VTK Surface

VTK Unstructured Grid

Individual surfaces can be imported from the VTK file format with the extension VTU. If the data in a surface is changing between time steps, a dictionary PVD file can be used. The PVD file contains time stamps and references to VTU files.

A VTK UnstructuredGrid file stores 3D mesh data with properties attached to cells or points. It’s an XML file that describes the shape of your mesh and the data values on it.

Example files are located at GitHub

Basic Structure

<VTKFile type="UnstructuredGrid" version="0.1" byte_order="LittleEndian">
  <UnstructuredGrid>
    <Piece NumberOfCells="24" NumberOfPoints="19">
      <!-- Data goes here -->
    </Piece>
  </UnstructuredGrid>
</VTKFile>

The file has one main piece with:

NumberOfCells: How many cells (triangles, quads, etc.) you have
NumberOfPoints: How many corner points your mesh has

Main Sections

1. CellData - Properties on cells

<CellData>
  <DataArray type="Float32" Name="ReservoirCell" format="ascii">
    83769 83769 83769 ...
  </DataArray>
</CellData>

This stores values that belong to each cell, like:

Cell IDs
Permeability values
Temperature, pressure, etc.

2. Points - Where your mesh corners are

<Points>
  <DataArray type="Float32" Name="Coordinates" NumberOfComponents="3" format="ascii">
    1095 1095 2604.45 1095 1095 2603.84 ...
  </DataArray>
</Points>

Lists x, y, z coordinates for every point in your mesh.

3. Cells - How points connect together

This has three parts:

Connectivity - Which points make each cell:

<DataArray type="Int32" Name="connectivity" format="ascii">
  0 1 2 0 2 3 0 3 4 ...
</DataArray>

Offsets - Where each cell ends:

<DataArray type="Int32" Name="offsets" format="ascii">
  3 6 9 12 15 18 ...
</DataArray>

Types - What kind of cell each one is:

<DataArray type="UInt8" Name="types" format="ascii">
  5 5 5 5 5 5 ...
</DataArray>

Cell Types

5 = Triangle
8 = Quad
10 = Tetrahedron
12 = Hexahedron (cube)

Data Types

Float32 - Regular numbers (coordinates, properties)
Int32 - Whole numbers (point indices)
UInt8 - Small positive numbers (cell types)

VTU File Format

<?xml version="1.0"?>
<VTKFile type="UnstructuredGrid" version="0.1" byte_order="LittleEndian">
  <UnstructuredGrid>
    <Piece NumberOfCells="24" NumberOfPoints="19">
      <CellData Scalars="ReservoirCell">
        <DataArray type="Float32" Name="ReservoirCell" NumberOfComponents="1" format="ascii">
          83769 83769 83769 83769 83769 83769 70169 83769 83769 83769 83769 83769
          83769 83769 77369 77369 83769 83769 83769 83769 83769 83769 83769 70169
        </DataArray>
        <DataArray type="Float32" Name="ReservoirPerm" NumberOfComponents="1" format="ascii">
          1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-2 1.2e-13 1.2e-13 nan nan 1.2e-13 1.2e-13
          1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13 1.2e-13
        </DataArray>
        <DataArray type="Float32" Name="ReservoirDist" NumberOfComponents="1" format="ascii">
          10 11 12 13 14 15 16 17 17 19 20 21
          22 23 24 25 26 27 28 29 30 31 32 33
        </DataArray>
      </CellData>
      <Points>
        <DataArray type="Float32" Name="Coordinates" NumberOfComponents="3" format="ascii">
          1095 1095 2604.45 1095 1095 2603.84 1094.47 1095 2604.15 1094.47 1095 2604.76
          1095 1095 2605.06 1095.53 1095 2604.76 1095.53 1095 2604.15 1095 1095 2603.23
          1094.39 1095 2603.39 1093.94 1095 2603.84 1093.78 1095 2604.45 1093.94 1095 2605.06
          1094.39 1095 2605.51 1095 1095 2605.67 1095.61 1095 2605.51 1096.06 1095 2605.06
          1096.22 1095 2604.45 1096.06 1095 2603.84 1095.61 1095 2603.39
        </DataArray>
      </Points>
      <Cells>
        <DataArray type="Int32" Name="connectivity" NumberOfComponents="1" format="ascii">
          0 1 2 0 2 3 0 3 4 0 4 5
          0 5 6 0 6 1 1 7 8 1 8 2
          2 8 9 2 9 10 2 10 3 3 10 11
          3 11 12 3 12 4 4 12 13 4 13 14
          4 14 5 5 14 15 5 15 16 5 16 6
          6 16 17 6 17 18 6 18 1 1 18 7
        </DataArray>
        <DataArray type="Int32" Name="offsets" NumberOfComponents="1" format="ascii">
          3 6 9 12 15 18 21 24 27 30 33 36
          39 42 45 48 51 54 57 60 63 66 69 72
        </DataArray>
        <DataArray type="UInt8" Name="types" NumberOfComponents="1" format="ascii">
          5 5 5 5 5 5 5 5 5 5 5 5
          5 5 5 5 5 5 5 5 5 5 5 5
        </DataArray>
      </Cells>
    </Piece>
  </UnstructuredGrid>
</VTKFile>

PVD File Format

<?xml version="1.0"?>
<VTKFile type="Collection"
         version="0.1"
         byte_order="LittleEndian"
         compressor="vtkZLibDataCompressor">
 <Collection>
   <DataSet timestep="1234567.123" file="fracture-00000.vtu"/>
   <DataSet timestep="7654321.765" file="fracture-00001.vtu"/>
 </Collection>
</VTKFile>