This manual is for GNU Ocrad (version 0.27, 10 January 2019).

Copyright © 2003-2019 Antonio Diaz Diaz.

This manual is free documentation: you have unlimited permission to copy, distribute and modify it.

Introduction
************

GNU Ocrad is an OCR (Optical Character Recognition) program and library based on a feature extraction method. It reads images in pbm (bitmap), pgm (greyscale) or ppm (color) formats and produces text in byte (8-bit) or UTF-8 formats. The pbm, pgm and ppm formats are collectively known as pnm.

Ocrad includes a layout analyser able to separate the columns or blocks of text normally found on printed pages.

For best results the characters should be at least 20 pixels high. If they are smaller, try the --scale option. Scanning the image at 300 dpi usually produces a character size good enough for ocrad.

Character sets
**************

The character set internally used by ocrad is ISO 10646, also known as UCS (Universal Character Set), which can represent over two thousand million characters (2^31).

As it is unpractical to try to recognize one among so many different characters, you can tell ocrad what character sets to recognize. You do this with the --charset option.

If the input page contains characters from only one character set, say ISO-8859-15, you can use the default byte output format. But in a page with ISO-8859-9 and ISO-8859-15 characters, you can't tell if a code of 0xFD represents a latin small letter i dotless or a latin small letter y with acute. You should use --format=utf8 instead. Of course, you may request UTF-8 output in any case.

NOTE: 10^9 is a thousand millions, a billion is a million millions (million^2), a trillion is a million million millions (million^3), and so on. Please, don't "embrace and extend" the meaning of prefixes, making communication among all people difficult. Thanks.

Invoking ocrad
**************

The format for running ocrad is:

     ocrad [OPTIONS] [FILES]

- used as a FILE argument means standard input. It can be mixed with other FILES and is read just once, the first time it appears in the command line. Ocrad can read concatenated files from standard input.

ocrad supports the following options:

-h
--help
     Print an informative help message describing the options and exit. ocrad --verbose --help describes also hidden options.

-V
--version
     Print the version number of ocrad on the standard output and exit. This version number should be included in all bug reports.

-a
--append
     Append generated text to the output file instead of overwriting it.

-c NAME
--charset=NAME
     Enable recognition of the characters belonging to the given character set. You can repeat this option multiple times with different names for processing a page with characters from different character sets.

     If no charset is specified, iso-8859-15 (latin9) is assumed.

     Try --charset=help for a list of valid charset names.

-e NAME
--filter=NAME
     Pass the output text through the given built-in postprocessing filter (see Filters). Several filters can be applied in sequence using as many --filter and --user-filter options as needed. The filters are applied in the order they appear on the command line.

     Try --filter=help for a list of valid filter names.

-E FILE
--user-filter=FILE
     Pass the output text through the postprocessing filter defined in FILE. See the chapter Filters (see Filters) for a description of the format of FILE. Several filters can be applied in sequence using as many --filter and --user-filter options as needed. The filters are applied in the order they appear on the command line.

-f
--force
     Force overwrite of output files.

-F NAME
--format=NAME
     Select the output format. The valid names are byte and utf8.

     If no output format is specified, byte (8 bit) is assumed.

-i
--invert
     Invert image levels (white on black).

-l
--layout
     Enable page layout analysis. Ocrad is able to separate blocks of text of arbitrary shape as long as they are clearly delimited by white space.

-o FILE
--output=FILE
     Place the output into FILE instead of into the standard output.

-q
--quiet
     Quiet operation.

-s VALUE
--scale=VALUE
     Scale up the input image by VALUE before layout analysis and recognition. If VALUE is negative, the input image is scaled down by -VALUE.

-t NAME
--transform=NAME
     Perform given transformation (rotation or mirroring) on the input image before scaling, layout analysis and recognition. Rotations are made counter-clockwise.

     Try --transform=help for a list of valid transformation names.

-T VALUE
--threshold=VALUE
     Set binarization threshold for pgm or ppm files or for --scale option (only for scaled down images). VALUE should be a rational number between 0 and 1, and may be given as a percentage (50%), a fraction (1/2), or a decimal value (0.5). Image values greater than threshold are converted to white. The default value is 0.5.

-u LEFT,TOP,WIDTH,HEIGHT
--cut=LEFT,TOP,WIDTH,HEIGHT
     Cut the input image by the rectangle defined by LEFT, TOP, WIDTH and HEIGHT. Values may be relative to the image size (-1.0 <= value <= +1.0), or absolute (abs( value ) > 1). Negative values of LEFT, TOP are relative to the right-bottom corner of the image. Values of WIDTH and HEIGHT must be positive. Absolute and relative values can be mixed. For example ocrad --cut 700,960,1,1 will extract from 700,960 to the right-bottom corner of the image.

