University of Bergen | Faculty of Mathematics and Natural Sciences | Department of Informatics | Visualization Group
You are here: Department of Informatics > Visualization Group > Teaching > INF219: Project in Visualization > Fall 2008 > Øyvind Sture
 > about
 > team & contact info
 > research
 > publications
 > projects
 > teaching
 --- > INF219: vis-project
 > seminars
 > resources
 > network
 > events
 > links

Øyvind Sture: Ambient occlusion shading of seismic data


This programming project consists of two parts. The first part deals with creating a file loader for volumetric seismic data that has been stored in the standardized SEG Y format. SEG Y data files are used to store geophysical data acquired from reflection seismology. The second part deals with visualizing the loaded SEG Y volume using the shading model called ’local ambient occlusion’. The renderings using the local ambient occlusion shading model is compared with renderings using the de facto Blinn Phong shading model. The comparisons show that the novel combination of seismic volumetric data with local ambient occlusion give superior results when it comes to revealing critical details in the seismic data.
Both the file loader and the shader are written as separate plugins for the Volumeshop software which is an interactive system for direct volume illustration. The programming is done on a Windows computer, using Visual C++ and OpenGL. The hardware consists of an NVIDIA Geforce 9600GT, 512Mb graphics card. The shading is done with a naive and nonoptimized implementation of local ambient occlusion.



Figure 5 and 6 shows a test dataset specifically created with a wedge, corners, flat areas and a hold to check that the ambient shading was calculated correctly. Figure 5 is rendered with Phong, and figure 6 is rendered with Local Ambient Occlusion. The cut-out parts of the object is where the shading effect is most visible.
Figure 7 shows the head dataset rendered with Phong shading. Figure 8 shows the difference between 14 rays for each voxel, and 26 rays for each voxel. The image rendered with 26 rays looks smoother than the image rendered with 14 rays. There are still some artifacts in the image, but one can see that more rays improve the image quality. In the paper by Hernell et al. they use up to 128 rays to create images.
Figure 9 shows the difference when the seismic dataset is shaded with Phong and with Local Ambient Occlusion, with 14 and 26 rays. The layers (so called horizons) in the seismic dataset are more clearly visible when the Local Ambient Occlusion model is used, and it is also easier to see where the dataset are more sparse. When comparing the image rendered with 14 rays and the image rendered with 26 rays, one can see that the top layer is smoother in the image with 26 rays. Figure 10 shows a split image with Phong shading on the left side and Local Ambient Occlusion on the right. Figure 11 shows a striped image with Phong shading and Local Ambient Occlusion. Figure 12 shows magnified selected areas of sections of figure 9. It can be seen that local ambient occlusion yields less noisy images with more clearly delineated layer structures.

Conclusion and problems

The implementation on the GPU without the use of precalculation led to some problems. The rendering of the objects are very slow, up to multiple seconds. One possible speedup would be to store the computed light values when they were computed, or to do this in a precalculation part, since the Local Ambient Occlusion model is independent of the view vector.
Another problem was that for large datasets, or when I tried with more rays, overloading the graphics card with intense calculations led to frequent bluescreens, or artifacts in the rendered images.
The use of a Local Ambient Occlusion model appears to have a big impact on visualization of noisy datasets like seismic dataset.

Related work

Ambient occlusion of medical data is discussed in [4] and quick ambient occlusion rendering is covered in [7].
Ambient occlusion applied on molecular data can be seen in [5]. In [6] it is discussed a variation of the phong shading model on seismic data by using a surface normal which is better suited for seismic data.



[3] Technical-Standards/Documents/seg y rev1.pdf
[4] F. Hernell, P. Ljung and A. Ynnerman, Efficient Ambient and Emissive Tissue Illumination using Local Occlusion in Multiresolution Volume Rendering, In Eurographics/IEEE-VGTC Symposium on Volume Graphics (2007)
[6] 3D Seismic Volume Rendering, Pedro Mário Silva, Marcos Machado, Marcelo Gattass, 8th International Congress of The Brazilian Geophysical Society, Rio de Janeiro, Brazil, 14-18 September 2003.
[7] Interactive Volume Rendering with Dynamic Ambient Occlusion and Color Bleeding. Timo Ropinski, Jennis Meyer-Spradow, Stefan Diepenbrock, Jörg Mensmann and Klaus Hinrichs. Eurographics 2008
[8] shading model
[12] seismology

Supervisor: Daniel Patel

The full report: INF219 project: Ambient occlusion shading of seismic data

Test data: here

Volumeshop Plugins: here

 Last change: Daniel Patel, 2009-04-06