributes can be used when necessary. For example, CSS supports writing-mode, direction, glyph-orientation [ISO15924]-based script (list of codes), text-indent, etc.

10. Alternative Segmentations / Readings

Delete x_cost

Alternative segmentations and readings are indicated by a  with class="alternatives". It must contains <ins> and <del> elements. The first contained element should be <ins> and represent the most probable interpretation, the subsequent ones <del>. Each <ins> and <del> element should have class="alt" and a property of either nlp or x_cost. These , <ins>, and <del> tags can nest arbitrarily.

<span class="alternatives">
<ins class="alt" title="nlp 0.3">hello</ins>
<del class="alt" title="nlp 1.1">hallo</del>
</span>

Whitespace within the  but outside the contained <ins>/<del> elements is ignored and should be inserted to improve readability of the HTML when viewed in a browser.

11. Grouped Elements and Multiple Hierarchies

The different levels of layout information (logical, physical, engine-specific) each form hierarchies, but those hierarchies may not be mutually compatible; for example, a single ocr_page may contain information from multiple sections or chapters. To represent both hierarchies within a single document, elements may be grouped together. That is, two elements with the same class may be treated as one element by adding a "groupid identifier" property to them and using the same identifier.

Grouped elements should be logically consistent with the markup they represent; for example, it is probably not sensible to use grouped elements to interleave parts of two different chapters. Therefore, grouped elements should usually be adjacent in the markup.

Applications using hOCR may choose to manipulate grouped elements directly, but the simplest way of dealing with them is to transform a document with grouped elements into one without grouped elements prior to further processing by first removing tags that are not of interest for the subsequent processing step, and then collapsing grouped elements into single elements. For example, output that contains both logical and physical layout information, where the logical layout information uses grouped elements, can be transformed by removing all the physical layout information, and then collapsing all split ocr_chapter elements into single ocr_chapter elements based on the groupid. The result is a simple DOM tree. This transformation can be provided generically as a pre-processor or Javascript.

The presence of grouped elements does not need to be indicated in the header; when it affects their operations, hOCR processors should check for the presence of grouped elements in the output and fail with an error message if they cannot correctly process the hOCR information.

12. Metadata

The creator of the hOCR document can indicate the following information information using meta tags in the head section.

ocr-system: Indicates software and version that generated the hOCR document; Every hOCR document must have exactly one ocr-system metadata field
ocr-capabilities: Features consumers of the hOCR document can expect; See §12.1 Capabilities for possible values; Every hOCR document must have exactly one ocr-capabilities metadata field
ocr-number-of-pages: The number of ocr_page in the document
ocr-langs: Use ISO 639-1 codes; Value may be unknown
ocr-scripts: Use ISO 15924 letter codes; Value may be unknown

12.1. Capabilities

Any program generating files in this output format must indicate in the document metadata what kind of markup it is capable of generating. This includes listing the exact set of markup sections that the system could have generated, even if it did not actually generate them for the particular document.

If a document lists a certain capabilities but no element or attribute is found that corresponds to that capability, users of the document may infer that the content is absent in the source document. If a capability is not listed, the corresponding element or attribute must not be present in the document.

The capability to generate specific properties is given by the prefix ocrp_...; the important properties are:

ocrp_lang: Capable of generating lang= attributes
ocrp_dir: Capable of generating dir= attributes
ocrp_poly: Capable of generating polygonal bounds
ocrp_font: Capable of generating font information (standard font information)
ocrp_nlp: Capable of generating nlp confidences
ocr_embeddedformat_<formatname>: The capability to generate other specific embedded formats is given by the prefix ocr_embeddedformat_<formatname>.
ocr_<tag>_unordered: If an OCR engine represents a particular tag but cannot determine reading order for that tag, it must must specify a capability of ocr_<tag>_unordered.

12.2. Document metadata

For document meta information, use the Dublin Core Embedding into HTML. See also Citation Guidelines for Dublin Core.

