Module: Volren ()

Description:

Direct Volume Rendering is a very intuitive method for visualizing 3D scalar fields. Each point in a data volume is assumed to emit and absorb light. The amount and color of emitted light and the amount of absorption is determined from the scalar data by using a Colormap which includes alpha values. Default colormaps for volume rendering are provided with the distribution and can be edited using the colormap editor. Then the resulting projection from the "shining" data volume is computed (see also Voltex).

The Volren module provides several visualization techniques: shaded and classical texture-based volume rendering (VRT), maximum intensity projection (MIP), and digitally reconstructed radiograph (DRR). The standard VRT and the its shaded version enable a direct 3D visualization with flexible color and transparency colormaps with virtual lighting effects for better rendering of complex spatial structures and enhancing fine detail. The MIP rendering allows the visualization of the highest or lowest intensity in a data volume along the current line of sight. The DRR simulates a radiograph display from arbitrary views using the loaded volume data. Furthermore, the Volren module enables you to render segmented regions at the same time with different colormaps. A LabelField can be attached to the Volren and each material of the label list can be rendered separately. In order to optimize the performance, the image volumes are represented at lower resolution in case of interactive rendering. For relatively large image volumes, you can also switch to a constant "Low Resolution" mode (see below). Volren is built for use with popular graphics accelerator technology such as the NVIDIA Quadro FX(TM) graphics boards.

Notes and Limitations

• On some systems a significant slowdown can occur if the data set is larger than the available texture memory (which is typically 4 - 16 MB). In this case, select the option LowRes only (see below).
• When two or more Volren modules are used to render intersecting volumes (multivolume rendering), some rendering inaccuracies may occur. In case of multivolume rendering the supported combinations of modes are one MIP together with one MIP or one VRT with one VRT (see below Mode). Other modes and combinations may lead to incorrect visual results.
• The Apply button is enabled only when the rendering must be recomputed.

Comparisons with the Volume Rendering module are described in a dedicated page.

Connections:

Data [required]
The 3D scalarfield to be visualized. Alternatively an RGBA data volume (Colorfield) can be connected. In this case, no colormap is used, but the color and opacity values are taken directly from the data.

Labels [optional]
Input LabelField to be rendered.

Roi [optional]
Connection to a module providing a region-of-interest, like SelectRoi. If such a module is connected, only the selected part of the volume will be displayed.

Roi Corner Cut [optional]
Connection to a CornerCut module providing a cutting box positioned at a specified corner of the bounding box. If such a module is connected, the selected corner region of the volume will be not displayed.

Color1, Color2, Color3, ... [optional]
Colormaps used to visualize the data.

Ports:

Materials

When a LabelField object is attached to the Volren module, the material list is shown in this group. For each material, a new port is opened. Clicking on the colormap icon allows you to select a colormap from the list in the Transfer Functions tab (see above). To create and use a new colormap, it is necessary to generate it directly in the Transfer Functions tab and to select it in the Materials tab. By activating the option Constant, the material volume will be rendered using a constant color instead of a colormap. To modify the color, click on the color icon. Note that if a volume data set is connected to the Volren, only LabelField objects of the same volume size can be connected.

Transfer Functions

Create

In this group you can add or manipulate the Volren colormaps. This is particularly useful when a label image is rendered together with the volume data. In this case, several different colormaps could be used at the same time (see Materials group below). Pressing the New button creates a new colormap which can be controlled by its Option, Color, and AlphaScale ports. To delete a colormap just press the Delete button.

Colormap

Port to select a colormap.

Opacity

A global factor to change the overall opacity of the object independent of the data value.

Advanced

The parameters used for the shading and transparency effects, and the rendering modes

Mode

VRT. Texture-based volume rendering, with optional shading effects. If VRT is selected the Shading port is activated.

DRR. Digitally reconstructed radiograph. This technique simulates a radiograph from arbitrary views using the loaded data. If this mode is selected, the Gamma port is shown.

MIP. Maximum intensity projection. When this option is selected, the brightest data value along each ray of sight is displayed instead of the result of the emission absorption model. This mode is especially useful for very sparse data sets, for example, angiographic data or images of neurons.

LowRes only. The image volumes are rendered always at lower resolution. For relatively large image volumes select this mode to optimize the computational efficiency.

Gamma

This port controls the shape of the transfer function when DDR mode is selected. The opacity value is taken to be proportional to the data values. The smaller the gamma value is, the more prominent regions with small data values will be.

Shading

This port is shown if VRT is selected in port Mode. If None is selected, no shading effects are visualized. With Diffuse and Specular, two pre-set shading effects are activated. Option Diffuse simulates a diffuse light source, while Specular simulates a specular one. The option User-defined opens the Coefficients port to specify the lighting parameters.

Coefficients

This port is enabled when the User-defined option in port Shading is selected. In this port you can specify the parameters of the light source. Ka, kd and ks are the coefficients for ambient, diffuse, and specular components of the illuminating light. To set the color of the light source, click on the light color icon.

Light Angle

This port is enabled when Diffuse, Specular, or User-defined options are selected in the Shading port. This port controls the position of the simulated light source.

Light Angle Yaw

This port is enabled when option User-defined in the port Light Angle is selected. It allows you to displace the light source on a horizontal semi circle around the volume data.

Light Angle Pitch

This port is enabled when the User-defined option in the Light Angle port is selected. It allows you to displace the light source on a vertical semi circle around the volume data.