unapper

[wanghaisheng]

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[wanghaisheng]

Basic Concepts

The terminology to describe unpaper makes heavy use of the paper
metaphor, because the software is mainly intended for post-processing
scanned images from printed paper documents.

Sheets and Pages

The very basic object unpaper operates on is a sheet. A sheet
is an initially blank image in the computer's memory. Think of a
sheet as an initially empty piece of paper on which something will
be printed later.

To do something useful with a sheet, you will at least want to place
one page onto a sheet. A page is a logical unit of a document
which takes up a rectangular area on a sheet. In the most simple
case, one sheet carries exactly one page, in other cases
(e.g. when using a double-page layout) there can be multiple pages
placed on one sheet.

Input and Output Image Files

unpaper can process either double-page layout scans or
individually scanned pages. It is up to the user's choice whether an
image-file carries a single page or a whole sheet with two
pages. The program can be configured to either join individual
image-files as multiple pages onto one sheet, or split sheets
containing multiple pages into several output image-files when
saving the output.

By default, unpaper places one input image-file onto a sheet,
and saves one output image-file per sheet. Alternatively, the
number of input or output image-files per sheet can be set to two
using the --input-pages 2 or --output-pages 2 options.

If two image-files are specified as input, they will successively be
placed on the left-hand half and the right-hand half of the
sheet.

In the same way, if two image-files are specified as output, the
sheet will be split into two halves which get saved as individual
files.

The default value both for --input-pages and --output-pages is 1.

File Formats

The image-file formats accepted by unpaper are those that
libav can handle. In particular it supports the
whole PNM-family: PBM, PGM and PPM. This ensures
interoperability with the SANE tools
under Linux. Support for TIFF and other complex file formats is not
guaranteed.

The output format is restricted to the PNM family of formats, and
conversions to other formats need to happen with tools such as
pnmtopng, pnmtotiff or pnmtojpeg. Alternatively you can use the
convert tool from ImageMagick.

Layouts and Templates

Built-In Layout-Templates

Layouts are the linking concept between physical sheets and
logical pages. A layout determines a set of rectangular areas at
which pages (or other parts of content) appear on a sheet. The
most common and simple layouts generally used are the single-page
layout (one page covers the whole sheet), and the double-page
layout (two pages are placed on the left-hand-side and the
right-hand-side of the sheet).

unpaper provides basic layout templates for the above types.
There are 2 layout templates built in, a third one deactivates any
template:

• single
• double
• none

A layout template is chosen by using the option --layout, e.g.

unpaper --layout double input%03d.pbm output%03d.pbm

Choosing a template with the --layout option is equivalent to
specifying a set of other options, e.g. setting --mask-scan-point.
In order to combine a template with other options, make sure that
the more specific options appear behind the --layout option, in
order to overwrite the template settings.

The default template is single, use none to deactivate this.

Note: A layout is completely independent from the number of
image-files used as input or output. That means, you can either
specify --layout double together with a single input image-file
(in cases where the input image-file already contains two scanned
pages in a double-page layout), or use it together with an
--input-pages 2 setting, in order to join two individually scanned
pages on one sheet.

Complex Layouts

Besides the built-in fixed templates, any kind of complex layout
can be handled by manually specifying either mask-scan-points using
the --mask-scan-point option, or setting masks at fixed
coordinates using the --mask option. Both the --mask-scan-point
and the --mask option may occur any number of times, in order to
declare as many masks in the layout as desired. See below for a
further explanation on masks.

Processing Multiple Files

In many cases, especially when post-processing scanned books, there
will be several input image-files to process in sequence within a
single run of unpaper, and several output image-files to be
generated. Processing of multiple files in a batch job is supported
through the use of wildcards in filenames, e.g.:

unpaper (...options...) input%03d.pbm output%03.pbm

This will successively read images from files input001.pbm,
input002.pbm, input003.pbm etc., and write output to the files
output001.pbm, output002.pbm, output003.pbm etc., until no more
input image-files with the current index number are available.