12.3. Example

<html>
  <head>
    <meta name="ocr-system" content="tesseract v3.03"/>
    <meta name="ocr-capabilities" content="ocr_page ocr_line ocrp_lang"/>
    <meta name="ocr-langs" content="aa la zu"/>
    <meta name="ocr-scripts" content="Arab Khmr"/>
    <meta name="ocr-number-of-pages" content="112"/>
    ...
  </head>
  ...
</html>

Indicate that the work this hOCR file represents:

was produced by Tesseract v3.03
will provide ocr_page and ocr_line elements with lang= attribute
contains text written in the Afar, Latin or Zulu languages
contains text written in Arabic and Khmer script
has 112 pages

13. Profiles

hOCR provides standard means of marking up information, but it does not mandate the presence or absence of particular kinds of information. For example, an hOCR file may contain only logical markup, only physical markup, or only engine-specific markup. As a result, merely knowing that OCR output is hOCR compliant doesn’t tell us whether that file is actually useful for subsequent processing.

OCR systems can use hOCR in various different ways internally, but we will eventually define some common profiles that mandate what kinds of information needs to be present in particular kinds of output.

Of particular importance are:

physical layout profile: OCR output in XHTML format with a defined set of common physical layout markup capabilities (page, carea, floats, line). Logical layout may be present as well, but the document tree structure must represent the physical layout structure, with logical layout elements split and grouped as needed.
logical layout profile: OCR output in XHTML format with a defined set of common logical layout markup capabilities (linear, chapter, section, subsection). Physical layout may be present as well, but the document tree structure must represent the logical layout structure, with logical layout elements split and grouped as needed.

Other possible profiles might be defined for specific engines or specific document classes:

common commercial OCR output (e.g., Abbyy)
- ocr_page
- ocrx_block, ocrx_line, ocrx_word
- ocrp_lang
- ocrp_font
book target
- all logical structuring elements (as applicable), except ocr_linear
- ocr_page
newspaper target
- all logical structuring elements (as applicable)
- articles map on ocr_linear
- ocr_page

14. HTML Markup

The HTML-based markup is orthogonal to the hOCR-based markup; that is, both can be chosen independent of one another. The only thing that needs to be consistent between the two markups is the text contained within the tags. hOCR and other embedded format tags can be put on HTML tags, or they can be put on their own div/span tags.

There are many different choices possible and reasonable for the HTML markup, depending on the use and further processing of the document. Each such choice must be indicated in the meta data for the document.

Many mappings derived from existing tools are quite similar, and most follow the restrictions and recommendations below already without further modifications.

Depending on the particular HTML markup used in the document, the document is suitable for different kinds of processing and use. The formats have the following intents:

html_none (see §14.3 HTML without logical markup): Straightforward equivalent of Goodoc or [XDOC]
html_simple: Target format for convenient on-line viewing and intermediate format for indexing
html_xytable_absolute, html_xytable_relative: Target format for layout-preserving on-screen document viewing
Formats defined in §14.5 HTML produced by OCR engines: Straightforward recording of commercial OCR system output
Formats defined in §14.6 HTML with absolute positioning: Target format for services like Google’s View as HTML

As long as a format contains the hOCR information, it can be reprocessed by layout analysis software and converted into one of the other formats. In particular, we envision layout analysis tools for converting any hOCR document into html_absolute, html_xytable_absolute, and html_simple. Furthermore, internally, a layout analysis system might use html_xytable_absolute as an intermediate format for converting hOCR into html_simple.

