sfrl/shortstraw

Python implementation of the ShortStraw algorithm for corner/curve point extraction

ShortStraw algorithm Python implementation

based on

A. Wolin, B. Eoff, and T. Hammond, “ShortStraw: A Simple and Effective Corner Finder for Polylines.” in SBM, 2008, pp. 33–40.

and

Y. Xiong and J. J. LaViola Jr, “Revisiting shortstraw: improving corner finding in sketch-based interfaces,” in Proceedings of the 6th Eurographics Symposium on Sketch-Based Interfaces and Modeling, 2009, pp. 101– 108.

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Use the function short_straw from shortstraw.py to calculate corner and curve points in a vector format image. A Jupyter notebook and a JSON file with an example dataset is included to demonstrate the functionality.

Parameters

• paths : array_like
  • expects an array in the form:

#Sketch array 
[
    #Path 1 array
    [ #Points 
        [x0,x1,x2 ,...] # x positions 
        [y0,y1,y2 ,...] # y positions
    ],
    #More paths
    [...] , [...] , ...
]

• t1 : float, optional
  • Threshold for first postprocessing iteration, should be between 0 and 1, default = 0.96
• t2max : float, optional
  • Upper threshold for second postprocessing iteration, should be be between 0 and 1, default = 0.95
• t2min : float, optional
  • Lower threshold for second postprocessing iteration, should be be between 0 and 1, default = 0.944

For a in depth description of the threshold, have a look at the Wolin et al. paper. If you just want to play around with the values, keep in mind that all values should be between 0 and 1 this should apply: t1 > t2max > t2min.

Returns

• all_corners: list
  • the indices of the points that are determined to be corner points for each path, in the form:

#Feature point array
[
    #Path 1 indices
    [index0,index1,index2,...],
    #More paths 
    [...],[...],...
]

• all_curves: list
  • the indices of the points that are determined to be curve points for each path. Same format as all_corners
• feature_info: dict
  • classifies angles and curve points and returns count. Have a look at the Xiong and LaViola paper for a definition of the curve angle.

Key	Description
straight	number of corner points with angle ≥ 0.99 * PI
obtuse	number of corner points with 0.55 * PI < angle < 0.99 * PI
right	number of corner points with 0.45 * PI ≤ angle ≤ 0.55 * PI
obtuse	number of corner points with angle < 0.45 * PI
wide_curve	number of curve points with angle > 0.5 * PI
narrow_curve	number of curve points with angle ≤ 0.5 * PI

• resampled_data: list
  • resampled version of the input data
• all_raw_angles: list
  • a list of floats representing the angle sizes for all corner points
• all_raw_curves: list
  • a list of floats representing the angle sizes for all curve points