Using a wildcard of the form %0nd will replace each occurrence of
the wildcard with an increasing index number, by default starting with
1 and counting up by 1 each time another files gets loaded. n
denotes the number of digits that the replaced number string is
supposed to have, and the 0 requests leading zeros. Thus "%03d" will
get replaced with strings in the sequence 001, 002, 003
etc. This way, a sequence of images named e.g. input001.pgm,
input002.pgm, input003.pgm... can be specified. There are two
separate index counters for input and output files which get increased
independently from each other.

Wildcards in filenames are also useful when combining a sequence of
individual pages onto double-page layouted sheets, or when
splitting double-page layouted sheets into individual output
files. When using two input or output image-files (by specifying
--input-pages 2 or --output-pages 2) the index number replaced for
the wildcard will generally not be the same as the sheet number in
the processing sequence, but will grow twice as fast.

The following example will combine single-page image-files onto a
double-page layout sheet:

unpaper -n --input-pages 2 singlepage%03d.pgm output%03d.pgm

This joins the input images singlepage001.pgm and
singlepage002.pgm on output001.pgm, singlepage003.pgm and
singlepage004.pgm on output002.pgm, and so on. Note that due to
the use of option -n (short for --no-processing), the images are
simply copied onto the left-hand half and the right-hand half of the
sheet without any processing regarding layout, mask-detection
etc.

Using multiple input image-files by setting --input-pages 2 is
independent from any layout possibly specified with the --layout
option. However, in order to use unpaper's post-processing features
for more than simply joining two image-files to one, you will most
likely want to combine the use of --input-pages 2 with the --layout double option, as in:

unpaper --layout double --input-pages 2 (...other options...) singlepage%03d.pgm output%03d.pgm

Similarly, it is also possible to split up a sheet into several
image-files when saving. The following line would be used to split
up a sequence of double-page layouted sheets into a sequence of
single-page output images, including full image processing (applying
masking, deskewing, border-aligning etc., see below) in order to
make sure that the pages in the double-page layout are really
placed fully on the left-hand half and the right-hand half of the
sheet before the sheet gets split up:

unpaper --layout double (...options...) --output-pages 2 doublepage%03d.pgm singlepage%03d.pgm

By default, processing of multiple sheets starts with sheet number
1, and also with input and output image-files number 1. unpaper
will run as long as input image-files with the current index number
can be found. If no more input files are available, processing stops.

Adjusting Indices

In order to start with a different sheet index, the --start-sheet
option can be set. Likewise, setting --end-sheet specifies a fix
sheet number that will the last one processed, even if more
input-files are available.

Using --sheet, a single sheet or a set of specific sheet numbers
to be processed can be specified. For example:

unpaper --sheet 7,12-15,31 --input-pages 2 (...options...) input%03d.pgm output%03d.pgm

This would generate the output-files output007.pgm, output012.pgm,
output013.pgm, output014.pgm, output015.pgm and output031.pgm,
reading input from the same files as if a whole sequence of sheets
and pages starting with index 1 had been processed, i.e. reading the
files input013.pgm and input014.pgm for sheet 7, input023.pgm
and input024.pgm for sheet 12, and so on.

To prevent some sheets from being processed (i.e., remove them from
the sequence), the option --exclude can be used. Note that this is
different from option --no-processing or -n, which still would
generate the output files but without applying any image processing to
them.

The input and output index numbers to start with can be adjusted using
the options --start-input and --start-output. These values apply
to the wildcard replacement in filenames only and are independent from
the sheet numbering. In other words, setting these options specifies
an offset at which the file numbering starts relative to
sheet 1. For example:

unpaper --input-pages 2 (...options...) --start-input 7 input%03d.pgm output%03d.pgm

These settings would cause the input-files input007.pgm and
input008.pgm to be used for sheet 1, input009.pgm and
input010.pgm for sheet 2, and so on. The default value for both
options is 1.