14.1. Restrictions on HTML Content

To avoid problems, any use of HTML markup must follow the following rules:

HTML content must not use class names that conflict with any of those defined in this document (ocr_*)
HTML content must not use the title= attribute on any element with an ocr_* class for any purposes other than encoding OCR-related properties as described in this document

14.2. Recommendations for Mappings

When possible, any mapping of logical structure onto HTML should try to follow the following rules:

the mapping should be "natural" -- similar to what an author of the document might have entered into a WYSIWYG content creation tool
text should be in reading order
all tags should be used for the intended purpose (and only for the intended purpose) as defined in the [HTML40] spec.
floats are contained in div elements with a style that includes a float attribute
repeating floating page elements (header/footer) should be repeated and occur in their natural location in reading order (e.g., between pages)
embedded images and SVG should be contained in files in the same directory (no / in the URL) and embedded with img and embed tags, respectively

Specifically

em and strong should represent emphasis, and are preferred to b, i, and u
b, i, and u should represent a change in the corresponding attribute for the current font (but an OCR font specification must still be given)
p should represent paragraph breaks
br should represent explicit linebreaks (not linebreak that happen because of text flow)
h1, ..., h6 should represent the logical nesting structure (if any) of the document
a should represent hyperlinks and references within the document
blockquote should represent indented quotations, but not other uses of indented text.
ul, ol, dl should represent lists
table should represent tables, including correct use of the th tag

If necessary, the markup may use the following non-standard tags:

nobr to indicate that line breaking is not permitted for the enclosed content
wbr to indicate that line breaking is permitted at that location

14.3. HTML without logical markup

The html_none format contains no logical markup at all; it is simply a collection of div and span elements with associated hOCR information. Note that such documents can still be rendered visually through the use of CSS.

14.4. HTML with limited logical elements

The html_simple format follows the restrictions and recommendations above, and only uses the following tags:

h1 ... h6
p, br
b, i, and u for appearance changes (bold, italic, underline)
font for any other appearance changes
a
div with a float style for floats
table for tables
img for images
all SVG must be externally embedded with the embed tag
the use of other embedded formats is permitted
all other uses of div, span, ins, and del only for hOCR tags or other embedded formats (hCard, …)

14.5. HTML produced by OCR engines

HTML markup produced by default by the OCR engine for the given document must follow the template html_ocr_<engine>.

Examples of possible values are:

html_ocr_unknown: The HTML was generated by some OCR engine, but it’s unknown which one
html_ocr_finereader_8
html_ocr_textbridge_11

14.6. HTML with absolute positioning

html_absolute: The HTML represents absolute positioning of elements on each page.

Possible subformats are:

html_absolute_cols: absolute positioning of cols
html_absolute_pars: absolute positioning of paragraphs
html_absolute_lines: absolute positioning of lines
html_absolute_words: absolute positioning of words
html_absolute_chars: absolute positioning of characters

The "View as HTML" for PDF files feature of Google Search uses html_absolute_lines; this is probably the most reasonable choice for approximating the appearance of the original document.

14.7. HTML as table

html_xytable: The HTML is a table that gives the XY-cut layout segmentation structure of the page in tabular form.; Note that in this format, text order does not necessarily correspond to reading order.; The format must contain one table of class ocr_xycut representing each page. The markup of the content of the table itself is as in html_simple.

Possible subformats are:

html_xytable_absolute: The table structure must represent the absolute size of the original page element.
html_xytable_relative: Table element sizes are expressed relative (percentages).

14.8. HTML from word processors

The HTML represents markup that follows the mappings of the given document processor to HTML.

Note that the document doesn’t actually need to have been constructed in the processor and that the processor doesn’t need to have been used to generate the HTML. For example, the html_latex2html tag merely indicates that, say, a scanned and ocr’ed article uses the same conventions for logical markup tags that an equivalent article actually written in LaTeX and actually converted to HTML would have used.

html_latex2html
html_msword: HTML mapping generated by “Save As HTML”
html_ooffice: HTML mapping generated by “Save As HTML”
html_docbook_xsl: HTML mapping generated by official XSL style sheets

15. Sample Usage

See also the hocr-tools for more samples.

