Spatial region filtering 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.
For example, to counts the photons within a circular region using the regcnts program, use:
$ regcnts snr.fits "circle(512,512,5)"This will filter a region of the image that looks like this:
$ regdisp foo.fits "circle 512 512 5" regions: circle(512,512,5) ............. ............. ....11111.... ...1111111... ..111111111.. ..111111111.. ..111111111.. ..111111111.. ..111111111.. ...1111111... ....11111.... ............. .............(NB: region displays are created using the regdisp program.)
# comment until end of line global keyword=value keyword=value ... # set global value(s) # include the following file in the region descriptor @file # 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 ... e.g.: ([+-]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 box xcenter ycenter xwidth yheight (angle) circle xcenter ycenter radius ellipse xcenter ycenter xwidth yheight (angle) field none line x1 y1 x2 y2 pie xcenter ycenter angle1 angle2 point x1 y1 polygon x1 y1 x2 y2 ... xn yn
In addition, the following regions accept accelerator syntax:
shape arguments ----- ------------------------------------------ annulus xcenter ycenter radius1 radius2 ... radiusn annulus xcenter ycenter inner_radius outer_radius n=[number] box xcenter ycenter xw1 yh1 xw2 yh2 ... xwn yhn (angle) box xcenter ycenter xwlo yhlo xwhi yhhi n=[number] (angle) circle xcenter ycenter r1 r2 ... rn # same as annulus circle xcenter ycenter rinner router n=[number] # same as annulus ellipse xcenter ycenter xw1 yh1 xw2 yh2 ... xwn yhn (angle) ellipse xcenter ycenter xwlo yhlo xwhi yhhi n=[number] (angle) pie xcenter ycenter angle1 angle2 (angle3) (angle4) (angle5) ... pie xcenter ycenter angle1 angle2 (n=[number]) point x1 y1 x2 y2 ... xn yn
Finally, the following are combinations of pie with different shapes (called "panda" for "Pie AND Annulus") allow for easy specification of radial sections:
shape: arguments: ----- --------- panda xcen ycen ang1 ang2 nang irad orad nrad # circular bpanda xcen ycen ang1 ang2 nang xwlo yhlo xwhi yhhi nrad (ang) # box epanda xcen ycen ang1 ang2 nang xwlo yhlo xwhi yhhi nrad (ang) # ellipseThe panda and cpanda specify combinations of annulus and circle with pie, respectively and give identical results. The bpanda combines box and pie, while epanda combines ellipse and pie.
See region examples for visual examples of region shapes.
The following "shapes" are ignored by generated by ds9 and ignored by the region filtering code:
shape: arguments: ----- --------- projection x1 y1 x2 y2 width # NB: ignored ruler x1 y1 x2 y2 # NB: ignored text x y # NB: ignored grid # NB: ignored tile # NB: ignored compass # NB: ignored
All arguments to regions are real values; integer values are automatically converted to real where necessary. All angles are in degrees and run counter-clockwise from the positive image x-axis. If a rotation angle is part of the associated WCS header, that angle is added implicitly as well.
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 overlapNote that the !region syntax must be combined with another region in order that we be able to assign a region id properly. That is,
!circle(512,512,10)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)
For example, two shapes connected by a boolean OR are given the same region value:
regions: circle(510,510,5)||circle(516,516,5) ................... ................... ..........11111.... .........1111111... ........111111111.. ........111111111.. ........111111111.. ........111111111.. ....1111111111111.. ...1111111111111... ..1111111111111.... ..111111111........ ..111111111........ ..111111111........ ..111111111........ ...1111111......... ....11111.......... ................... ...................On the other hand, the two shapes separated by a comma are given different region values:
regions: circle(510,510,5),circle(516,516,5) ................... ................... ..........22222.... .........2222222... ........222222222.. ........222222222.. ........222222222.. ........222222222.. ....1111122222222.. ...1111111222222... ..1111111112222.... ..111111111........ ..111111111........ ..111111111........ ..111111111........ ...1111111......... ....11111.......... ................... ...................
Of course these two examples will both mask the same image pixels. However, in programs that distinguish region id's (such as regcnts ), they will act differently. The explicit OR operator will result in one region expression consisting of two shapes having the same region id and regcnts will report a single region. The comma operator will cause regcnts to report two region expressions, each with one shape, in its output.
