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Image Processing Features

The automatic processing will sometimes fail with the standard parameter settings. It is always a good idea to manually control the results of crass and adjust the parameter settings according to the requirements of the input.

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 deskewing (optional)
    1. save deskewed image file
    2. load deskewed image file
  3. find the linecoordination informations.
  4. compute masks
  5. cropping the areas under the masks
  6. save single segments and debuginformation (optional)
  7. load segments
  8. splice segments to a new image
  9. save output image file

Processing Order

Disabling Processing Steps

Each processing step can be disabled individually by a corresponding --xxx option (where xxx stands for the feature to disable).

./crass (...options...) --deskew

This will disable the deskew.

Multiprocessing

You can fully leverage multiple processors on a given machine with the feature "parallel" and the number of process. Recommended is the number of available processors - 1 (which is used by the OS).

./crass (...options...) --parallel 3  # for 4 processors

Threshold - Sauvola

Sauvola threshold is a local thresholding technique that is useful
for images where the background is not uniform, especially for text
recognition. Instead of calculating a single global threshold
for the entire image, several thresholds are calculated for every pixel by
using specific formulae that take into account the mean and standard
deviation of the local neighborhood (defined by a window centered around
the pixel).

In the original method a threshold T is calculated for every pixel in the
image using the following formula:

[T = m(x,y) * (1 + k * ((s(x,y) / R) - 1))] 2

where m(x,y) and s(x,y) are the mean and standard deviation of pixel (x,y)
neighborhood defined by a rectangular window with size w times w centered
around the pixel. k is a configurable parameter that weights the effect of
standard deviation. R is the maximum standard deviation of a greyscale image.

Linecoord Analyse

The different linetypes to look for

hor -> Horizontal line ver -> Vertical line

xxxver
xxxhor

(where xxx stands for the feature to set)

The analyse of the Linecoordination can be influenced with the following parameters:

minwidth of the linetype + factor of the image width
maxwidth of the linetype + factor of the image width
minheight of the linetype + factor of the image height
maxheight of the linetype + factor of the image height

./crass (...options...) --minwidthver 0.5

This will set the minwidth of the vertical lines to find to 50% of the image width.

Mask options (search area) for the different linetypes:

Horizontal:

minheighthormask + factor of the image height
maxheighthormask + factor of the image height

0.0 -> 0% height -> top of the image
1.0 -> 100% height -> bottom of the image

./crass (...options...) --minheightplmask 0.0 --maxheightplmask 0.3

This will set the mask (search area) between 0% (top of the image) and 30% of the images height.

Vertical:

minwidthver + factor of the image width
maxwidthver + factor of the image width

0.0 -> 0% width -> left side of the image
1.0 -> 100% width -> right side of the image

./crass (...options...) --minwidthvermask 0.3 --maxwidthvermask 0.7

This will set the mask (search area) between 30% and 70% of the image.

Setting Clipping Masks

Clipping mask mark the area to be crop out. This area will be compute automatically but the user can set some extra parameters.

Add startheight

Addstartheight expands the mask of either a&b or c to the top of the image.

 ./crass (...options...) --addstartheightab 50
 ./crass (...options...) --addstartheightc 50

This will expand the area 50 pixels to the top.

Add stopheight

Addstopheight expands the mask of either a&b or c to the bottom of the image.

 ./crass (...options...) --addstopheightab 50
 ./crass (...options...) --addstopheightc 50

This will expand the area 50 pixels to the bottom.

Splice Pattern

Splicetypes

There are 5 types of segments:

  • h = header
  • a = left side separated by a vertical line
  • b = right side separated by a vertical line
  • c = space between header and vertical line or vertical line and another vertical line
  • f = footer
./crass (...options...) --splicetypes a,b,f

Only the segments of the types a, b and f will be considered in the splicing process. (Default. a,b and c)

Splicemaintype

The splicemaintype starts or ends (depending on the splicemaintypestart-Option) the splicepattern.

Splicemaintypestop

The splicemaintypestop set the maintype to the end of each segment not the start (default).

./crass (...options...) --splicetypes a,b,h
./crass (...options...) --splicemaintype h

Only the segments of the types a, b and h will be considered in the splicing process. The pattern will start with a h-segment. There can be several a and b segments in between.