Documentation/UserGuide/Image Files

Two dimensional images of any variable can be created by Athena in two formats: ppm (portable pixel map) and pgm (portable gray map). The former should be used for color images (a corresponding palette must be specified), and the latter for grayscale images.

Since images are more compact than storing the floating point data directly (e.g. as binary or vtk files), the image formats can be used to create high time resolution animations of any variable. However, accuracy is lost by not storing the floating point data, therefore image files cannot be used for further quantitative analysis.

The advantage of using the ppm image format over the many others available (e.g. gif, jpg, tif, etc.) is that no external libraries are required to create the images. Moreover, since they are not compressed, there is no loss of information, on the other hand this often makes ppm images much larger than other formats.

It is easy to post-process the image files created by Athena into other formats, including mpeg (or other format) animations, using image conversion software such as ImageMagick.

Both ppm and pgm image files contain a snapshot of the data at a particular time. New files are created whenever the integration time exceeds an integer multiple of <output>/dt. At the end of execution, the lesser of tlim/dt or <time>/nlim sequentially numbered files will be created.

ppm image files

An example of an <output> block in an input file that generates ppm images of the pressure is given below:

out_fmt = ppm
dt      = 0.004
out     = P
id      = P
dmin    = 0.01
dmax    = 0.70
palette = rainbow

Note the extra parameters required for ppm image files.

dmin and dmax specify the global minimum and maximum applied to all the images (image colors are scaled to these values). If these values are not specified, each image is auto-scaled to the min/max at that time, however this is not generally useful for animations. Athena will print the min/max over all time for each output variable in a diagnostic message at the end of execution, so low resolution runs can be used to provide a first guess for the proper values of dmin/dmax.

palette specifies which color palette to use, examples are given in Color Palettes in Athena.

Slicing is very useful for creating two-dimensional images from 3D runs, see Specifying Output Slices, and the example using the pgm format below.

New (user-defined) variables can be output as images, see User-defined Output Variables, and the example using the pgm format below.

pgm image files

Grayscale images can be created using the pgm format, rather than specifying a grayscale palette in the ppm format. An example of an <output> block which creates a pgm image of a 2D slice in a 3D calculation using a user-defined variable is given below.

out_fmt = pgm                # pgm image file
out     = dVy                # user-defined variable: fluctuations in V3
id      = dVy                # file id
usr_expr_flag = 1            # user-defined variable
dt      = 62.831853          # time step between output of delta V3
dmin    = -0.0006            # min value for imaging delta V3
dmax    =  0.0006            # max value for imaging delta V3
x2      = 0.0                # slice in X-Z plane at Y=0

The variable dVy would have to be calculated in a special function added to the problem generator file.