     The cutting is performed before any other transformation (rotation or mirroring) on the input image, and before scaling, layout analysis and recognition.

-v
--verbose
     Verbose mode.

-x FILE
--export=FILE
     Write (export) OCR results file to FILE (see OCR results file). -x - writes to stdout, overriding text output except if output has been also redirected with the -o option.

Exit status: 0 for a normal exit, 1 for environmental problems (file not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or invalid input file, 3 for an internal consistency error (eg, bug) which caused ocrad to panic.

Postprocessing the produced text
********************************

Filters replace some characters in the text output with different characters and remove some other characters from the output. For example, when recognizing a text that is known to contain just numbers, any character recognized as a Z will probably be a 2.

Filters don't enable the recognition of characters, just filter them from the output. Use --charset to enable the recognition of a character set different from the default ISO-8859-15.

Ocrad provides both built-in filters and user-defined filters.

User-defined filters
====================

The format of a user-defined filter file (see --user-filter) is very simple. Each line contains either a character conversion or a word that specifies the default behaviour for unlisted characters.

A character conversion is a comma-separated list of quoted characters ('c'), character sets ([0-9A-Z]), character codes (U0063), or character ranges (U0000 - UFFFF), and an optional conversion (an equal sign (=) followed by a quoted character or a character code). The characters in the list are converted to the character in the conversion. If no conversion is specified, the character is left unmodified (converted to itself).

The default behaviour is to discard unlisted characters, i.e. those characters not appearing in the file, either by themselves or included in a set or range. If a line containing just the word leave is found in the file, unlisted characters are left unmodified. If the word is mark, unlisted characters are marked as unrecognized.

The destination character of a conversion is considered as listed by default. Every character may be listed more than once, even as part of different conversions. The last conversion affecting a given character is the one that is performed.

Character sets and quoted characters may contain escape sequences.

The character # at begin of line or after whitespace starts a comment that extends to the end of the line.

Ranges of characters may be specified in character sets by writing the starting and ending characters with a - between them. Thus, [A-Z] matches any ASCII uppercase letter. - may be specified by placing it first or last. ] may be specified by placing it first. If the first character after the left bracket is ^, it indicates a "complemented set", which matches any character except the ones between the brackets.

Literals (quoted characters and character sets) are decoded as ISO-8859-15. Character codes are decoded as UCS2. Thus, a latin capital letter y with diaeresis is specified in a set as [\xBE], but its code is U0178.

Spaces and control characters are unaffected by filters, except that leadind, trailing, and duplicate spaces produced by the removal of other characters will be themselves removed.

Here is an example user-defined filter file equivalent to the built-in filter numbers:

     leave                      # remove this line to get numbers_only
     'D', 'O', 'Q', 'o' = '0'
     'I', 'L', 'l', '|' = '1'
     'Z', 'z'           = '2'
                          '3'
     'A', 'q'           = '4'
     'S', 's'           = '5'
     'G', 'b', U00F3    = '6'   # latin small letter o with acute
     'J', 'T'           = '7'
     '&', 'B'           = '8'
     'g'                = '9'

Built-in filters
================

Ocrad provides the following built-in filters (see --filter):

--filter=letters
     Forces every character that resembles a letter to be recognized as a letter. Other characters will be output without change.

--filter=letters_only
     Same as --filter=letters, but other characters will be discarded.

--filter=numbers
     Forces every character that resembles a number to be recognized as a number. Other characters will be output without change.

--filter=numbers_only
     Same as --filter=numbers but other characters will be discarded.

--filter=same_height
     Discards any character (or noise) whose height differs in more than 10 percent from the median height of the characters in the line.

--filter=text_block
     Discards any character (or noise) outside of a rectangular block of text lines.

--filter=upper_num
     Forces every character that resembles a uppercase letter or a number to be recognized as such. Other characters will be output without change.

--filter=upper_num_mark
     Same as --filter=upper_num, but other characters will be marked as unrecognized.

--filter=upper_num_only
     Same as --filter=upper_num, but other characters will be discarded.

Library version
***************

 -- Function: const char * OCRAD_version ( void )
     Returns the library version as a string.

 -- Constant: const char * OCRAD_version_string
     This constant is defined in the header file ocradlib.h.

The application should compare OCRAD_version and OCRAD_version_string for consistency. If the first character differs, the library code actually used may be incompatible with the ocradlib.h header file used by the application.

     if( OCRAD_version()[0] != OCRAD_version_string[0] )
       error( "bad library version." );

Library functions
*****************

These are the OCRAD library functions. In case of error, all of them return -1 or a null pointer, except OCRAD_open whose return value must be verified by calling OCRAD_get_errno before using it.

 -- Function: struct OCRAD_Descriptor * OCRAD_open ( void )
     Initializes the internal library state and returns a pointer that can only be used as the OCRDES argument for the other OCRAD functions, or a null pointer if the descriptor could not be allocated.

