In this paper, we present an effcient way to visualize Super Carbon Nanotubes (SCNTs). SCNTs are complex, hierarchical structures and their models easily consist of more than 1 million atoms. Our SCNTs are modeled as graphs of uniform nodes and egdes. We show that OpenGL instancing is a very suitable technique for rendering such large graphs, because they only consist of two different types of geometry. Our visualizer software exploits this property and we demonstrate that it allows to render the tubes in a fashion that is time- and space-effective. We implemented auto-tuning of the model to the underlying graphics card through adaptive mesh-resolution-choices. We also designed and implemented our own shading programs in the OpenGL Shading Language (GLSL) to realize a sufficient but performant and configurable lighting computation. This allows us to render big models even on laptop GPUs and to cope with models that consist of 150 million triangles, which is still a challenging amount for most of today's graphics cards

Bischof, Christian

Burger, Michael

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C 4 Using Instancing to EfficientlyRender Super Carbon NanotubesAuthors: Burger, Michael, Bischof, Christian205Noname manuscript No.(will be inserted by the editor)Using Instancing to efficiently render Super CarbonNanotubesMichael Burger · Christian BischofReceived: date / Accepted: dateAbstract In this paper, we present an efficient way to visualize Super Car-bon Nanotubes (SCNTs). SCNTs are complex, hierarchical structures andtheir models easily consist of more than 1 million atoms. Our SCNTs aremodeled as graphs of uniform nodes and egdes. We show that OpenGL in-stancing is a very suitable technique for rendering such large graphs, becausethey only consist of two different types of geometry. Our visualizer softwareexploits this property and we demonstrate that it allows to render the tubesin a fashion that is time- and space-effective. We implemented auto-tuning ofthe model to the underlying graphics card through adaptive mesh-resolution-choices. We also designed and implemented our own shading programs in theOpenGL Shading Language (GLSL) to realize a sufficient but performant andconfigurable lighting computation. This allows us to render big models evenon laptop GPUs and to cope with models that consist of 150 million triangles,which is still a challenging amount for most of today’s graphics cards.1 Introduction to Super Carbon Nanotubes and OpenGLInstancingCarbon Nanotubes (CNTs) are an interesting field of research, because theycan be used in a lot of applications. New synthesis approaches allow the com-position of tubes with Y-shaped junctions to structures that possess again theMichael BurgerFG Scientific Computing, TU DarmstadtMornewegstrasse 30, 64293 DarmstadtTel.: +49 6151 16-76991E-mail: michael.burger@sc.tu-damrstadt.deChristian BischofFG Scientific Computing, TU DarmstadtTel.: +49 6151 16-71001E-mail: christian.bischof@sc.tu-damrstadt.deUrheberrechtlich geschützt / In Copyright2 Michael Burger, Christian Bischofshape of a tube. The resulting tubes are called Super Carbon Nanotubes (SC-NTs) ([SW13]). This process can be repeated an arbitrary number of times.Figure 1 shows a CNT. A Y-shaped junction to construct tubes of level 1 isdepicted in figure 2.Fig. 1 SCNT of level 0 Fig. 2 Junction as basic building block ofa SCNT of level 1.The shape is defined by a quadruple of values (D0, L0, DX , LX) whichdetermines the following properties:– D0: Diameter of the resulting SCNT, L0: Length of the resulting SCNT– DX : Thickness of the Y-junction arms, LX : Length of the junction armsEach valid combination of these parameters, together with the tube level,results in a unique tube. Even for small CNTs this procedure leads very fastto structures with high atom-count. This makes it challenging to cope withsuch a high amount of data and to render these structures.Our visualization tool uses OpenGL and instancing [SSKLK13, p. 128 -139]. This allows to draw several elements with the same geometry with slightdifferences. The technique makes it possible to reduce the OpenGL API-callsby only using one instanced draw-call for n instances of an object. This dras-tically reduces the CPU load, as the host only needs to pass the mesh itselfand additional information like the position of the models, their material, etc.2 Features of our toolOur program is able to zoom into and to arbitrarily rotate the scene. It caninteractively visualize the constructed tubes within the main program by ex-ploiting the graph properties that are suitable for instancing. Compared toa data export to a general purpose visualization tool, this saves time andmemory.2.1 Auto-performance-tuning by adaptive meshesThe