region

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

Polygon filling algorithm.

NOTE: Algorithm description can be found, e.g., here:

http://www.cs.rit.edu/~icss571/filling/how_to.html http://www.cs.uic.edu/~jbell/CourseNotes/ComputerGraphics/PolygonFilling.html

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

Edge representation

An edge has “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 romancal.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 romancal.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