File-Sequence Patterns

More sophisticated file-sequence patterns can be specified using
the --input-file-sequence or --output-file-sequence options. In
cases where the input files are named after a pattern like
e.g. left01.pbm, right01.pbm, left02.pbm, right02.pbm etc.,
the use of --input-pages 2 together with --input-file-sequence left%02d.pbm right%02d.pbm will load to the desired images. The index
counter with which the wildcards in the filenames get replaced is
increased every time the file-sequence pattern is iterated through,
it will not be increased after each single replacement of a wildcard.

Note that it would also be possible to use file-sequence patterns of
different lengths than the number of pages per sheet. In case an
input file-sequence like e.g. a%d.pbm b%d.pbm c%d.pbm is specified
together with --input-pages 2, the input image-files used for the
first sheet would be a1.pbm and b1.pbm, the input image-files
used for the second sheet would be c1.pbm and a2.pbm (!), for
the third sheet they would be b2.pbm and c2.pbm, and so on. It's
up to the user whether it makes sense to use file-sequence patterns
of different length than the corresponding number of input
image-files or output image-files per sheet.

Specifying a filename as the very last argument on the command-line is
equivalent to using --output-file-sequence <file> (a sequence of
length 1), specifying a filename as the last-but-one argument on the
command line is equivalent to using --input-file-sequence <file>.

Inserting Blank Content

Input file-sequences may be forced to use completely blank images at
some index positions. The --insert-blank option allows to specify
one or more input indices at which no file is read, but instead a
blank image is inserted into the sequence of input images. The input
image that would have been loaded at this index position in the
sequence will be used at the following non-blank index position
instead, thus the following indices get shifted to make room for the
blank image inserted.

The --replace-blank option also allows to insert blank images into
the sequence, but it suppresses the images that would have been loaded
at the specified index positions and ignores them. No index positions
get shifted to make room for the blank image.

Masks

Masks are rectangular areas on a sheet that are affected by
several of the processing steps unpaper performs. Although there
may be as many masks on a sheet as desired, in most cases it will
be useful to operate with either one or two masks per sheet
only. A single-page layout would operate on only one mask covering
the whole page, a double-page layout would make use of two
masks, one placed somewhere in the left-hand half of a sheet, the
other somewhere in the right-hand half.

Automatic Mask-Detection

Masks can be set directly by specifying pixel coordinates using the
--mask option, but in most cases it is desirable to detect masks
automatically. Automatic mask-detection allows input images to
contain content which is not perfectly placed at fix areas, but
probably differs slightly in position from sheet to sheet (which
is usually the case when books are scanned or photocopied manually).

Automatic mask-detection uses a starting point somewhere on the
sheet called mask-scan-point, which marks a position estimated
to be somewhere inside the mask to be detected. (When detecting
masks that cover a whole page, it is useful to place the
mask-scan-point right in the center of the sheet's half on which
the page appears.) Beginning from the mask-scan-point, the image
content is virtually scanned in either the two horizontal directions
(left and right), or the two vertical directions (up and down), or all
four directions, until no more dark pixels are found which means an
edge of the mask is considered to have been found.

Several parameters control the process of mask-detection. At first,
mask-scan-points to start detection at get specified either using
the --layout option (which automatically sets one mask-scan-point
for single-page layouts, and two mask-scan-points for double-page
layouts) or manually with the option --mask-scan-point.

Mask-detection is performed by the use of a 'virtual bar' which
covers an area of the sheet under which the number of dark pixels is
counted. The 'virtual bar' is moved towards the directions specified
by --mask-scan-direction. (Those directions not given via
--mask-scan-direction will use up the whole sheet's size in these
directions for the detected result.)

While moving the 'virtual bar' the number of dark pixels below it is
continually compared to the number that has been counted at the very
first position of the 'virtual bar' above the mask-scan-point when
detection started. Once the number of dark pixels drops below the
relative value given by --mask-scan-threshold, mask-detection
stops and an edge of the mask is considered to have been found.

