The region module provides a polygon filling algorithm used by skymatch to create data masks.

Polygon filling algorithm.

Authors:Nadezhda Dencheva, Mihai Cara (contact:
class jwst.skymatch.region.Region(rid, coordinate_system)[source]

Base class for regions.

  • rid (int or string) – region ID
  • coordinate_system (astropy.wcs.CoordinateSystem instance or a string) – in the context of WCS this would be an instance of wcs.CoordinateSysem

Sets mask values to region id for all pixels within the region. Subclasses must define this method.

Parameters:mask (ndarray) – a byte array with the shape of the observation to be used as a mask
Returns:mask – pixels which are not included in any region).
Return type:array where the value of the elements is the region ID or 0 (for
class jwst.skymatch.region.Edge(name=None, start=None, stop=None, next=None)[source]

Edge representation

An edge has a “start” and “stop” (x,y) vertices and an entry in the GET table of a polygon. The GET entry is a list of these values:

[ymax, x_at_ymin, delta_x/delta_y]


Compute the entry for an edge in the current Active Edge Table

[ymax, x_intersect, 1/m] note: currently 1/m is not used


Compute the entry in the Global Edge Table

[ymax, x@ymin, 1/m]

class jwst.skymatch.region.Polygon(rid, vertices, coord_system='Cartesian')[source]

Represents a 2D polygon region with multiple vertices

  • rid (string) – polygon id
  • vertices (list of (x,y) tuples or lists) – The list is ordered in such a way that when traversed in a counterclockwise direction, the enclosed area is the polygon. The last vertex must coincide with the first vertex, minimum 4 vertices are needed to define a triangle
  • coord_system (string) – coordinate system

Create a list of Edge objects from vertices


This is the main function which scans the polygon and creates the mask

  • data (array) – the mask array it has all zeros initially, elements within a region are set to the region’s ID
  • Algorithm
  • Set the Global Edge Table (GET) (-) –
  • Set y to be the smallest y coordinate that has an entry in GET (-) –
  • Initialize the Active Edge Table (AET) to be empty (-) –
  • For each scan line (-) –
    1. Add edges from GET to AET for which ymin==y
    2. Remove edges from AET fro which ymax==y
    3. Compute the intersection of the current scan line with all edges in the AET
    4. Sort on X of intersection point
    5. Set elements between pairs of X in the AET to the Edge’s ID
update_AET(y, AET)[source]

Update the Active Edge Table (AET)

Add edges from GET to AET for which ymin of the edge is equal to the y of the scan line. Remove edges from AET for which ymax of the edge is equal to y of the scan line.