Defining a Region

Summary:

• To define a data region (by default, an "Include in Source" region):
  • Click on the Cursor Pull-down Menu to select a region shape. Circle, Rectangle, and Ellipse, Polygon, and Point are supported.
  • Move to the desired image location and click the left mouse button. Optionally, you can drag the mouse with the left button pressed to specify the initial size of the region.
  • Move to the interior of the region, click and drag the left mouse button to move to another image location.
  • Move to the perimeter of the region, click and drag the left mouse button to change the region size.
  • Use the Shift key and left button together to rotate ellipses, rectangles, and polygons.
  • Use the Shift key and middle button together to activate regions which are contained within other regions. The active region gets a thickened border and can be moved, resized, etc.

• To change region type, move cursor to the interior of the region and:
  • Type "b" to change from source to background region.
  • Type "s" to change from background to source region.
  • Type "e" to change from "include data with region" to "exclude data within region".
  • Type "i" to change from "exclude data with region" to "include data within region".

• To delete region, move to the interior of the region and use Delete key.

Contents of This Section

• Using Regions to Extract Data from Images
• Region Descriptors

Using Regions to Extract Data from Images

Regions provide a means for marking particular areas of an image for further analysis. A common example is to mark a circular region within which counts from an x-ray source are accumulated, and an adjacent annular region within which background counts are accumulated. Net source counts can then be determined by subtracting the background counts, normalized to the source region area, from source counts. In addition to "source" and "background" attributes, regions may have "include/exclude" attributes, meaning include/exclude data in this region in the analysis. For example, an annular region may be described as an "include" circle + an "exclude" circle at the same location but with a smaller radius.

Regions may be created by clicking the left mouse button in the display window, and repositioned by clicking and dragging the left mouse button in the region interior. They may be moved on a fine scale by the use of the l(eft), r(ight), u(p), and d(own) keys (one image pixel per stroke) or the L, R, U, D keys (4 pixels per stroke). Circles and ellipses may be resized by clicking and dragging the left mouse button on the region perimeter. Note that for ellipses, dragging vertically or horizontally changes the aspect ratio of the figure. Rectangles may be resized by clicking and dragging the left mouse button while on a corner of the figure.

Region attributes may be changed with the s(ource), b(ackground), i(nclude), and e(xclude) keys, while the cursor is in the region interior. Different colors correspond to different region attributes - green for "source include", red for "source exclude", blue for "background include", and yellow for "background exclude".

Regions may be saved to files by selecting either Save Regions from the Markers Pull-down Menu.

Region Descriptors

Region definitions are not used within SAOtng itself. Rather, they are communicated to external programs which understand them, either through the X Public Access Mechanism, or via files. Therefore, they must be output or converted to a format with which the external programs are familiar. In particular, SAOtng regions follow the syntax defined in the IRAF/PROS data analysis system. The following descriptors are defined:

• +/-circle(x center, y center, radius)
• +/-box(x center, y center, x half-width, y half-width, rotation angle)
• +/-ellipse(x center, y center, x half-width, y half-width, rotation angle)

In all cases, "+" refers to "include" regions and "-" to "exclude" regions. Coordinates are given in physical pixels of the current image. N.B. The origin of the pixel coordinate system is at (1,1), following IRAF conventions.

Last Updated May 26, 1995