T8: Visualization of Galaxies in cosmological simulations

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

How to Prepare:

(This has to be done only once at the beginning !)

Copy over the spltch code:cp -r ~/splotch .
Change into the splotch directory cd splotch
Edit the Makefile vi Makefile
uncomment the OPT += -DLONGIDS option
comment the #OPT += -DUSE_MPI option
compile the code make -j
Change back to the directory once above cd ..
Download a example parameterfile wget https://www.usm.uni-muenchen.de/~dolag/Lille2026/T08/demo.par

How to do your first image:

The galaxies_box4_uhr.txt and the galaxies_box4_uhr_ideal.txt files contains 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:

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, select a galaxy of the list, download the file needed and edit the demo.par file:

Copy the simulation data: wget https://www.usm.uni-muenchen.de/~dolag/Box4uhr/snapdir_[SNR]/snap_[SNR].[SUB] where you replace the bracketed labels with the numbers from the file for the selected galaxy.
In the demo.par do the same replacement to specity the file to be read: infile=snap_[SNR].[SUB]
Then also set the focus of the camera to the point where you want to look at by setting lookat_x=[X], lookat_y=[Y] and lookat_z=[Z], where you replace the bracketed values by the numbers for that galaxy from the file.
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: ./splotch/Splotch6-generic demo.par
You should have an image demo0000.tga which you can watch.
As the .tga image format is not recognized by all image viewers, you can convert it into more known frmats by:
convert demo0000.tga demo0000.png
You should now have an image demo0000.png which you can watch.

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: