Particle tracking using IDL -- John C. Crocker and Eric R. Weeks
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polycut.pro

Similar to eclip, this is useful when extracting portions of a data set. Suppose we have an array, such as a pretrack array, with a bunch of columns:

IDL> help,pta 

PTA             FLOAT     = Array[6, 717892]

Typically pta(0,*) is the x-coordinate, pta(1,*) is the y-coordinate, pta(2,*) is the brightness of the particle, pta(3,*) is the radius of the particle, pta(4,*) is the eccentricity of the particle, and pta(5,*) is the time step. One thing you typically do when you start data analysis is look at the spread of data in (brightness, radius) space:
particle clouds IDL> plot,pt(2,*),pt(3,*),psym=3

Suppose it's clear that we want the particles contributing to the distinct donut shape, and not the outlying particles. Use polycut to draw a convex polygon around the data, and then only the particles inside the polygon will be accepted:
particle clouds IDL> ptb=polycut(pt,f1=2,f2=3) 
% ECUT: Draw a convex polygon on the data...
% GET_POLYGON: Left Button to add vertex, Right to quit

As you select vertices, a polygon is drawn around your data. The "f1=2,f2=3" commands select fields 2 and 3 as the relevant fields to plot, in other words, the plot command that polycut is using is

IDL> plot,pt(f1,*),pt(f2,*),psym=3

where f1 and f2 are defined when you call polycut.

particle clouds When you are done, a right-click of your mouse closes the polygon, then the data within the polygon is selected and returned. Also, the coordinates of the polygon you drew are printed out: 

   2935.66      12.9114
   2935.66      8.31852
   24465.9      6.80524
   37633.2      9.35391
   14590.5      12.4070

Also, if you want, you can specify a polygon yourself, and polycut will just proceed using that polygon:

IDL> ptb=polycut(pt,mypolygon,f1=2,f2=3)

If you include the "mypolygon" variable but it's undefined, then you can proceed as normal (drawing a polygon using the mouse) and then the variable will contain the polygon you have drawn.

Keywords:

John wrote this routine originally in a different context, and Eric modified the core routine into this particular format. Eric added /INVERT and /WHEREFLAG.

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