The width of the 'virtual bar' can be configured using the
--mask-scan-size option, the length of it by setting
--mask-scan-depth. Adjusting the 'virtual bar's' width can help to
fine-tune the process of mask detection according to the content that
is being scanned. The wider the 'virtual bar' is, the more tolerant
the detection process becomes with respect to small gaps in the
content (which is e.g. needed if a page is made up of multiple
columns). However, if the 'virtual bar' is too wide, detection might
not stop properly when a mask's edge should have been found.

Mask-detection can be disabled using the --no-mask-scan option,
optionally followed by the sheet numbers to disable the filter for.

Mask-Centering

Masks that have been automatically detected or manually set will be
used for several further processing steps. At first they provide the
basis for properly centering the content on the corresponding page
area on the sheet.

This allows unpaper to automatically correct imprecise positions of
page content in scanned sheets and shift the content to a
normalized position. Especially when processing multiple pages, this
leads to more regular positions of pages in the sequence of
resulting sheets.

Mask-centering can be suppressed using --no-mask-center,
optionally followed by the sheet numbers to disable the filter for.

Borders

Unlike masks, borders are detected by starting at the outer
edges of the sheet (or left/right halves of the sheet, in a
double-page layout), and then scanning towards the middle until some
content-pixels are reached. Again, a 'virtual bar' is used for
detection, the width of which can be set using the option
--border-scan-size, and the step-distance with which to move it by
setting the option --border-scan-step. The option
--border-scan-threshold determines the maximum absolute number of
pixels which are tolerated to be found below the 'virtual bar' until
border-detection stops and one edge of the border area is
considered to have been found.

Border-Aligning

Borders serve two different purposes: First, the area outside the
detected border on the sheet will be wiped out, which is another
mechanism to clean the outer sheet boundary from unwanted pixels.

Second, a detected border can optionally be aligned towards one edge
of the sheet. Border-aligning means shifting the area inside the
border towards one edge of the sheet. The edge towards which to
shift the border is specified with the option --border-align.
Additionally, a fixed distance from the edge is kept, which can be set
via --border-margin.

This way, it can be assured that e.g. all pages of a scanned book
regularly start 2 cm below the upper sheet edge.

Note that border-aligning is not performed by default, it needs to
be explicitly activated by setting the option --border-align to one
of the edge names top, bottom, left or right, and by setting
--border-margin to the desired distance which is to be kept to this
edge.

Use --no-border-scan to disable border-detection, or
--no-border-align to prevent border-aligning on specific sheets,
both optionally followed by the sheet numbers to disable the filters
for.

Size Values

Whenever an option expects a size value, there are three possible
ways to specify that:

• as absolute pixel values, e.g. --sheet-size 4000,3000
• as length measurements on one of the scales cm, mm, in,
  e.g. --size 30cm,20cm or also --size 10in,250mm
• using one of the following size names:
  • a5
  • a4
  • a3
  • letter
  • legal
  • a5-landscape (horizontally oriented A5)
  • a4-landscape (horizontally oriented A4)
  • a3-landscape (horizontally oriented A3)
  • letter-landscape (horizontally oriented letter)
  • legal-landscape (horizontally oriented legal)
    Examples: --sheet-size a4, --post-zoom letter``-landscape

Using one of the last two ways, length measurements get internally
converted to absolute pixel values based on the resolution set via the
option --dpi. If the default of 300 DPI should be changed, this
option must appear on the command line before using a length
measurement value. --dpi may also appear multiple times, e.g. if the
size values of the output image(s) should be based on a different
resolution than those of the input file(s).

Note that using the --dpi option will have no effect on the
resolution of the image-files that get written as output. (The PNM
format is not capable of storing information about the image
resolution.) The value set via --dpi will only have effect on
unpaper's internal conversion of length measurements to absolute
pixel values when size values are specified using length
measurements or size names.

