Introduction — GMT 5.1.1 documentation

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# Introduction

Here we will explore what is involved in creating animations (i.e., movies). Of course, an animation is nothing more than a series of individual images played back in an orderly fashion. Here, these images will have been created with GMT. To ensure a smooth transition from frame to frame we will be following some general guidelines when writing our scripts. Since there is no “movie” mode in GMT we must take care of all the book-keeping in our script. Thus, animations may require a bit of planning and may use more advanced scripting than the previous static examples. Note: This is a new chapter introduced with the 4.4.0 version and should be considered work in progress.

Most, if not all, animation scripts must deal with several specific phases of movie making:

1. Define parameters that determine the dimension of the final movie.
2. Pre-calculate all variables, data tables, grids, or background map layers that are independent of your time variable.
3. Have a frame-number loop where each frame is created as a PostScript plot, then rasterized to a TIFF file of chosen dimension.
4. Convert the individual frames to a single movie of suitable format.
5. Clean up temporary files and eventually the individual frames.

We will discuss these phases in more detail before showing our first example.

1. There are several coordinates that you need to consider when planning your movie. The first is the coordinates of your data, i.e., the user coordinates. As with all GMT plots you will transform those to the second set of plot coordinates in inches (or cm) by applying a suitable region and map projection. As before, you normally do this with a particular paper size in mind. When printed you get a high-resolution plot in monochrome or color. However, movies are not device-independent and you must finally consider the third set of pixel coordinates which specifies the resolution of the final movie. We control the frame size by selecting a suitable dpi setting that will scale your physical dimensions to the desired frame size in pixels. If you decide up front on a particular resolution (e.g., 480 by 320 pixels) then you should specify a paper size and dpi so that their product yields the desired pixel dimensions. For instance, here it might make sense to plan your plotting on a 4.8 by 3.2 inch “paper” and use 100 dpi to convert it to pixels, but you are free to use any combination that multiplies to the desired dimensions. After deciding on frame size you need to consider how many frames your movie should have. This depends on lots of things such as how patient you are, how many frames per second you need and the time range of your animation. We recommend you use variables to specify the items that go into computing the number of frames so that you can easily test your script with a few frames before changing settings and running the full Hollywood production overnight.
2. Depending on what you want to display, there are usually many elements that do not change between frames. Examples include a coastline base map for background, an overlay of text legends, perhaps some variables that hold information that will be used during the movie, and possibly subsets of larger data sets. Since movie-making can take a long time if you are ambitious, it is best to compute or plot all the elements that can be done outside your main frame-loop rather than waste time doing the same thing over and over again. You are then ready for the main loop.
3. Initialize a frame counter to 0 and have a loop that continues until your frame counter equals the desired number of frames. You must use your frame counter to create a unique file name for each frame image so that the series of images can be lexically arranged. We recommend using the GMT shell function gmt_set_framename to format the frame counter with an adequate number of leading zeros; see our examples for details. The bulk of your main loop involves create the single PostScript plot for this particular frame (time). This can be trivial or a serious scripting exercise depending on what you want to show. We will give a few examples with increasing complexity. Once the PostScript plot is created you need to rasterize it; we recommend you use ps2raster to generate a TIFF image at the agreed-upon resolution. We also recommend that you place all frame images in a sub-directory. You may increment your frame counter using gmt_set_framenext.
4. Once you have all your frames you are ready to combine them into an animation. There are two general approaches. (a) If your image sequence is not too long then you can convert the images into a single animated GIF file. This file can be included in PowerPoint presentations or placed on a web page and will play back as a movie by pausing the specified amount between frames, optionally repeating the entire sequence one or more times. (b) For more elaborate projects you will need to convert the frames into a proper movie format such as Quicktime, AVI, MPEG-2, MPEG-4, etc., etc. There are both free and commercial tools that can help with this conversion and they tend to be platform-specific. Most movie tools such as iMovie or MovieMaker can ingest still images and let you specify the frame duration. Under OS X we prefer to use Quicktime. [1] Free tools exist to call the Quicktime library functions from the command line as we prefer to do in our scripts. Another choice is to use the free mencoder. You will find yourself experimenting with compression settings and movie formats so that the final movie has the resolution and portability you require.
5. Finally, when all is done you should delete any temporary files created. However, since creating the frames may take a lot of time it is best to not automatically delete the frame sub directory. That way you can redo the frames-to-movie conversion with different settings until you are satisfied.

[1] | While Quicktime is free you must upgrade to QuickTime Pro (USD 30) to use the authoring functions.

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