region

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

Polygon filling algorithm.

class jwst.skymatch.region.Edge(name=None, start=None, stop=None, next=None)

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_AET_entry(edge)

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_GET_entry()

Compute the entry in the Global Edge Table

[ymax, x@ymin, 1/m]

intersection(edge)

is_parallel(edge)

property next

property start

property stop

property ymax

property ymin

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

Represents a 2D polygon region with multiple vertices

Parameters

• 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

get_edges()

Create a list of Edge objects from vertices

scan(data)

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

Parameters

• data (array) – the mask array it has all zeros initially, elements within a region are set to the region’s ID

• Algorithm –

• (GET) (- Set the Global Edge Table) –

• GET (- Set y to be the smallest y coordinate that has an entry in) –

• empty (- Initialize the Active Edge Table (AET) to be) –

• line (- For each scan) –

  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)

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.

class jwst.skymatch.region.Region(rid, coordinate_system)

Base class for regions.

Parameters

• 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

scan(mask)

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