[wanghaisheng]

Image Processing Features

Blackfilter

Sometimes it is desirable to automatically remove large black areas
which originate from bad photocopies or other optical influences. The
blackfilter can help to find large areas of black and wipe them
out automatically.

Be careful with pictures in scanned documents, especially with
diagrams. Some diagrams intentionally contain large areas of dark
color, which might be affected by automatic wipe-out of the
blackfilter. In order to prevent actual page content from being
wiped out, the option --blackfilter-scan-exclude allows to specify
areas on the sheet which should not be taken into account by the
blackfilter. When using one of the default layout templates set
via the --layout option, the inner area of each page will
automatically be excluded from black-filtering.

The blackfilter can be disabled by the option --no-blackfilter,
optionally followed by the sheet numbers to disable the filter for.

Noisefilter

The noisefilter removes small clusters of pixels ("noise") from
the sheet. The maximum pixel-size of clusters to be removed can be
set via --noisefilter-intensity. This value must not be chosen too
high in order not to remove relevant elements of page content,
e.g. normal text-points ("."). As a consequence, this option might
have to be adjusted on images with a low scan resolution.

Disable with --no-noisefilter, optionally followed by the sheet
numbers to disable the filter for.

Blurfilter

The blurfilter removes "lonely" clusters of pixels, i.e. clusters
which have only very few other dark pixels in their neighborhood.

The size of the neighborhood to be searched and the amount of other
dark pixels accepted in the neighborhood below which the area gets
wiped out can be adjusted with the options --blurfilter-size,
--blurfilter-step and --blurfilter-intensity. Additionally,
--blurfilter-step also determines the step-size with which the
neighborhood-area is moved across the image while filtering.

Disable with --no-blurfilter, optionally followed by the sheet
numbers to disable the filter for.

Grayfilter

The grayfilter removes areas which are gray-only, that means it
wipes out all those areas which do not contain a maximum relative
amount of non-dark pixels. The size of the local area the grayfilter
operates on can be set using --grayfilter-size, and the granularity
of detection is controlled via --grayfilter-step. The maximum
relative amount of non-dark pixels that are still considered to be
deletable can be set using --grayfilter-threshold.

Be careful with the grayfilter when processing color scans, because
any bright color might be considered as gray and be wiped out. It
might be a good idea to disable the grayfilter when processing color
scans.

Disable with --no-grayfilter, optionally followed by the sheet
numbers to disable the filter for.

Deskewing (Auto-Straightening)

The deskewing performed by unpaper is actually a rotation to
automatically straighten rectangular content areas on the sheet. It
is applied to any mask that has been found during mask-detection
or that has been set directly via the --mask option.

The algorithm that detects the angle of skew works better the more
regular and solid the edges of the area's content are. It works as
follows: A 'virtual line' is moved from the outside of one edge inside
the rectangular area. This happens several times, gradually changing
the rotation of the 'virtual line'. (Called 'virtual', because there
is of course no visible line drawn in the image.) The algorithm will
count the number of dark pixels along the line as it is virtually
moved.

Some parameters control the the size of the 'virtual line' and its
movement: --deskew-scan-size: the height/width of the 'virtual line'
used for scanning (the length of the line at rotation angle 0)
--deskew-scan-range: the absolute value of degrees between the
negative and positive value of which the line will be rotated (i.e.,
the default value 5.0 will cause the 'virtual line' to be rotated in
several small steps between -5.0 degrees and 5.0 degrees).
--deskew-scan-step: the step size with which to iterate between the
bounds set by --deskew-scan-range (I.e., a value of 0.1 will lead to
the virtual line being successively rotated with
0.0, 0.1, -0.1, 0.2, -0.2, 0.3, -0.3 ... degrees.)

