# Volumetric Rendering on Wavelet-Based Adaptive Grid

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## Abstract

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## 1. Introduction

## 2. Wavelet Based Solution of PDEs

## 3. Volume Rendering in Compression Domain

## 4. Numerical Results and Discussion

#### Available Volume Rendering Software

## 5. The Proposed Rendering Techniques

#### 5.1. Direct Summation of Wavelets

#### 5.2. Ray Casting

#### 5.3. Data Structure

#### 5.4. Adaptive Wavelet Grid and AMR

#### 5.5. Completing Wavelet Based Grid to AMR

#### 5.6. AMR Volume Rendering

`VTK_VOXEL`cell type, six correspondent tetrahedra, or

`VTK_TETRA`cells, will be: 0125, 1325, 0245, 2645, 3725, 7625 [VTK, 2000]. It should be noted that such a triangulation is not compatible across the neighboring cells (i.e., T-junctions may appear) and therefore is potentially subject to various rendering artifacts, including gap (empty space) appearance during isosurface generation [46]. Note when a size of the input file is a concern one may save AMR data in a

`VTK_VOXEL`cell format instead of

`VTK_TETRA`cell format [VTK, 2000]. It might be convenient for slice or isosurface generation. As for the volume rendering, VTK seems to perform it on an unstructured tetrahedral mesh only; hence the volume is still to be tetrahedralized (and the additional memory for the tetrahedra to be requested).

## 6. Results

`vtkRenderWindow`class command

`Render`. We note that the rendering studies were performed on old generation system and the reported timings would be much shorter for systems with more modern processors.

#### 6.1. Convection Data Sets

#### 6.2. Hydrogen 3D Orbital Data Sets

#### 6.3. Rendering Results

## 7. Conclusions and Future Work

## Author Contributions

## Funding

## Conflicts of Interest

## Abbreviations

AMR | adaptive mesh refinement |

PDE | partial differential equation |

VTK | Visualization Toolkit |

VT VOXEL | orthogonal parallelepiped-type VTK cell |

VTK TETRA | tetrahedra type VTK cell |

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**Figure 1.**A 4-th order interpolating scaling function $\varphi \left(\xi \right)$ and wavelet $\psi \left(\xi \right)$ vs. non-dimensional coordinate $\xi $.

**Figure 2.**Data structure used for volume rendering. Scaling coefficients are stored as ${N}^{3}$ array of “Coarse Mesh Nodes” ($N=64$ in the current implementation). Wavelet coefficients inside a coarse mesh block are stored in a dynamically allocated array of “Wavelet Nodes”.

**Figure 3.**Completing wavelet based grid (filled circles) to AMR in two-dimensional case by adding the required nodes (white circles). Total number of cells in the final AMR grid is 23.

**Figure 4.**Tetrahedral subdivision of cubic cell of AMR grid. The tetrahedra are: 0125, 1325, 0245, 2645, 3725, 7625.

**Figure 5.**Direct summation rendering time ${T}_{wlt}$ vs. data compression $\sigma $ for the convection (

**a**) and electron density (

**c**) data sets. The coarse mesh is ${64}^{3}$, the number of rays is ${500}^{2}$, the average number of points per ray is 170. The cutoff limits for the scaling and wavelet functions are 1 and 2, respectively. VTK projected tetrahedra rendering time ${T}_{PT}$ vs. the number of tetrahedra in the correspondent AMR grid for for the Rayleigh–Benard convection (

