# IBM Visualization Data Explorer Programmer's Reference

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# Chapter 9. Using the Pick Structure

Partial Table-of-Contents

- 9.1 The Pick Structure
- 9.2 ShowPick Module Example--Using Color to Show a Picked Object

  Data Explorer includes a tool for "picking" points in Objects in an image.

  This tool, Pick, creates a structure that can be used to perform various functions (e.g., to display the data value at picked points). But you can also write your own module to perform different functions if you like.

  ## 9.1 The Pick Structure

  Picking is the selection of a location on an object in an image by use of the mouse. A straight line from the camera through the location selected constitutes a "poke," which may intersect the object in the image in one or more places or in none at all. The intersections are called "picks." For example, a poke through a spherical isosurface results in two picks: one on the "front" surface and one on the "back". Picks differ from probes in that probes may be present anywhere in 3-dimensional space, picks only on the surface of an object.

  The pick structure is a Field, and the picked points are listed in its "positions" component. A number of routines in Data Explorer allow you to query the pick structure output by the Pick tool and to traverse a picked Object. (See 14.6 , "Pick-Assistance Routines" for details.) The structure includes information on how to traverse the picked Object to reach the picked element. It also identifies:

  - the connection in which the picked point resides (the element ID)
  - the vertex of the picked element closest to the picked point (the vertex ID)
  - the position of the picked point itself.

  If the picked Object has no connections, the element ID and the vertex ID both refer to the position closest to the picked point. Other information can be accessed with pick-assistance routines.

  Note: For a module that uses the pick structure, the Object displayed in the image being picked must (1) be the same as the Object passed to the module or (2) have a matching Object hierarchy. The reason for this requirement is that the output of the Pick tool describes the location of the picked Object as it exists in the hierarchy of the rendered Object. To use the pick structure, therefore, requires an Object with a matching structure.

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