At each of these rotation steps, the following is done: The rotated
'virtual line' gets moved (again 'moved virtually') towards the center
of the rectangular area on which detection gets performed. Movement
is performed pixel by pixel, it starts with the line completely
outside the rectangular area, not yet reaching inside the area. At
each movement-step, the number of dark pixels covered by the virtual
line is counted and is accumulated as the total sum of dark
pixels. For each rotation angle at which this is done, the maximum
difference in the accumulated sum of dark pixels occurring between a
previous movement-step and the next one gets calculated. The rotation
angle for which this maximum difference becomes maximal will be the
rotation angle detected for deskewing.

The relative amount of dark pixels to accumulate before shifting the
'virtual line' is stopped (and continued with the next rotation-step)
is given by --deskew-scan-depth. This value is relative to the
number of pixels that the 'virtual line' covers in total, i.e. for
the default deskew-scan-size of 1500 and the default deskew-scan-depth
of 0.66, shifting at each rotation step stops after 1000 dark pixels
have been counted in sum (or, if not enough pixels are met, when the
'virtual line' has reached the center of the rectangular area).

Sometimes, trying out different deskew-scan-depth values, either lower
than the default of 0.66, or higher, can noticeably increase detection
quality. Which value is best is merely coincidental, depending on the
shape of the outer edges of each very first character in each line of
a text area.

The above described the detection process starting at one single edge
of a 4-edged rectangular area (e.g. the left edge, as displayed in the
above image). However, the overall rotation angle detection uses
results of up to all four edges. Which edges to use can be specified
by --deskew-scan-direction. The final rotation angle will then be
the average value of all rotation angles detected at each
edge. Usually, the individually detected values can be expected to be
almost the same at each edge, if the rectangular area to be deskewed
has a regular shape. If, however, the individual values differ too
much, it can be concluded that something went wrong with the
detection, and no deskewing should be performed. (E.g., if the
rotation at the left edge appears to be -0.5 degrees, but at the right
edge results in 1.9 degrees, one should better not use the average
value, because with that big difference something seems to have gone
wrong with the detection.) So, before using the average of all
individually detected values, their statistical standard-deviation is
calculated, which is √((a-average)² + (b-average)² + ...).

If the standard-deviation among the detected angles exceeds the value
specified by --deskew-scan-deviation, the total result is considered
to be wrong and no deskewing is performed.

Deskewing can be disabled with --no-deskew, optionally followed by
the sheet numbers to disable it for.

Processing Order

Processing of the filters and auto-corrections is performed in a fixed
order according to the following sequence:

1. load image file(s)
2. perform pre-rotate, pre-mirror etc. actions on the
   individual input files (if specified)
3. place on the sheet (multiple input-files are placed as tiles),
   auto-determine sheet size by the size of the input image-file(s)
   if not specified explicitly
4. apply noisefilter and blurfilter to remove small bits of
   unwanted pixels
5. apply blackfilter and grayfilter to remove larger areas of
   unwanted pixels
6. detect masks starting from specified mask-scan-points
7. perform deskewing on each detected or directly specified
   mask
8. re-detect masks again to get precise masks after deskewing
9. center masks on the corresponding page's area on the sheet
10. perform border-detection
11. align the detected borders
12. save output image file(s), possibly perform post-rotate,
    post-mirror etc. actions on the individual output files before
    saving

Disabling Processing Steps

Each processing step can be disabled individually by a corresponding
--no-xxx option (where xxx stands for the feature to disable,
e.g. --no-grayfilter, --no-mask-scan etc.). If such an option is
followed by a sheet number, or a comma-separated list of multiple
sheet numbers, the filter gets disabled only for those sheets
specified. Otherwise (if no sheet number follows), the filter is
disabled for all sheets. Instead of specifying individual sheet
numbers, also a range of numbers can be given, e.g. "10-20" to
represent all sheet numbers between 10 and 20. Example:

unpaper (...options...) --no-blackfilter 3,15,21-28,40 (...)

This will disable the blackfilter on the sheets 3, 15, 21, 22, 23,
etc. until 28, and 40.