Topology-guided analysis and visualization of charge density fields

Sammanfattning: Direct volume rendering techniques for scalar fields make use of transfer functions to map optical properties to the field; the field can subsequently be visualized through the drawing of isosurfaces in the volume spanned by the field. The utility of this approach is limited in the case of nested or clustered structures with the same isovalue and further does not easily allow for quantitative measurements of the visualized data. This report explores the use of topological structures (contour trees and Morse-Smale complexes) as an augmentation of traditional direct volume rendering and describes a fully functional implementation in the visualization software Inviwo. The implementation is evaluated through analysis of valency charge density fields in cubic MgO2 and FeO2. It is demonstrated that both contour trees and Morse-Smale complexes provide information and segmentation of initial volume data that allows for selective transfer function application (based on the segmentation), on-demand information on critical points and an overview of the scalar field through a topological representation embedded in the visualized volume. Analysis of the provided charge density fields show that contour trees generate physically irrelevant artefacts and thus are ill-suited for analysing highly symmetric data. On the other hand, the Morse-Smale complex approach is used to extract information of the bond strength of O-O contacts in MgO2 and FeO2 consistent with previous findings, as well as information on electronic charge configuration consistent with previous findings on MgO2. In the case of FeO2, the electronic configuration results are not consistent. This is speculated to be due to a combination of factors, most notably the lack of periodic boundary conditions in the implementation and the more complicated structure of FeO2.   In light of the partially accurate data analysis, as well as the added functionality and utility provided to visualization software, this approach to topology-guided visualization is considered promising and worthy of further study and/or development.