graph is rendered in a two step approach. First, the connections betweenthe nodes are drawn as GL LINES, a primitive within OpenGL. GL LINES areUsing Instancing to efficiently render Super Carbon Nanotubes 3determined by their end points and the graphics driver and card are responsiblefor rasterizing the line. The second step adds the atoms. These are representedas spheres, modeled by a triangle-mesh. To be able to run on different systems,the visualizer adapts the resolution of the triangle meshes, depending on thegraphics card it is executing on. This change is transparent to the user. Ourvisualizer monitors the frame rate and automatically reduces the details if it istoo slow for a predefined time interval. The current version includes five levelsof detail for the spheres, plus a sixth level that draws no spheres at all.Example screenshots for this procedure are shown in figure 3 and figure 4.Figure 3 shows a scene with the highest resolution, e.g. level 5, while figure 4represents the same scene with detail-level 1.Fig. 3 Full detail Fig. 4 Reduced detailThe visualizer is configured to target about 12 frames per second (fps).From experiments we noticed that this rate is sufficient for practical use.2.2 GLSL shading programsThe model-transformations are applied within our GLSL-vertex-shader pro-grams by the GPU. This moves work load from the CPU to the graphics card.Lighting is also done by our shading programs. We developed two differentshaders for the two distinct components of the model. The one for spheres isbased on Phong-shading-model. The lines are drawn in a predefined color.3 Test Systems and ResultsThe tests show that our visualization software is able to render all structuresthat are of relevance for us. For the measurements we used a desktop systemequipped with an Intel Xeon 1230v2 processor, 16 GB of DDR3 RAM and aAMD R280x graphics card. For comparison, a laptop with an Intel i7-3520M,integrated HD4000 graphics and 8 GB of DDR3 main memory is employed.The visualizer is able to render a (16, 16, 1, 2) tube of level 2 on a R280xwith 24 fps with detail level 1 and still 11 fps with detail level 2. The HD4000is able to reach a frame rate > 12 when falling back to detail-level 0. In thatcase the scene is rendered with about 20 fps. This scene consists of 50 milliontriangles for the spheres in detail level 1. This value rises to about 150 million4 Michael Burger, Christian Bischoftriangles for the detail level 2. This tube is also one of the most complexstructures that our software package for constructing SCNTs can cope withat the moment. Table 1 summarizes the results for two smaller tubes.level 2 (6, 6, 1, 3) tube, 1179648 atomsL5 L4 L3 L2 L1 L0R280x 1.99 5.61 14.97 27.07 63.76 284.35HD4000 0.45 1.33 3.51 5.62 11.19 23.04level 1 (10, 8, 1, 11) tube, 81920 atomsL5 L4 L3 L2 L1 L0R280x 28.57 80.21 211.53 380.66 868.03 3336.66HD4000 6.48 18.15 45.01 66.13 113.38 177.91Table 1 Summary of the achieved frames per second for a two different tubes.As can be seen in table 1, the R280x allows the use of detail-level 3 for thelevel 2 (6, 6, 1, 3) tube case. For HD4000, the visualizer chooses level 1. It canbe noticed that the scaling between the distinct level of details of the R280xis much better than the one of the HD4000. This can be explained with theperformance for rasterizing lines. Considering the change from level 1 to 0,the rendering speed of the R280x increases by the factor of 3 when completelydisabling sphere-rendering, the frame rate of the HD4000 only doubles.Another test is the (10, 8, 1, 11) level 1 tube. As figure 5 shows, it has theproperty that some parts of it are very dense (the”dark lines“), while theothers contain no atoms (the rest). Figure 6 depicts such a”dark line“ indetail.Fig. 5 Level 1 tube with longerarms at the junctionsFig. 6 Single line within the tubeThe R280x has no problems in visualizing the tube at all. The fact of higherline-rendering speed of the R280x is confirmed. While the performance whenneglecting spheres triples for the R280x, the HD4000 increases by a factor of1.57. The overlaying lines in the dense parts seem to be the reason.References[SSKLK13] Shreiner, Dave ; Sellers, Graham ; Kessenich, John ; Licea-Kane, Bill:OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version4.3. 8th. Boston, MA, USA : Addison-Wesley Longman Publishing Co., Inc.,2013. – ISBN 978–0–321–77303–6[SW13] Schroeppel, Christian ; Wackerfuss, Jens: Constructing meshes based onhierarchically symmetric graphs. In: Internal Report, TU Darmstadt 0 (2013)

Using Instancing to Efficiently Render Carbon Nanotubes

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Using Instancing to Efficiently Render Carbon Nanotubes

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