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NAME

       i.segment  - Identifies segments (objects) from imagery data.

KEYWORDS

       imagery, segmentation, classification, object recognition

SYNOPSIS

       i.segment
       i.segment --help
       i.segment  [-dwap]  group=name[,name,...] output=name  [band_suffix=name]  threshold=float
       [radius=float]   [hr=float]    [method=string]    [similarity=string]    [minsize=integer]
       [memory=integer]    [iterations=integer]    [seeds=name]   [bounds=name]   [goodness=name]
       [--overwrite]  [--help]  [--verbose]  [--quiet]  [--ui]

   Flags:
       -d
           Use 8 neighbors (3x3 neighborhood) instead of the default 4 neighbors for each pixel

       -w
           Weighted input, do not perform the default scaling of input raster maps

       -a
           Use adaptive bandwidth for mean shift
           Range (spectral) bandwidth is adapted for each moving window

       -p
           Use progressive bandwidth for mean shift
           Spatial bandwidth is increased,  range  (spectral)  bandwidth  is  decreased  in  each
           iteration

       --overwrite
           Allow output files to overwrite existing files

       --help
           Print usage summary

       --verbose
           Verbose module output

       --quiet
           Quiet module output

       --ui
           Force launching GUI dialog

   Parameters:
       group=name[,name,...] [required]
           Name of input imagery group or raster maps

       output=name [required]
           Name for output raster map

       band_suffix=name
           Suffix for output bands with modified band values
           Name for output raster map

       threshold=float [required]
           Difference threshold between 0 and 1
           Threshold = 0 merges only identical segments; threshold = 1 merges all

       radius=float
           Spatial radius in number of cells
           Must be >= 1, only cells within spatial bandwidth are considered for mean shift
           Default: 1.5

       hr=float
           Range (spectral) bandwidth [0, 1]
           Only  cells  within  range  (spectral)  bandwidth are considered for mean shift. Range
           bandwidth is used as conductance parameter for adaptive bandwidth

       method=string
           Segmentation method
           Options: region_growing, mean_shift
           Default: region_growing

       similarity=string
           Similarity calculation method
           Options: euclidean, manhattan
           Default: euclidean

       minsize=integer
           Minimum number of cells in a segment
           The final step will merge small segments with their best neighbor
           Options: 1-100000
           Default: 1

       memory=integer
           Memory in MB
           Default: 300

       iterations=integer
           Maximum number of iterations

       seeds=name
           Name for input raster map with starting seeds

       bounds=name
           Name of input bounding/constraining raster map
           Must be integer values, each area will be segmented independent of the others

       goodness=name
           Name for output goodness of fit estimate map

DESCRIPTION

       Image segmentation or object recognition is the process of grouping  similar  pixels  into
       unique  segments,  also  referred to as objects.  Boundary and region based algorithms are
       described in  the  literature,  currently  a  region  growing  and  merging  algorithm  is
       implemented. Each object found during the segmentation process is given a unique ID and is
       a collection of contiguous pixels meeting some criteria.  Note  the  contrast  with  image
       classification  where  all pixels similar to each other are assigned to the same class and
       do not need to be contiguous.  The image segmentation results can be useful on their  own,
       or  used  as a preprocessing step for image classification. The segmentation preprocessing
       step can reduce noise and speed up the classification.

NOTES

   Region Growing and Merging
       This segmentation algorithm sequentially examines all current segments in the raster  map.
       The  similarity  between  the  current  segment  and  each  of its neighbors is calculated
       according to the given distance formula. Segments will be merged if they meet a number  of
       criteria, including:

       1      The  pair  is  mutually most similar to each other (the similarity distance will be
              smaller than to any other neighbor), and

       2      The similarity must be lower than the input  threshold.  The  process  is  repeated
              until no merges are made during a complete pass.

   Similarity and Threshold
       The  similarity  between  segments and unmerged objects is used to determine which objects
       are merged. Smaller distance values indicate a closer match, with a  similarity  score  of
       zero for identical pixels.

