T12: Visualization of Galaxies in cosmological simulations

We are going to use Splotch to visualize galaxies out of the Magneticum simulations.

How to start:

Log in to USM from your outside connection via ssh -Y numprakt@moon.usm.uni-muenchen.de, you might remember the password from the numerical practicum, otherwise, ask us.
Log in to our central workstation via ssh -Y numprakt@dorc1
Change to the directory /ptmp2/T12 and create a new sub-directory for you.
Copy from the WhatYouNeed directory Splotch6-generic and demo.par into your directory, as well as the galaxies_*.txt files.

How to do your first image:

The galaxies_*.txt files contains a PATH in the first line and then a list of interesting objects, defined by 5 numbers: SNR and SUB defines the file and X, Y and Z the coordinate. Here an example:

/HydroSims/Magneticum/Box4/uhr_test/
SNR SUB X Y Z
136 12 44319.5 11750.2 15495.4
136 9 24555.5 37925.5 41413.5
036 10 32042.6 44672.2 22093.5

Now, edit the demo.par file:

Construct the file name to be read for the object of interest: infile=[PATH]/snapdir_[SNR]/snap_[SNR].[SUB]
Define the point where you want to look at by setting lookat_x=[X], lookat_y=[Y] and lookat_z=[Z].
Finally place the Camera at a useful position, e.g.: camera_x=[X], camera_y=[Y]+200 and camera_z=[Z].
Now execute Splotch by: ./Splotch6-generic demo.psr

You should have an image demo0000.tga which you can watch by xv demo.tga or display demo.tga.

How to improve your image:

Now you have a large range of possibilities to influence the appearance of the result. In general, lines starting with # as well as unused or unknown parameters in the parameter file are ignored. ptypes=2 defines that you want to visualize two particles species and ptype0=0 defines that the first species are the gas particles of the simulation and ptype1=4 defines that the second ones are the star particles in the simulation. Accordingly, all parameters ending with 0 refer to the gas particles and the one ending with 1 are referring to the stellar particles.

brightness[0/1]=[value] controls the overall brightness a component becomes. If you set it to 0 for a component then this component disappears.
palette[0/1]=[filename] defines the color table used to translate a value to RGB color. Have a look on the files in /ptmp2/T13/WhatYouNeed/palettes/ to see what you could us.
outfile=[filename] controls the name of the image files produced and xres=[value] and yres=[value] control the dimension of the produced image.
pic_gamma=[value], pic_brightness=[value] and pic_contrast=[value] allow to manipulate the final image. Note that gamma here is the real gamma correction, while programs like photo shop or similar typically interpret gamma as 1/gamma, see also here.
You have various ways to change the appearance of your image by controlling the quantity which is used to get visualized.
color_[min/max][0/1]=[value] allows to set the min/max values used. When you run Splotch it displays you the underlying min/max values of all particles.
color_log[0/1]=[TRUR/FALSE] allows to use the logarithm of the quantity (if you change, do not forget to adapt the min/max values accordingly.

How to make a movie:

Splotch allows you to create image sequences directly in a very simple way. You create just a file with the evolution of some of the parameters and Splotch will produce an image for each line in the file. So simply:

Create a simple text file with the first line stating the parameters you want to modify and the consecutive lines with the values for them. For example move the camera:
camera_x camera_y
34555.5 38125.5
34559.0 38125.5
34562.5 38125.4
Give the file name as in the parameter file as scene_file=[filename]. You can additional control how the file is read by setting scene_start=[value] and scene_incr=[value].

When you now run Splotch files with increasing numbers will be created, which you can glue together as a movie after wards. For example you can set the camera position as a circle around the object, see the movie below.

How to explore the unknown: