Volume Rendering in VAPOR is controlled by the DVR tab, which stands for Direct Volume Rendering. Volume Rendering is used to visualize 3D variables as volume densities. The opacity and color are controlled by a transfer function. The transfer function can be used to make opaque the features of most interest, and color can be controlled to understand the data values in the regions of interest. The following is a typical use of volume rendering in observing turbulent flow:
The above screenshot is a volume rendering of the vertical component of momentum in a turbulent flow. The transfer function has been edited to highlight the strongest upward and downward momentum, and to make transparent the smaller values of vertical momentum. The colors have been mapped to make the negative momentum yellow to red, and the positive momentum blue to violet. A color bar is used to show the color map. Region mode is enabled, so that the current region of interest (where the volume rendering is occurring) is outlined with a red box.
Transfer function editing: The DVR Transfer function is coupled with a color selector that is placed above it. To edit a color, first highlight the color control point, then click in the color selector at the desired color.
Use of DVR with other renderers: VAPOR GUI's volume rendering does not support more than one DVR being enabled at the same time. If you enable a second DVR in the same visualizer, other enabled DVR's will be disabled. There are also deficiencies in using other transparent volumes in the same volume. The DVR does not work correctly if an Isosurface, Probe, 2D, or Image renderer, using transparency, renders into the same region.
Bits per pixel: The DVR can use either 8 or 16-bits per voxel. Using 16 bits results in smoother images, with a moderate increase in memory space. There is a "Bits per voxel" selector near the bottom of the DVR tab.
Lighting: The volume rendering can often be improved by enabling lighting. This is enabled with a check box near the bottom of the DVR tab. When enabled, the volume rendering simulates the presence of one light source, based on the light settings in the View tab. Only the first light source is used.
Region extents define the volume to be rendered. For efficient volume rendering, the necessary data can be limited, by controlling the Region size, as well as by using LOD and Refinement level control.
Pre-integration, when checked, can sometimes improve the performance and appearance of the volume rendering.