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Correlated Photon Mapping for Interactive Global Illumination of Time-Varying Volumetric Data
Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, Faculty of Science & Engineering. (C-Research)ORCID iD: 0000-0002-5220-633X
Linköping University, Department of Science and Technology, Media and Information Technology. Linköping University, Faculty of Science & Engineering. (C-Research)
2016 (English)In: IEEE Transactions on Visualization and Computer Graphics, ISSN 1077-2626, E-ISSN 1941-0506Article in journal (Refereed) Epub ahead of print
Abstract [en]

We present a method for interactive global illumination of both static and time-varying volumetric data based on reduction of the overhead associated with re-computation of photon maps. Our method uses the identification of photon traces invariant to changes of visual parameters such as the transfer function (TF), or data changes between time-steps in a 4D volume. This lets us operate on a variant subset of the entire photon distribution. The amount of computation required in the two stages of the photon mapping process, namely tracing and gathering, can thus be reduced to the subset that are affected by a data or visual parameter change. We rely on two different types of information from the original data to identify the regions that have changed. A low resolution uniform grid containing the minimum and maximum data values of the original data is derived for each time step. Similarly, for two consecutive time-steps, a low resolution grid containing the difference between the overlapping data is used. We show that this compact metadata can be combined with the transfer function to identify the regions that have changed. Each photon traverses the low-resolution grid to identify if it can be directly transferred to the next photon distribution state or if it needs to be recomputed. An efficient representation of the photon distribution is presented leading to an order of magnitude improved performance of the raycasting step. The utility of the method is demonstrated in several examples that show visual fidelity, as well as performance. The examples show that visual quality can be retained when the fraction of retraced photons is as low as 40%-50%.

Place, publisher, year, edition, pages
Institute of Electrical and Electronics Engineers (IEEE), 2016.
Keyword [en]
Volume rendering, photon mapping, global illumination, participating media
National Category
Media Engineering
URN: urn:nbn:se:liu:diva-131022DOI: 10.1109/TVCG.2016.2598430PubMedID: 27514045OAI: diva2:957940

Funding Agencies|Swedish e-Science Research Centre (SeRC)||Swedish Research Council (VR) grant 2016-05462||Knut and Alice Wallenberg Foundation (KAW) grant 2016-0076||

Available from: 2016-09-05 Created: 2016-09-05 Last updated: 2016-09-19Bibliographically approved
In thesis
1. Enhancing Salient Features in Volumetric Data Using Illumination and Transfer Functions
Open this publication in new window or tab >>Enhancing Salient Features in Volumetric Data Using Illumination and Transfer Functions
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The visualization of volume data is a fundamental component in the medical domain. Volume data is used in the clinical work-flow to diagnose patients and is therefore of uttermost importance. The amount of data is rapidly increasing as sensors, such as computed tomography scanners, become capable of measuring more details and gathering more data over time. Unfortunately, the increasing amount of data makes it computationally challenging to interactively apply high quality methods to increase shape and depth perception. Furthermore, methods for exploring volume data has mostly been designed for experts, which prohibits novice users from exploring volume data. This thesis aims to address these challenges by introducing efficient methods for enhancing salient features through high quality illumination as well as methods for intuitive volume data exploration.

Humans are interpreting the world around them by observing how light interacts with objects. Shadows enable us to better determine distances while shifts in color enable us to better distinguish objects and identify their shape. These concepts are also applicable to computer generated content. The perception in volume data visualization can therefore be improved by simulating real-world light interaction. This thesis presents efficient methods that are capable of interactively simulating realistic light propagation in volume data. In particular, this work shows how a multi-resolution grid can be used to encode the attenuation of light from all directions using spherical harmonics and thereby enable advanced interactive dynamic light configurations. Two methods are also presented that allow photon mapping calculations to be focused on visually changing areas.The results demonstrate that photon mapping can be used in interactive volume visualization for both static and time-varying volume data.

Efficient and intuitive exploration of volume data requires methods that are easy to use and reflect the objects that were measured. A value that has been collected by a sensor commonly represents the material existing within a small neighborhood around a location. Recreating the original materials is difficult since the value represents a mixture of them. This is referred to as the partial-volume problem. A method is presented that derives knowledge from the user in order to reconstruct the original materials in a way which is more in line with what the user would expect. Sharp boundaries are visualized where the certainty is high while uncertain areas are visualized with fuzzy boundaries. The volume exploration process of mapping data values to optical properties through the transfer function has traditionally been complex and performed by expert users. A study at a science center showed that visitors favor the presented dynamic gallery method compared to the most commonly used transfer function editor.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 61 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1789
National Category
Media and Communication Technology Computer Science Media Engineering Other Computer and Information Science
urn:nbn:se:liu:diva-131023 (URN)10.3384/diss.diva-131023 (DOI)9789176856895 (Print) (ISBN)
Public defence
2016-10-21, Domteatern, Visualiseringscenter C, Kungsgatan 54, Norrköping, 09:30 (English)
Available from: 2016-10-04 Created: 2016-09-05 Last updated: 2016-10-04Bibliographically approved

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