Foreground/background balance options are the most important. They determine the tradeoff between oversegmentation (putting too much in the foreground) and under-segmentation (putting too much in the background). Their values typically range from 0 (all foreground) to 100 (all background).
The pix-filter-level parameter defines the balance between foreground and background pixels. A value of 0 indicates that the maximum number of pixels should be placed into the foreground, and a value of 100 indicates that the maximum number of pixels should be placed in the background. A value of 25 is the optimum setting for most 300 dpi documents.
Increase to remove very small shapes from the foreground, smooth out edges, and remove half-toning. Decrease to avoid dropping of characters. This is the most efficient option level to manage the foreground/background tradeoff; always try to tune it first.
Option type
Quality slider
Range
0..100
0
Indicates that the maximum number of pixels should be placed into the
foreground
100
Indicates that the maximum number of pixels should be placed in the
background
Default
25
Optimization
Default chosen to avoid dropping of characters (it is only acceptable to
drop dots). Generally, the optimal pix-filter-level corresponds to the smaller
file size, but the optimum varies by document. For most documents it is 50, but
it is 25 for 200dpi documents.
Command
line
Yes
Expert
The pix-filter-level parameter corresponds to a penalty added each time there is a transition from foreground to background (if two contiguous pixels belong to the same layer, there is no penalty). Reasons why it would decide not to segment foreground are that there are too many such transitions, or that the difference in luminance is not large enough.
Impact on
text
A value lower than 25 may improve segmentation of characters whose color is close to that of the background
Impact on
photos and images
Higher values avoid small speckles to be segmented
The threshold-level parameter interacts with the pix-filter-level and shape-filter-level options to determine the balance between foreground and background. A threshold level of 50 gives equals room for the foreground and the background to fluctuate around their local average value. In practice a threshold of 75 is preferred, which gives much more room for the background to fluctuate.
It is advisable to decrease the threshold when parts of characters are lost in the background.
Option type
Quality slider
Range
0..100
0
Maximum foreground
100
Maximum background
Default
75
Command
line
Yes
Interacts
with
pix-filter-level, shape-filter-level
Expert
The threshold-level parameter corresponds to the difference
between the foreground and the background standard deviations used by the foreground/background
algorithm, as illustrated in the table below. Take for example a pixel whose
luminance is between the background average and the foreground average. Its
relative luminance is a number between 0 (background) and 100 (foreground). This
pixel is classified as foreground only if its relative luminance is larger than
its threshold-level. In other words, pixels with luminance<threshold are
background, while pixels with luminance>threshold are foreground. As a
consequence, high threshold levels favor the background in the transition
zones.
threshold-level |
bg-stdev |
fg-stdev |
bg-stdev/fg-stdev |
comment |
0 |
50 |
250 |
0.2 |
untested |
25 |
100 |
200 |
0.5 |
Poorly scanned
documents |
50 |
150 |
150 |
1 |
Manuscripts |
75 |
200 |
100 |
2 |
Default |
100 |
250 |
50 |
5 |
untested |
Impact on
text
Lower values may improve segmentation when characters are very thin. Higher values avoid a noisy background to be segmented with the foreground.
Impact on
photos and images
Higher values minimize unwanted segmentation of some parts of the image.
The shape-filter-level parameter specifies the balance between placing ambiguous shapes in the foreground and background layers. An ambiguous shape has both foreground and background layer characteristics. During segmentation, this shape may appear in the inappropriate layer because Document Express must "guess" where to place it. Objects in the foreground have lower color resolution, so ambiguous shapes placed in this layer may stand out unnaturally. (Eyebrows, for example, will appear as if someone drew them in by hand.) Objects in the background have lower overall resolution, so the edges of ambiguous shapes placed in this layer will appear blurry.
Assign the shape-filter-level option a whole number value between 0 and 100. A value of 0 indicates that every ambiguous shape should be placed in the foreground. A value of 100 indicates that every ambiguous shape should be placed in the background.
Determining which ambiguous shapes appear better in the foreground and background layers requires experimentation. If you are unsatisfied with the appearance of an encoded image, encode it again using the shape-filter-level option with different values to find the appropriate balance between foreground and background shapes.
Option type
Quality slider
Range
0..100
0
Maximum foreground (filter is off)
100
Maximum background
Default
50
Optimization
Because of tremendous improvements in the foreback module, thus parameter is no longer so critical. Has not been optimized recently.
Command
line
No
Interacts
with
Expert
Shape (i.e., connected components) with a score higher than this filter
level are kept. This score is the difference of two coding costs:
· Coding cost of everything as a smooth background
· Coding cost of putting the shape in the foreground: code for the shape and the color separately
This parameter has very little influence over text and drawings. It is mostly useful to minimize the segmentation of photos and images, at very little risk for text. In this case, one should increase it from its default of 50 to something like 75.
In some very rare cases (to segment very small dots), there would be good reasons to decrease this parameter.
Impact on
text
Minimal
Impact on
photos and images
Higher values improve filtering of speckles
The inhibit-foreback-level parameter corresponds to an a priori cost added to the decision to perform foreground/background separation (vs. deciding that a given block should not be segmented). The segmentation algorithm calculates a "cost" of segmenting a block between foreground and background. If the cost is too high, the block is not segmented; therefore, adding to the cost decreases the likelihood that the block will be segmented.
It is advisable to increase the inhibit-foreback level to remove isolated speckles.
Option type
Quality slider
Range
0..100
0
Maximum text
100
Maximum background
Default
40
Command
line
No
Impact on
text
Lower values may improve segmentation of very low contrasted text
Impact on
photos and images
Lower values may result in undesired segmentation
The inversion-level parameter controls how the Document Express segmenter decides that the text is inverted (white text on a black background). A value of 0 indicates that all text is black on a white background. A value of 100 indicates that all text is white on a black background. The default is 25.
Option type
Quality slider
Range
0..100
0
No inversion
100
All inversion
Default
25
Command
line
Yes
Interacts
with
Render-size: many of the inversion heuristics rely on render-size when
deciding to invert large foreground shapes
Impact on
text
Inverted test is the only reason we need this parameter
Impact on
drawing
Very hard to control: set inversion-level to 0 if there is no need for
inversion
Impact on
photos and images
Again, may produce strange artifacts