VisIVO Extracting Galaxies

Transcription

1 VO Day Usecase 4 VisIVO Extracting Galaxies The result of a Cosmological N Body SPH simulation is a set of millions (sometimes billions) of particles. From this complex distribution, we show how we can identify a galaxy and extract the corresponding data for further investigation

2 1. Load Data File Menu >Import >Binary Open small10m.bin file (not the.head file, the.bin file with this demo you load 10 millions particles, quite a lot so you need a good computer. Otherwise use small1m.bin, just 1 million particles however some of the next settings could be slightly different) Select all listed fields (3D coordinates, density, temperature and tipical size of each particle) OK The data node on the control tree panel (hereafter tree) appears

3 2. Visualize Particles Select the small10m node, clicking on it Open the Create Visual Object panel clicking onthe icon with three small spheres (or, equivalently, Operations >Table Data >Create Visual Object) X, Y, Z should be already selected as cartesian coordinates (if not, select them) OK work on it!!! At this point the Render Window will open, with all the points visualized. At this stage you get a just monocromatic spot on the screen.

4 3. Improving Rendering 1/3 Select Visual Points 3D in the tree Click on Rendering in the lower panel Play with the Opacity slider. The response could be slow depending on your PC. You will see particles become transparent and structures t appear Go to Object Fields panel, clicking on the corresponding tab Select "temperature" as Active Scalar (which will be used to color points)

5 3. Improving Rendering 2/3 Go back to Rendering panel Click on Use LUT and Log. Scale. The points will be coloured by the temperature

6 3. Improving Rendering 3/3 At this point the image is again quite fuzzy. Let's improve it!!! Click on the colour bar in the "Rendering" panel. The LUT editor will open On the top, select the temperature from the colours menu (at the beginning default is active) set the "mapped value range" between (min.) and 2e6 (max.) Apply Now let's set the transparency of each colour select the first (white) square on the top left of the colours array (click on it) drag the round selector in the vertical coloured rectangle (below Fade colours button) to the base of the rectangle (max transparency) select all the squares on the colour array, passing on them with the mouse with the left button pushed When all squares are selected, click on Fade ONLY alpha. The transparency of each colour will be linearly interpolated between 0 and 1 OK At this point you should ldhave a nice image where low density regions are blue and galaxies are red and yellow. At this point, you can have fun, moving the volume, rotating it (move the mouse keeping the lfb left button pushed), zooming (move the mouse keeping the right button pushed) or translating (move the mouse keeping the central button pushed).

7 4. Computing Particle masses Gl Galaxies are identified d as regions with higher masses. To do that we need the mass of the particles. We must derive them from the available fields as density*volume(=smooth^3). Select small10m in the tree Select the f(x) button on the top (or Operations >Table Data >Math Op.) In the Expression field write: density*smooth*smooth*smooth smooth smooth smooth Set the Name to Mass (or whatever you like) OK Select Visual Points 3D in the tree Select Object Fields In the scalars list, the new Mass field is now present

8 5. Identify Particles 1/3 - Select Visual Points - 3D in the tree - Calculate the continuous mass field by smoothing the mass on a support computational mesh by selecting Operations->Point Functions->Point Distribute - Select Mass as Field - Set Resolution to OK The Distributed points field appears in the tree.

9 5. Identify Particles 2/3 Let's define galaxies as regions in which the mass is above a given threshold (in arbitrary units). To visualize these regions, we use isosurfaces. Volumes inside the surfaces are above the given threshold. - Select Distributed ib t points - Select Operations->Volume Functions- >Extract IsoSurface - Set contour value to 5000 (500 if you use samll1m) - OK

10 5. Identify Particles 3/3 -TheDistributed points Isosurface 5000 field appears in the tree, visualize it by clicking on the square to the left - Galaxies are points clusters insideid the isosurfaces - Find one you like and go close to it

11 6. Extract the galaxy - Select Distributed points Isosurface 5000 in the tree - Select Operations->Surface Functions- >Extract Cluster - Click on the selected isosurface (galaxy) which will change colour (this operation could take some time) - OK Points inside the surface will be identified and extracted, a new dataset (Sampled Cluster Points) appears on the tree. You can treat it as the initial dataset, visualizing i it in a new render window, that you can open from View->Add View- >3D View

12 7. Play with the galaxy Since the points of the galaxy are few, you can visualize them as coloured spheres. - Select Visual Points associated to the new dataset -Go to Object Fields panel and select temperature -Go to Rendering - Click on Use LUT - Click on Sphere - Set the Scale field to Select Sampled Cluster Points - Save the new dataset in a file: File->Export

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