Spatial Region Filtering

This document contains a summary of the user interface for spatial region filtering images and tables.

Spatial region filtering allows a program to select regions of an image or rows of a table (e.g., X-ray events) to process using simple geometric shapes and boolean combinations of shapes. When an image is filtered, only pixels found within these shapes are processed. When a table is filtered, only rows found within these shapes are processed. Spatial region filtering for images and tables is accomplished by means of region specifications. A region specification consists of one or more region expressions, which are geometric shapes,combined according to the rules of boolean algebra. Region specifications also can contain comments and local/global processing directives. Typically, region specifications are specified using bracket notation appended to the filename of the data being processed:

It is also possible to put region specification inside a file and then pass the filename in bracket notation:

When region filters are passed in bracket notation in this manner, the filtering is set up automatically when the file is opened and all processing occurs through the filter. Programs also can use the filter library API to open filters explicitly.

Region Expressions

More specifically, region specifications consist of one or more lines containing:
  # comment until end of line
  global   keyword=value keyword=value  ... # set global value(s)
  # include the following file in the region descriptor
  # each region expression contains shapes separated by operators
  [region_expression1], [region_expression2], ...
  [region_expression], [region_expression], ...
A single region expression consists of:
  # parens and commas are optional, as is the + sign
  [+-]shape(num , num , ...) OP1 shape num num num OP2 shape ...
  ([+-]shape(num , num , ...) && shape num  num || shape(num, num)
  # a comment can come after a region -- reserved for local properties
  [+-]shape(num , num , ...)  # local properties go here, e.g. color=red

Thus, a region descriptor consists of one or more region expressions or regions, separated by comas, new-lines, or semi-colons. Each region consists of one or more geometric shapes combined using standard boolean operation. Several types of shapes are supported, including:

shape:		arguments:
-----		---------
ANNULUS		xcenter ycenter inner_radius outer_radius (outer_radius) ...
ANNULUS		xcenter ycenter inner_radius outer_radius (n=)
BOX		xcenter ycenter xwidth yheight (optional angle)
CIRCLE		xcenter ycenter radius
ELLIPSE		xcenter ycenter xwidth yheight angle
FIELD		none
LINE		x1 y1 x2 y2
PANDA		xcenter ycenter angle1 angle2 nangle iradius oradius nradius
PIE		xcenter ycenter angle1 angle2 (angle3) (angle4) (angle5) ...
PIE		xcenter ycenter angle1 angle2 (n=)
POINT		x1 y1
POINT		x1 y1 x2 y2 ... xn yn
POLYGON		x1 y1 x2 y2 ... xn yn

All arguments to regions are real values; integer values are automatically converted to real where necessary. All angles are in degrees and specify angles that run counter-clockwise from the positive y-axis. Note that 3-letter abbreviations are supported for all shapes, so that you can specify "circle" or "cir".

Region Algebra

(See also region algebra for more complete information.)

Region shapes can be combined together using Boolean operators

Symbol		Operation	Use
--------	---------	---

  !     	not		Exclude this shape from this region
  & or &&	and		Include only the overlap of these shapes
  | or ||	inclusive or	Include all of both shapes
  ^  		exclusive or	Include both shapes except their overlap

Note that the !region syntax must be combined with another region in order that we be able to assign a region id properly. That is,
is not a legal region because there is no valid region id to work with. To get the full field without a circle, combine the above with field(), as in:
  field() && !circle(512,512,10)

Region Separators Also Are Operators

As mentioned previously, multiple region expressions can be specified in a region descriptor, separated by commas, new-lines, or semi-colons. When such a separator is used, the boolean OR operator is automatically generated in its place but, unlike explicit use of the OR operator, the region ID is incremented (starting from 1).

For example, the two shapes specified in this example are given the same region value:

On the other hand, the two shapes defined in the following example are given different region values:

Of course these two examples will both mask the same table rows or pixels. However, in programs that distinguish region id's (such as funcnts ), they will act differently. The explicit OR operator will result in one region expression consisting of two shapes having the same region id and funcnts will report a single region. The comma operator will cause funcnts to report two region expressions, each with one shape, in its output.

In general, commas are used to separate region expressions entered in bracket notation on the command line:

  # regions are added to the filename in bracket notation
New-lines are used to separate region expressions in a file:
  # regions usually are separated by new-lines in a file
  # use @filename to include this file on the command line
Semi-colons are provided for backward compatibility with the original IRAF/PROS implementation and can be used in either case.

If a pixel is covered by two different regions expressions, it is given the mask value of the first region that contains that pixel. That is, successive regions do not overwrite previous regions in the mask, as was the case with the original PROS regions. In this way, an individual pixel is covered by one and only one region. This means that one must sometimes be careful about the order in which regions are defined. If region N is fully contained within region M, then N should be defined before M, or else it will be "covered up" by the latter.