**b**) and electron density (

**d**) data sets.

**Figure 6.**Convection data set. Direct wavelet summation adaptive mesh rendering (

**a**) vs. Amira volume rendering (after extrapolation to a regular ${256}^{3}$ grid) (

**b**) vs. VTK’s projected tetrahedra volume rendering (after completing an AMR grid) (

**c**). Wavelet compression is ∼4.5% relative to ${256}^{3}$ regular mesh.

**Figure 7.**3D electron density data set. Direct wavelet summation adaptive mesh rendering (

**a**) vs. Amira volume rendering (after extrapolation to a regular ${256}^{3}$ grid) (

**b**) vs. VTK’s projected tetrahedra (

**c**), ray casting (

**d**), and z-sweep (

**e**) volume rendering. Wavelet compression is ∼1.2% relative to ${256}^{3}$ regular mesh.

**Table 1.**Data set properties and timings: $\u03f5$ is the threshold, $\sigma $ is the compression for the original wavelet based grid, ${\sigma}_{amr}$ is the compression for the correspondent AMR grid, F and ${F}_{vtk}$ are binary file sizes (in megabytes) to store the original wavelet and tetrahedralized AMR grid in VTK format, respectively. ${T}_{wlt}$, ${T}_{PT}$, ${T}_{RC}$, and ${T}_{ZS}$ are the rendering times (in seconds) for the direct wavelet summation algorithm and for the VTK library unstructured grid renderers: ProjectedTetrahedra, RayCast, and ZSweep, respectively. A hyphen (-) implies the lack of RAM for the rendering.

Data Set | $\mathit{\u03f5}$ | $\mathit{\sigma}$, % | ${\mathit{\sigma}}_{\mathbf{amr}}$, % | F, M | ${\mathit{F}}_{\mathbf{vtk}}$, M | ${\mathit{T}}_{\mathbf{wlt}}$ | ${\mathit{T}}_{\mathbf{PT}}$ | ${\mathit{T}}_{\mathbf{RC}}$ | ${\mathit{T}}_{\mathbf{ZS}}$ |
---|---|---|---|---|---|---|---|---|---|

0.0005 | 22.5 | 87.1 | 101 | 2199 | 254 | - | - | - | |

0.001 | 16.7 | 74.9 | 75 | 1847 | 198 | - | - | - | |

0.005 | 6.85 | 39.9 | 31 | 915 | 94 | 149 | - | - | |

Convection | 0.01 | 4.51 | 27.3 | 21 | 607 | 70 | 96 | - | - |

0.05 | 1.99 | 8.15 | 9.0 | 167 | 42 | 25 | 100 | - | |

0.1 | 1.72 | 3.82 | 7.8 | 83 | 41 | 10 | 58 | - | |

0.5 | 1.56 | 1.55 | 7.1 | 38 | 39 | 4.8 | 43 | 97 | |

${10}^{-7}$ | 8.12 | 28.8 | 37 | 706 | 107 | 106 | - | - | |

${10}^{-6}$ | 3.69 | 12.9 | 17 | 306 | 58 | 46 | - | - | |

${10}^{-5}$ | 1.84 | 4.71 | 8.3 | 109 | 37 | 16 | 54 | 7400 | |

3D electron | ${10}^{-4}$ | 1.21 | 1.65 | 5.5 | 41 | 32 | 5.2 | 35 | 210 |

${10}^{-3}$ | 0.83 | 1.04 | 3.8 | 26 | 31 | 3.6 | 29 | 78 | |

${10}^{-2}$ | 0.41 | 0.60 | 1.9 | 15 | 29 | 2.1 | 25 | 57 | |

${10}^{-1}$ | 0.10 | 0.22 | 0.5 | 5 | 19 | 0.7 | 22 | 38 |

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**MDPI and ACS Style**

Vezolainen, A.V.; Erlebacher, G.; Vasilyev, O.V.; Yuen, D.A. Volumetric Rendering on Wavelet-Based Adaptive Grid. *Fluids* **2022**, *7*, 245.
https://doi.org/10.3390/fluids7070245

**AMA Style**

Vezolainen AV, Erlebacher G, Vasilyev OV, Yuen DA. Volumetric Rendering on Wavelet-Based Adaptive Grid. *Fluids*. 2022; 7(7):245.
https://doi.org/10.3390/fluids7070245

**Chicago/Turabian Style**

Vezolainen, Alexei V., Gordon Erlebacher, Oleg V. Vasilyev, and David A. Yuen. 2022. "Volumetric Rendering on Wavelet-Based Adaptive Grid" *Fluids* 7, no. 7: 245.
https://doi.org/10.3390/fluids7070245