Using IDL 8 Graphics (a.k.a. New Graphics)

I’ve been using Direct Graphics (DG) in IDL since 1993. I was initially hesitant about IDL 8 Graphics (christened “New Graphics” on the comp.lang.idl-pvwave newsgroup; I’ll refer to them as NG), but I’ve been pleasantly surprised by how well they work. In particular, I like how their syntax mirrors DG and how I can use them programmatically. I also like how they handle color and how easily they make publication-ready PostScript and PNG files (among other formats). Here’s a short example of working with NG. If you’d like, please try these commands in IDL. Start by making variables to represent one cycle of a sine and a cosine wave:

 t = findgen(361)*!dtor x = sin(t) y = cos(t)

Display the sine wave:

 p = plot(t, x)

Note how I called PLOT as a function. (You can still call the DG PLOT routine as a procedure.) The PLOT function returns a reference—here, the variable “p” —that can be used to control the graphic. For example, I can use “p” to change the line color to red:

 p.color = 'red'

and the graphic updates immediately. I specified the color red by name; you could also use an RGB triple. Next, I’ll add titles to the plot:

 p.xtitle = 'Time' p.ytitle = 'Amplitude' p.title = 'Sine and Cosine Waves'

I could’ve set these properties through identical keywords in the call to PLOT. I can display the cosine wave on the same axes as the sine wave with another call to PLOT:

 q = plot(t, y, color='blue', /overplot)

The OVERPLOT keyword is the NG analog to the DG OPLOT procedure. I’ll also display a zero line:

 !null = plot(p.xrange, [0,0], linestyle='dotted', /overplot)

Note how I used “p” to get the current x-axis range of the plot. Last, I’ll add a legend positioned in the lower left corner of the graphic. Don't include the zero line in the legend.

 p.name = 'Sine' q.name = 'Cosine' pq_lgd = legend(target=[p,q], position=[2.0,-0.6], /data)

Here’s a screenshot of the window that results from these steps (this is from the Linux side of my laptop): The graphic can be saved to a JPEG file with one command:

 p.save, 'plot_ex.jpg'

IDL uses the extension on the file name to determine the file type. The default resolution of the image is 600 dpi, which makes a 2598 x 2079 image on my laptop! NG won’t replace DG in every instance. For example, NG are typically slower than DG. This is especially true when displaying more than about 10⁴ points. NG may not be for everyone. If you’re already comfortable with DG, please continue to use them. But when I think back to when I was learning how to go beyond the basics in DG, I remember it wasn’t always easy. (In particular, I remember the first time my advisor asked me to print a plot. That was a frustrating day. Or two.) I hope that new users of IDL will try NG, and I hope that NG just makes sense to them. If you’d like to learn more about using NG, please continue to follow me on this blog; I’ll post numerous examples of using them in the coming weeks and months.