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#### A.10.1.6 Display Image Region

Configuration for Display Image Region action

The Display Image Region action behaves like Display Image, but additionally identifies an X/Y position on the displayed image, in pixel coordinates. This is currently done by scrolling the image if necessary to ensure the identified position is in view, and highlighting it with green crosshairs.

Configuration:

Image Location
  Gives the URL or filename of the image to display for each row. A column name may be selected from the drop-down list, or the expression language may be used to assemble a location string from other columns.
X Offset
  Horizontal offset in pixels from the left of the image for the position of interest. In most cases you will select a column from the drop-down list, but this could also be a constant or algebraic expression.
Y Offset
  Vertical offset in pixels from the top of the image for the position of interest. In most cases you will select a column from the drop-down list, but this could also be a constant or algebraic expression. For most image formats, Y values increasing downwards is the natural direction; for FITS however Y is normally considered to increase upwards. This viewer displays FITS images upside down so that the offsets probably do what you want, but the images may be inverted. This somewhat confusing behaviour may (or may not) be changed in the future.
Image Viewer
  Selects which of the internal image viewers will be used for display. The following options are available;
  - Basic: A minimal image viewer.
  - Basic (allow preprocessing): The same image viewer as Basic, but the image location is permitted to contain a preprocessing system command, for instance "`< convert " + filename + " -normalize png:-`", using the ImageMagick `convert` command. This rather advanced usage may only work on Un*x-like OSs. This option is usually not required, and has security implications; if it is used in an action loaded from a session file, the security panel will be displayed.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk