In the first part of this two-part post we’ll learn to use different tools and techniques to visualize and save animations (second part here). This first part focuses on VisIt, an open-source visualization software developed by the Lawrence Livermore Laboratory – and many others (…that’s an advantage of being open-source). There are a lot of cool things about VisIt, and you can read about that here or here, and probably in future posts. But for today, we’ll just learn how to use VisIt to play and save animations. VisIt supports over a hundred file formats, and this post focuses on reading spatially distributed and time-varying data stored in the netCDF format (more about netCDF here).
Let us assume we have VisIt installed and the VisIt path has been added to our bash profile. Then we just need to
cd into the directory where our data are stored, and type
visit in the command line to launch the program. VisIt comes with two windows, one for manipulating files and the data stored in them, and another where plots are drawn (it is possible to draw more plots in additional windows, but in this tutorial we’ll stick to one window). Both windows are visible in the screenshot below.
It may be difficult to see, but the left-hand window is the space for managing files and plots. The active “Plot” is also displayed. Here the active file is “air.2m.2000.nc” where nc is th extension for netCDF files. The plot is a “Pseudocolor” of that data, the equivalent of “contourf” in Matlab or Python – Matplotlib. It is plotted in the right-hand window. It is the worldwide distribution of daily air temperature (in Kelvin) with a 1 degree resolution for January 1, 2000 on the left pane (Data from NCEP). Note the toolbar above the plot: it contains “play / stop” buttons that can allow us to play the 365 other days of year 2000 (the cursor is on “stop” in the screenshot). We are going to save the animation of the whole year with the VisIt movie wizard.
But first, let us change the specifications of our plot. In the left-hand window, we go to PlotAtts – > Pseudocolor (recall that is the plot type that we have). We can change the colorbar, set minima and maxima so that the colorbar does not change for each day of the year when we save the movie, and even make the scale skewed so we see more easily the differences in temperatures in non-polar regions where people live (on the image above the temperature in the middle of the colorbar is 254.9 K, which corresponds to -18.1C or roughly -1F). The screenshot below shows our preferences. We can click “Apply” for the changes to take effect, and exit the window.
Now we can save the movie. We go to File – > Save Movie and follow the VisIt Movie Wizard. You will find it very intuitive and easy. We save a “New Simple Movie” in DVIX (other formarts are available), with 20 frames (i.e. 20 days) per second. The resulting movie is produced within 2 minutes, but since WordPress does not support DIVX unless I am willing to pay for premium access, I had to convert the result as a GIF. To be honest, this is a huge letdown as the quality suffers compared to a video (unless I want to upload a 1GB monster). But the point of this post is that VisIt is really easy to use, not that you should convert its results into GIF.
In this animation it is easy to see the outline of land masses as they get either warmer or cooler than the surrounding oceans! Videos would look neater: you can try them for yourself! (at home, wherever you want).
It is actually possible to command VisIt using Python scripts, but I haven’t mastered that yet so it will be a tale for another post.