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Volume Ray Casting in WebGL

2012; Springer Nature; Linguagem: Inglês

10.5772/34878

1431-1380

John Congote, Luis Kabongo, Aitor Moreno, Álvaro Segura, Andoni Beristain, Jorge Posada, Oscar Ruiz-Salguero,

Computational Geometry and Mesh Generation

Computer Graphics scalar ( f : R 3 → R) or vector ( f : R 3 → R 3 ) fields.For the purpose of this chapter, we will use the term volumetric data sets to refer to scalar fields and will ignore for the time being vector fields.Surface-based raster rendering techniques are obviously not suitable for visualizing such datasets and specific Direct Volume Rendering algorithms are needed, which are not available for the Web.Ray Casting is a common technique for volume visualization which displays the saliend characteristics of the volume set, although it is not photo-realistic.Therefore, our work uses Volume Ray-Casting, which is a common technique in Computer Graphics for volume visualization.Originally presented by Kajiya Kajiya & Von Herzen (1984) as an extension of Ray-tracing algorithm, then Levoy Levoy (1988) defines the volume rendering.This has been further studied in Hadwiger et al. (2009).The aforementioned rendering is not photo-realistic, however it shows important characteristics of the dataset.In medical imaging, diagnostic techniques, such as computer tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET), produce sets of parallel slices that form a volumetric dataset.Volume rendering is a common technique for visualizing volumetric datasets along with multi-planar reconstructions (MPR).Storage and distribution of these 3D images usually requires a Picture Archiving and Communication Systems (PACS), which normally uses specialized workstation software (Meyer-Spradow et al. (2009), Fogal & Kruger (2010)) for interactive visualization (Mahmoudi et al. (2009)).Reference Kabongo et al. (2009) presents on some of the few implementations of volumetric data displays.WebGL is a new standard for accelerated 3D graphics rendering on the Web that complements other technologies in the future HTML5 standard (Marrin ( 2011)).Some of the major Web browsers, including Google Chrome, Mozilla Firefox, WebKit, Safari and Opera have already implemented WebGL in their latest releases or release candidates.WebGL is basically a JavaScript binding of the OpenGL ES API and enables low level imperative graphics rendering based on programmable shaders.We present in this chapter an implementation of a volume rendering system for the Web, based on the Volume Ray-casting algorithm and implemented on WebGL.The system is capable of obtaining interactive visualization with diverse volume datasets (Figure 1).The original Volume Ray-casting algorithm was slightly modified to work with the input structures needed for the Web environment.Special care was taken to avoid the use of dynamic server content.This avoidance allows for the algorithm to be used without increasing the demands on the server and shifts, as much as possible, the processing to the client.This work was tested in bioinformatic scenarios with volumetric datasets such as medical imaging.Also, a metereological prototype was developed to visualize doppler radar datasets.This chapter is organized as follows.Section 2 presents a brief status of the different technologies present in this work: Volume Rendering, Web rendering, medical and confocal visualization.Section 3 presents our methodology for volume rendering with special attention to the modifications of the algorithm for the Web environment.Section 4 shows the implementation of volume rendering for doppler wheather radars.Section 5 presents the output obtained by the implemented algorithm and the performance values in different conditions.Section 7 presents the conclusions of our work and future directions.Another important evolution will be the integration of surface rendering within volume-rendered scenes in order to visualize, for example, segmented areas in medical images or terrain surfaces.Some tests have already been performed on desktop prototypes.This chapter lays the technical ground that would make the integration of surface render in volume-rendering (via WebGL) possible and reasonable. 8.