Region Exclusion

Shapes also can be globally excluded from all the region specifiers in a region descriptor by using a minus sign before a region:

operator	arguments:
--------	-----------
  -		Globally exclude the region expression following the '-' sign
		from ALL regions specified in this file
The global exclude region cannot be used by itself; it requires at least one include region. That is,
is not a legal region because there is no valid region id to work with. To get the full field while excluding the circle, combine the above with field(), as in:

A global exclude differs from the local exclude (i.e. a shape prefixed by the logical not "!" symbol) in that global excludes are logically performed last, so that no region will contain pixels from a globally excluded shape. A local exclude is used in a boolean expression with an include shape, and only excludes pixels from that include shape. Global excludes cannot be used in boolean expressions.

Include Files

The special @filename directive specifies an include file containing region expressions. This file is processed as part of the overall region descriptor:


Global and Local Properties of Regions

The ds9 image display program describes a host of properties such as color, font, fix/free state, etc. Such properties can be specified globally (for all regions) or locally (for an individual region). The global keyword specifies properties and qualifiers for all regions, while local properties are specified in comments on the same line as the region:

  global color=red
  circle(20,20,3) # color=blue
The first and third circles will be red, which the second circle will be blue. Note that funtools currently ignores region properties, as they are used in display only.

Coordinate Systems

For each region, it is important to specify the coordinate system used to interpret the region, i.e., to set the context in which position and size values are interpreted. For this purpose, the following keywords are recognized:
PHYSICAL		# pixel coords of original file using LTM/LTV
IMAGE			# pixel coords of current file
FK4, B1950		# sky coordinate systems
FK5, J2000		# sky coordinate systems
GALACTIC		# sky coordinate systems
ECLIPTIC		# sky coordinate systems
ICRS			# currently same as J2000
LINEAR			# linear wcs as defined in file
AMPLIFIER		# mosaic coords of original file using ATM/ATV
DETECTOR		# mosaic coords of original file using DTM/DTV

Specifying Positions, Sizes, and Angles

The arguments to region shapes can be floats or integers describing positions and sizes. They can be specified as pure numbers or using explicit formatting directives:
position arguments
[num]			# context-dependent (see below)
[num]d			# degrees
[num]r			# radians
[num]p			# physical pixels
[num]i			# image pixels
[num]:[num]:[num]	# hms for 'odd' position arguments
[num]:[num]:[num]	# dms for 'even' position arguments
[num]h[num]m[num]s	# explicit hms
[num]d[num]m[num]s	# explicit dms

size arguments
[num]			# context-dependent (see below)
[num]"			# arc sec
[num]'			# arc min
[num]d			# degrees
[num]r			# radians
[num]p			# physical pixels
[num]i			# image pixels
When a "pure number" (i.e. one without a format directive such as 'd' for 'degrees') is specified, its interpretation depends on the context defined by the 'coordsys' keyword. In general, the rule is:

All pure numbers have implied units corresponding to the current coordinate system.

If no such system is explicitly specified, the default system is implicitly assumed to be PHYSICAL. In practice this means that for IMAGE and PHYSICAL systems, pure numbers are pixels. Otherwise, for all systems other than linear, pure numbers are degrees. For LINEAR systems, pure numbers are in the units of the linear system. This rule covers both positions and sizes. The input values to each shape can be specified in several coordinate systems including:

IMAGE			# pixel coords of current file
LINEAR			# linear wcs as defined in file

FK4, B1950		# various sky coordinate systems
FK5, J2000

PHYSICAL		# pixel coords of original file using LTM/LTV
AMPLIFIER		# mosaic coords of original file using ATM/ATV
DETECTOR		# mosaic coords of original file using DTM/DTV

If no coordinate system is specified, PHYSICAL is assumed. PHYSICAL or a World Coordinate System such as J2000 is preferred and most general. The coordinate system specifier should appear at the beginning of the region description, on a separate line (in a file), or followed by a new-line or semicolon; e.g.,

  global coordsys physical
  circle 6500 9320 200
The use of celestial input units automatically implies WORLD coordinates of the reference image. Thus, if the world coordinate system of the reference image is J2000, then
  circle 10:10:0 20:22:0 3'
is equivalent to:
  circle 10:10:0 20:22:0 3' # j2000
Note that by using units as described above, you may mix coordinate systems within a region specifier; e.g.,
  circle 6500 9320 3' # physical

Note that, for regions which accept a rotation angle:

ellipse (x, y, r1, r2, angle)
box(x, y, w, h, angle)
the angle is relative to the specified coordinate system. In particular, if the region is specified in WCS coordinates, the angle is related to the WCS system, not x/y image coordinate axis. For WCS systems with no rotation, this obviously is not an issue. However, some images do define an implicit rotation (e.g., by using a non-zero CROTA value in the WCS parameters) and for these images, the angle will be relative to the WCS axes. In such case, a region specification such as:
fk4;ellipse(22:59:43.985, +58:45:26.92,320", 160", 30)
will not, in general, be the same region specified as:
physical;ellipse(465, 578, 40, 20, 30)
even when positions and sizes match. The angle is relative to WCS axes in the first case, and relative to physical x,y axes in the second.

More detailed descriptions are available for: region geometry, region algebra, and region coordinates. See the coordinates for a description of how pixels around the edge of a region that are partially in regions are included or excluded from the mask.

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Last updated: March 30, 2001