The HTML format described here may seem fairly complicated and difficult to parse, but because there are lots of tools for manipulating HTML documents, they’re actually pretty easy to manipulate. Here are some examples:

import libxml2,re,os,string

# convert the HTML to XHTML (if necessary)
os.system("tidy -q -asxhtml < page.html > page.xhtml 2> /dev/null")

# parse the XML
doc = libxml2.parseFile('page.xhtml')

# search all nodes having a class of ocr_line
lines = doc.xpathEval("//*[@class='ocr_line']")

# a function for extracting the text from a node
def get_text(node):
    textnodes = node.xpathEval(".//text()")
    s = string.join([node.getContent() for node in textnodes])
    return re.sub(r'\s+',' ',s)

# a function for extracting the bbox property from a node
# note that the title= attribute on a node with an ocr_ class must
# conform with the OCR spec

def get_bbox(node):
    data = node.prop('title')
    bboxre = re.compile(r'\bbbox\s+(\d+)\s+(\d+)\s+(\d+)\s+(\d+)')
    return [int(x) for x in bboxre.search(data).groups()]

# this extracts all the bounding boxes and the text they contain
# it doesn’t matter what other markup the line node may contain
for line in lines:
    print get_bbox(line),get_text(line)

Note that the OCR markup, basic HTML markup, and semantic markup can co-exist within the same HTML file without interfering with one another.

16. IANA Considerations

XML namespace for hOCR HTML?

What DOCTYPE for hOCR HTML?

16.1. Media Type

In accordance to [RFC2048]

correct MIME type for hOCR?

MIME media type name: text
MIME subtype name:: vnd.hocr+html
Required parameters:
Optional parameters:
Encoding considerations:: hOCR documents should be encoded as UTF-8
Security considerations:
Interoperability considerations:
Applications which use this media type:
File extension(s):: *.html, *.hocr

hOCR - OCR Workflow and Output embedded in HTML

Living Standard, 20 October 2016

Abstract

1. Revision History

2. Introduction

3. Terminology and Representation

3.1. General Properties

3.1.1. bbox

3.1.2. textangle

3.2. Non-recommended general properties

3.2.1. poly

3.2.2. order

3.2.3. presence

3.2.4. cflow

3.2.5. baseline

4. Logical Structuring Elements

5. Typesetting Related Elements

5.1. Classes for typesetting elements

5.1.1. ocr_page

5.1.2. ocr_column

5.1.3. ocr_carea

5.1.4. ocr_line

5.1.5. ocr_separator

5.1.6. ocr_noise

5.2. Recommended Properties for typesetting elements

5.2.1. bbox (typesetting)

5.2.2. image

5.2.3. imagemd5

5.2.4. ppageno

5.2.5. lpageno

5.3. Optional Properties for typesetting elements

5.3.1. scan_res

5.3.2. x_scanner

5.3.3. x_source

5.3.4. hardbreak

5.4. Classes for floats

5.4.1. ocr_float

5.4.2. ocr_separator

5.4.3. ocr_textfloat

5.4.4. ocr_textimage

5.4.5. ocr_image

5.4.6. ocr_linedrawing

5.4.7. ocr_photo

5.4.8. ocr_header

5.4.9. ocr_footer

5.4.10. ocr_pageno

5.4.11. ocr_table

6. Inline Representations

6.1. Classes for Inline Representation

6.1.1. ocr_glyph

6.1.2. ocr_glyphs

6.1.3. ocr_dropcap

6.1.4. ocr_chem

6.1.5. ocr_math

6.1.6. Superscript and Subscript

6.1.7. Non-breaking space

6.1.8. Non-default spaces

6.1.9. Hyphenation

6.1.10. Ruby characters

7. Character Information

7.1. Classes for Character Information

7.1.1. ocr_cinfo

7.2. Properties for Character Information

7.2.1. cuts

7.2.2. nlp

8. OCR Engine-Specific Markup

8.1. Classes for engine specific markup

8.1.1. ocrx_block

8.1.2. ocrx_line

8.1.3. ocrx_word

8.2. Properties for engine-specific markup

8.2.1. x_font

8.2.2. x_fsize

8.2.3. x_bboxes

8.2.4. x_confs

8.2.5. x_wconf

9. Font, Text Color, Language, Direction

10. Alternative Segmentations / Readings

11. Grouped Elements and Multiple Hierarchies

12. Metadata

5.2.1. `bbox (typesetting)`