     The returned pointer must be verified by calling OCRAD_get_errno before using it. If OCRAD_get_errno does not return OCRAD_ok, the returned pointer must not be used and should be freed with OCRAD_close to avoid memory leaks.

 -- Function: int OCRAD_close ( struct OCRAD_Descriptor * const OCRDES )
     Frees all dynamically allocated data structures for this descriptor. After a call to OCRAD_close, OCRDES can no more be used as an argument to any OCRAD function.

 -- Function: enum OCRAD_Errno OCRAD_get_errno ( struct OCRAD_Descriptor * const OCRDES )
     Returns the current error code for OCRDES (see Library error codes).

 -- Function: int OCRAD_set_image ( struct OCRAD_Descriptor * const OCRDES, const struct OCRAD_Pixmap * const IMAGE, const bool INVERT )
     Loads IMAGE into the internal buffer. If INVERT is true, image levels are inverted (white on black). Loading a new image deletes any previous text results.

 -- Function: int OCRAD_set_image_from_file ( struct OCRAD_Descriptor * const OCRDES, const char * const FILENAME, const bool INVERT )
     Loads a image from the file FILENAME into the internal buffer. If INVERT is true, image levels are inverted (white on black). Loading a new image deletes any previous text results.

 -- Function: int OCRAD_set_utf8_format ( struct OCRAD_Descriptor * const OCRDES, const bool UTF8 )
     Set the output format to byte (if UTF8=false) or to utf8. By default ocrad produces byte (8 bit) output.

 -- Function: int OCRAD_set_threshold ( struct OCRAD_Descriptor * const OCRDES, const int THRESHOLD )
     Set binarization threshold for greymap or RGB images. THRESHOLD values between 0 and 255 set a fixed threshold. A value of -1 sets an automatic threshold. Pixel values greater than the resulting threshold are converted to white. The default threshold value if this function is not called is 127.

 -- Function: int OCRAD_scale ( struct OCRAD_Descriptor * const OCRDES, const int VALUE )
     Scale up the image in the internal buffer by VALUE. If VALUE is negative, the image is scaled down by -VALUE.

 -- Function: int OCRAD_recognize ( struct OCRAD_Descriptor * const OCRDES, const bool LAYOUT )
     Recognize the image loaded in the internal buffer and produce text results which can be later retrieved with the OCRAD_result functions. The same image can be recognized as many times as desired, for example setting a new threshold each time for 3D greymap recognition. Every time this function is called, the produced text results replace any previous ones. If LAYOUT is true, page layout analysis is enabled, probably producing more than one text block.

 -- Function: int OCRAD_result_blocks ( struct OCRAD_Descriptor * const OCRDES )
     Returns the number of text blocks found in the image, or 0 if no text was found. The returned value is usually 1, but can be larger if layout analysis was requested.

 -- Function: int OCRAD_result_lines ( struct OCRAD_Descriptor * const OCRDES, const int BLOCKNUM )
     Returns the number of text lines contained in the given text block.

 -- Function: int OCRAD_result_chars_total ( struct OCRAD_Descriptor * const OCRDES )
     Returns the total number of text characters contained in the recognized image.

 -- Function: int OCRAD_result_chars_block ( struct OCRAD_Descriptor * const OCRDES, const int BLOCKNUM )
     Returns the number of text characters contained in the given text block.

 -- Function: int OCRAD_result_chars_line ( struct OCRAD_Descriptor * const OCRDES, const int BLOCKNUM, const int LINENUM )
     Returns the number of text characters contained in the given text line.

 -- Function: const char * OCRAD_result_line ( struct OCRAD_Descriptor * const OCRDES, const int BLOCKNUM, const int LINENUM )
     Returns the line of text specified by BLOCKNUM and LINENUM.

 -- Function: int OCRAD_result_first_character ( struct OCRAD_Descriptor * const OCRDES )
     Returns the byte result for the first character in the image. Returns 0 if the image has no characters or if the first character could not be recognized. This function is a convenient short cut to the result for images containing a single character.

Library error codes
*******************

Most library functions return -1 or a null pointer to indicate that they have failed. But this return value only tells you that an error has occurred. To find out what kind of error it was, you need to verify the error code by calling OCRAD_get_errno.

Library functions don't change the value returned by OCRAD_get_errno when they succeed; thus, the value returned by OCRAD_get_errno after a successful call is not necessarily OCRAD_ok, and you should not use OCRAD_get_errno to determine whether a call failed. If the call failed, then you can examine OCRAD_get_errno.

The error codes are defined in the header file ocradlib.h.

 -- Constant: enum OCRAD_Errno OCRAD_ok
     The value of this constant is 0 and is used to indicate that there is no error.

