Operator Reference
dyn_threshold (Operator)
dyn_threshold — Segment an image using a local threshold.
Signature
dyn_threshold(OrigImage, ThresholdImage : RegionDynThresh : Offset, LightDark : )
Description
dyn_threshold selects from the input image those
regions in which the pixels fulfill a threshold condition. Let
g_{o} = g_{OrigImage}, and g_{t} =
g_{ThresholdImage}. Then the condition for
LightDark = 'light' is:
For LightDark = 'dark' the condition is:
For LightDark = 'equal' it is:
Finally, for LightDark = 'not_equal' it is:
Typically, the threshold images are smoothed versions of the
original image (e.g., by applying mean_image,
binomial_filter, gauss_filter, etc.). Then the
effect of dyn_threshold is similar to applying
threshold to a highpass-filtered version of the original
image (see highpass_image).
With dyn_threshold, contours of an object can be extracted,
where the objects' size (diameter) is determined by the mask size of
the lowpass filter and the amplitude of the objects' edges:
The larger the mask size is chosen, the larger the found regions
become. As a rule of thumb, the mask size should be about twice the
diameter of the objects to be extracted. It is important not to set
the parameter Offset to zero because in this case too
many small regions will be found (noise). Values between 5 and 40
are a useful choice. The larger Offset is chosen, the
smaller the extracted regions become.
All points of the input image fulfilling the above condition are
stored jointly in one region. If necessary, the connected
components can be obtained by calling connection.
Attention
If Offset is chosen from -1 to 1
usually a very noisy region is generated, requiring large storage.
If Offset is chosen too large (> 60, say) it may happen
that no points fulfill the threshold condition (i.e., an empty
region is returned). If Offset is chosen too small (<
-60, say) it may happen that all points fulfill the threshold
condition (i.e., a full region is returned).
Execution Information
- Multithreading type: reentrant (runs in parallel with non-exclusive operators).
- Multithreading scope: global (may be called from any thread).
- Automatically parallelized on tuple level.
- Automatically parallelized on domain level.
Parameters
OrigImage (input_object) singlechannelimage(-array) → object (byte / int2 / uint2 / int4 / real)
Input image.
ThresholdImage (input_object) singlechannelimage(-array) → object (byte / int2 / uint2 / int4 / real)
Image containing the local thresholds.
RegionDynThresh (output_object) region(-array) → object
Segmented regions.
Offset (input_control) number → (real / integer)
Offset applied to ThresholdImage.
Default: 5.0
Suggested values: 1.0, 3.0, 5.0, 7.0, 10.0, 20.0, 30.0
Value range:
-255.0
≤
Offset
≤
255.0 (lin)
Minimum increment: 0.01
Recommended increment: 5
LightDark (input_control) string → (string)
Extract light, dark or similar areas?
Default: 'light'
List of values: 'dark', 'equal', 'light', 'not_equal'
Example (HDevelop)
* Looking for regions with the diameter D mean_image(Image,Mean,D*2+1,D*2+1) dyn_threshold(Image,Mean,Seg,5,'light') connection(Seg,Regions)
Complexity
Let A be the area of the input region. Then the runtime complexity is O(A).
Result
dyn_threshold returns 2 (
H_MSG_TRUE)
if all parameters are
correct. The behavior with respect to the input images and output
regions can be determined by setting the values of the flags
'no_object_result', 'empty_region_result', and
'store_empty_region' with set_system.
If necessary, an exception is raised.
Possible Predecessors
mean_image,
smooth_image,
binomial_filter,
gauss_filter
Possible Successors
connection,
select_shape,
reduce_domain,
select_gray,
rank_region,
dilation1,
opening,
erosion1
Alternatives
See also
Module
Foundation