       During normal processing, merges are only allowed when the similarity between two segments
       is lower than the given threshold value. During the final  pass,  however,  if  a  minimum
       segment  size  of 2 or larger is given with the minsize parameter, segments with a smaller
       pixel count will be merged with their most similar neighbor  even  if  the  similarity  is
       greater than the threshold.

       The  threshold  must be larger than 0.0 and smaller than 1.0. A threshold of 0 would allow
       only identical valued pixels to be merged, while a threshold of 1 would  allow  everything
       to  be merged. The threshold is scaled to the data range of the entire input data, not the
       current computational region.  This allows  the  application  of  the  same  threshold  to
       different  computational  regions  when  working  on  the same dataset, ensuring that this
       threshold has the same meaning in all subregions.

       Initial empirical tests indicate threshold values of 0.01 to 0.05 are reasonable values to
       start.  It  is  recommended  to  start  with  a  low  value,  e.g.  0.01, and then perform
       hierarchical segmentation by using the output of the last run as seeds for the next run.

   Calculation Formulas
       Both Euclidean and Manhattan distances use the normal definition, considering each  raster
       in  the  image  group  as a dimension.  In future, the distance calculation will also take
       into account the shape characteristics of the segments.  The  normal  distances  are  then
       multiplied  by  the  input  radiometric  weight. Next an additional contribution is added:
       (1-radioweight) * {smoothness * smoothness weight + compactness * (1-smoothness  weight)},
       where  compactness  =  Perimeter Length / sqrt( Area ) and smoothness = Perimeter Length /
       Bounding Box. The perimeter length is estimated as the number of pixel sides  the  segment
       has.

   Seeds
       The  seeds  map  can be used to provide either seed pixels (random or selected points from
       which to start the segmentation process) or seed segments. If the seeds are the results of
       a  previous segmentation with lower threshold, hierarchical segmentation can be performed.
       The different approaches are automatically detected by the program: any pixels  that  have
       identical seed values and are contiguous will be assigned a unique segment ID.

   Maximum number of segments
       The  current  limit  with  CELL storage used for segment IDs is 2 billion starting segment
       IDs. Segment IDs are assigned whenever a yet unprocessed  pixel  is  merged  with  another
       segment.  Integer  overflow  can happen for computational regions with more than 2 billion
       cells and very low threshold values, resulting  in  many  segments.  If  integer  overflow
       occurs   durin  region  growing,  starting  segments  can  be  used  (created  by  initial
       classification or other methods).

   Goodness of Fit
       The goodness of fit for each pixel is calculated as 1 -  distance  of  the  pixel  to  the
       object  it  belongs  to. The distance is calculated with the selected similarity method. A
       value of 1 means identical values, perfect fit, and a value of 0  means  maximum  possible
       distance, worst possible fit.

   Mean shift
       Mean  shift  image  segmentation  consists  of  2  steps:  anisotrophic  filtering  and 2.
       clustering. For anisotrophic filtering new cell values are calculated from all pixels  not
       farther  than  hs  pixels  away  from the current pixel and with a spectral difference not
       larger than hr. That means that pixels that are too different from the current  pixel  are
       not  considered  in  the  calculation  of new pixel values.  hs and hr are the spatial and
       spectral (range) bandwidths  for  anisotrophic  filtering.  Cell  values  are  iteratively
       recalculated  (shifted  to  the  segment’s mean) until the maximum number of iterations is
       reached or until the largest shift is smaller than threshold.

       If input bands have been  reprojected,  they  should  not  be  reprojected  with  bilinear
       resampling because that method causes smooth transitions between objects. More appropriate
       methods are bicubic or lanczos resampling.

   Boundary Constraints
       Boundary constraints limit the adjacency  of  pixels  and  segments.   Each  unique  value
       present  in  the  bounds  raster  are  considered as a MASK. Thus no segments in the final
       segmentated map will cross a boundary, even if their spectral data is very similar.

   Minimum Segment Size
       To reduce the salt and pepper effect, a minsize greater than 1  will  add  one  additional
       pass  to  the processing. During the final pass, the threshold is ignored for any segments
       smaller then the set size, thus forcing very small  segments  to  merge  with  their  most
       similar  neighbor. A minimum segment size larger than 1 is recommended when using adaptive
       bandwidth selected with the -a flag.