As a matter of practice, commas or semi-colons are used to separate region expressions entered on the command line:
# regions are added to the filename in bracket notation regcnts foo.fits "circle(512,512,100),circle(400,400,20)"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 circle(512,512,100) circle(400,400,20)
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. 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:
$ regdisp foo.fits "circle 512 512 5; circle 512 512 3" regions: circle 512 512 5; circle 512 512 3 ............. ............. ....11111.... ...1111111... ..111111111.. ..111111111.. ..111111111.. ..111111111.. ..111111111.. ...1111111... ....11111.... ............. ............. $ regdisp foo.fits "circle 512 512 3; circle 512 512 5" regions: circle 512 512 3; circle 512 512 5 ............. ............. ....22222.... ...2222222... ..221111122.. ..221111122.. ..221111122.. ..221111122.. ..221111122.. ...2222222... ....22222.... ............. .............
The order in which mask values are assigned can be reversed by setting the REGIONS_PAINT environment variable to true. This puts region filtering into "paint mode", so that new regions overwrite old regions:
$ env REGIONS_PAINT=true regdisp foo.fits "circle 512 512 5; circle 512 512 3" regions: circle 512 512 5; circle 512 512 3 ............. ............. ....11111.... ...1111111... ..112222211.. ..112222211.. ..112222211.. ..112222211.. ..112222211.. ...1111111... ....11111.... ............. .............
operator arguments:
-------- -----------
- Globally exclude the region expression following '-' sign
from ALL regions specified in this file
The global exclude region can be used by itself; in such a case, field() is
implied.
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:
regdisp foo.fits "-circle 512 512 3; circle 512 512 5; circle 514 512 5" regions: -circle 512 512 3; circle 512 512 5; circle 514 512 5 ............... ............... ....1111122.... ...111111122... ..11.....1122.. ..11.....1122.. ..11.....1122.. ..11.....1122.. ..11.....1122.. ...111111122... ....1111122.... ............... ...............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.
regdisp foo.fits '!circle 512 512 3&&circle 512 512 5; circle 514 512 5' regions: !circle 512 512 3&&circle 512 512 5; circle 514 512 5 ............... ............... ....1111122.... ...111111122... ..11222221122.. ..11222221122.. ..11222221122.. ..11222221122.. ..11222221122.. ...111111122... ....1111122.... ............... ...............Here, the local exclude applied to the region at 512,512 and the space cleared was then filled in by the region at 514, 512.
Note that in REGIONS_PAINT mode, excludes are done in the order in which they appear, not globally at the end. Therefore, order matters:
env REGIONS_PAINT=true regdisp foo.fits "-circle 512 512 3; circle 512 512 5; circle 514 512 5" regions: -circle 512 512 3; circle 512 512 5; circle 514 512 5 ............... ............... ....1122222.... ...112222222... ..11222222222.. ..11222222222.. ..11222222222.. ..11222222222.. ..11222222222.. ...112222222... ....1122222.... ............... ............... env REGIONS_PAINT=true regdisp foo.fits "circle 512 512 5; circle 514 512 5; -circle 512 512 3" regions: circle 512 512 5; circle 514 512 5; -circle 512 512 3 ............... ............... ....1122222.... ...112222222... ..11.....2222.. ..11.....2222.. ..11.....2222.. ..11.....2222.. ..11.....2222.. ...112222222... ....1122222.... ............... ...............
regcnts foo.fits "@foo"
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(10,10,2) circle(20,20,3) # color=blue circle(30,30,4)The first and third circles will be red, which the second circle will be blue. Note that regions currently ignores these properties, as they are used in display only.
name description ---- ------------------------------------------ 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
position arguments description ------------------ ------------------------------ [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 description -------------- ----------- [num] context-dependent (see below) [num]" arc seconds [num]' arc minutes [num]d degrees [num]r radians [num]p physical pixels [num]i image pixelsWhen 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:
name description ---- ---------------------------- IMAGE pixel coords of current file LINEAR linear wcs as defined in file FK4, B1950 various sky coordinate systems FK5, J2000 GALACTIC ECLIPTIC ICRS PHYSICAL pixel coords of original file using LTM/LTV
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 200The 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' # j2000Note 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.