 -- Constant: enum OCRAD_Errno OCRAD_bad_argument
     At least one of the arguments passed to the library function was invalid.

 -- Constant: enum OCRAD_Errno OCRAD_mem_error
     No memory available. The system cannot allocate more virtual memory because its capacity is full.

 -- Constant: enum OCRAD_Errno OCRAD_sequence_error
     A library function was called in the wrong order. For example OCRAD_result_line was called before OCRAD_recognize.

 -- Constant: enum OCRAD_Errno OCRAD_library_error
     A bug was detected in the library. Please, report it (see Problems).

Image format conversion
***********************

There are a lot of image formats, but ocrad is able to decode only three of them; pbm, pgm and ppm. In this chapter you will find command examples and advice about how to convert image files to a format that ocrad can manage.

.png
     Portable Network Graphics file. Use the command pngtopnm filename.png | ocrad.

     In some cases, like the ocrad.png icon, you have to invert the image with the -i option: pngtopnm filename.png | ocrad -i.

.ps
.pdf
     Postscript or Portable Document Format file. Use the command gs -sPAPERSIZE=a4 -sDEVICE=pnmraw -r300 -dNOPAUSE -dBATCH -sOutputFile=- -q filename.ps | ocrad.

     You may also use the command pstopnm -stdout -dpi=300 -pgm filename.ps | ocrad,

     but it seems not to work with pdf files. Also old versions of pstopnm don't recognize the -dpi option and produce an image too small for OCR.

.tiff
     TIFF file. Use the command

     tifftopnm filename.tiff | ocrad.

.jpg
     JPEG file. Use the command djpeg -greyscale -pnm filename.jpg | ocrad.

     JPEG is a lossy format and is in general not recommended for text images.

.pnm.gz
     Pnm file compressed with gzip. Use the command gzip -cd filename.pnm.gz | ocrad

.pnm.lz
     Pnm file compressed with lzip. Use the command lzip -cd filename.pnm.lz | ocrad

Algorithm
*********

Ocrad is mainly a research project. Many of the algorithms ocrad uses are ad hoc, and will change in successive releases as I myself gain understanding about OCR issues.

The overall working of ocrad may be described as follows:

1) Read the image.

2) Optionally, perform some transformations (cut, rotate, scale, etc).

3) Optionally, perform layout detection.

4) Remove frames and pictures.

5) Detect characters and group them in lines.

6) Recognize characters (very ad hoc; one algorithm per character).

7) Correct some ambiguities (transform l.OOO into 1.000, etc).

8) Optionally, apply one or more filters to the text.

9) Output text result.

Ocrad recognizes characters by its shape, and the reason it is so fast is that it does not compare the shape of every character against some sort of database of shapes and then chooses the best match. Instead of this, ocrad only compares the shape differences that are relevant to choose between two character categories, mostly like a binary search.

As there is no such thing as a free lunch, this approach has some drawbacks. It makes ocrad very sensitive to character defects, and makes difficult to modify ocrad to recognize new characters.

OCR results file
****************

Calling ocrad with option -x produces an OCR results file (ORF), that is, a parsable file containing the OCR results. The ORF format is as follows:

  - Any line beginning with # is a comment line.

  - The first non-comment line has the form source file FILENAME, where FILENAME is the name of the file being processed (- for standard input). This is the only line guaranteed to exist for every input file read without errors. If the file, or any block or line, has no text, the corresponding part in the ORF file will be missing.

  - The second non-comment line has the form total text blocks N, where N is the total number of text blocks in the source image.

For each text block in the source image, the following data follows:

  - A line in the form text block I X Y W H. Where I is the block number and X Y W H are the block position and size as described below for character boxes.

  - A line in the form lines N. Where N is the number of lines in this block.

For each line in every text block, the following data follows:

  - A line in the form line I chars N height H, where I is the line number, N is the number of characters in this line, and H is the mean height of the characters in this line (in pixels).

  - N lines (one for every character) in the form X Y W H; G[, 'C'V]..., where:

     X is the left border (x-coordinate) of the char bounding box in the source image (in pixels).

     Y is the top border (y-coordinate).

     W is the width of the bounding box.

     H is the height of the bounding box.

     G is the number of different recognition guesses for this character.

     The result characters follow after the number of guesses in the form of a comma-separated list of pairs. Every pair is formed by the actual recognised char C enclosed in single quotes, followed by the confidence value V, without space between them. The higher the value of confidence, the more confident is the result.

Running ./ocrad -x test.orf testsuite/test.pbm in the source directory will give you an example ORF file.

Reporting bugs
**************

There are probably bugs in ocrad. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.

If you find a bug in GNU Ocrad, please send electronic mail to bug-ocrad@gnu.org. Include the version number, which you can find by running ocrad --version.