EXAMPLES

   Segmentation of RGB orthophoto
       This example uses the ortho photograph included in  the  NC  Sample  Dataset.  Set  up  an
       imagery group:
       i.group group=ortho_group input=ortho_2001_t792_1m@PERMANENT

       Set the region to a smaller test region (resolution taken from input ortho photograph).
       g.region -p raster=ortho_2001_t792_1m n=220446 s=220075 e=639151 w=638592
       Try out a low threshold and check the results.
       i.segment group=ortho_group output=ortho_segs_l1 threshold=0.02

       From  a  visual  inspection,  it  seems this results in too many segments.  Increasing the
       threshold, using the previous results as seeds, and setting a minimum size of 2:
       i.segment group=ortho_group output=ortho_segs_l2 threshold=0.05 seeds=ortho_segs_l1 min=2
       i.segment group=ortho_group output=ortho_segs_l3 threshold=0.1 seeds=ortho_segs_l2
       i.segment group=ortho_group output=ortho_segs_l4 threshold=0.2 seeds=ortho_segs_l3
       i.segment group=ortho_group output=ortho_segs_l5 threshold=0.3 seeds=ortho_segs_l4

       The output ortho_segs_l4 with threshold=0.2 still has too many segments,  but  the  output
       with  threshold=0.3  has  too  few  segments. A threshold value of 0.25 seems to be a good
       choice. There is also some noise in the image, lets next force all segments  smaller  than
       10  pixels  to  be  merged into their most similar neighbor (even if they are less similar
       than required by our threshold):

       Set the region to match the entire map(s) in the group.
       g.region -p raster=ortho_2001_t792_1m@PERMANENT

       Run i.segment on the full map:
       i.segment group=ortho_group output=ortho_segs_final threshold=0.25 min=10

       Processing the entire ortho image with nearly 10 million pixels took about 450 times  more
       then for the final run.

   Segmentation of panchromatic channel
       This  example  uses  the  panchromatic channel of the Landsat7 scene included in the North
       Carolina sample dataset:
       # create group with single channel
       i.group group=singleband input=lsat7_2002_80
       # set computational region to Landsat7 PAN band
       g.region raster=lsat7_2002_80 -p
       # perform segmentation with minsize=5
       i.segment group=singleband threshold=0.05 minsize=5 \
         output=lsat7_2002_80_segmented_min5 goodness=lsat7_2002_80_goodness_min5
       # perform segmentation with minsize=100
       i.segment group=singleband threshold=0.05 minsize=100
         output=lsat7_2002_80_segmented_min100 goodness=lsat7_2002_80_goodness_min100

       Original panchromatic channel of the Landsat7 scene

       Segmented panchromatic channel, minsize=5

       Segmented panchromatic channel, minsize=100

TODO

   Functionality
           ·   Further testing of the shape  characteristics  (smoothness,  compactness),  if  it
               looks good it should be added.  (in progress)

           ·   Malahanobis distance for the similarity calculation.

   Use of Segmentation Results
           ·   Improve  the  optional  output  from  this module, or better yet, add a module for
               i.segment.metrics.

           ·   Providing updates to i.maxlik to ensure the segmentation output  can  be  used  as
               input for the existing classification functionality.

           ·   Integration/workflow for r.fuzzy (Addon).

   Speed
           ·   See create_isegs.c

REFERENCES

       This   project  was  first  developed  during  GSoC  2012.  Project  documentation,  Image
       Segmentation references, and other information is at the project wiki.

       Information about classification in GRASS is at available on the wiki.

SEE ALSO

        g.gui.iclass, i.group, i.maxlik, i.smap, r.kappa

AUTHORS

       Eric Momsen - North Dakota State University
       Markus Metz (GSoC Mentor)

       Last changed: $Date: 2017-11-18 09:59:58 +0100 (Sat, 18 Nov 2017) $

SOURCE CODE

       Available at: i.segment source code (history)

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       © 2003-2019 GRASS Development Team, GRASS GIS 7.6.1 Reference Manual