UnityVolumeRendering Documentation

The project handles coordinate systems for DICOM and NRRD formats, converting them to Unity's coordinate system using meter units.

By default, imported datasets have their size normalized. If you need to maintain the original physical size (for example, to ensure multiple datasets align correctly in world space), select the outer GameObject (the one containing the VolumeRenderedObject component) and set its Scale to (1, 1, 1).

The Render mode setting determines the fundamental visualization technique used for the volume:

• Direct Volume Rendering: Uses raymarching combined with transfer functions to visualize density.
• Maximum Intensity Projection: Displays only the maximum density along each ray.
• Isosurface Rendering: Uses raymarching but stops the ray once it hits a specific surface threshold.

The Cross section plane is a simple plane that can be moved and rotated to clip the volume at any arbitrary position and angle.

Note for developers: If you require axis-aligned clipping planes, you should implement your own tool by implementing the CrossSectionObject interface and adding it to the CrossSectionManager.

Shadow volumes are implemented using a 3D texture generated for the entire dataset. A compute shader updates this volume by casting rays from the light source through the dataset to store shadow information.

To maintain performance and handle platforms where async compute is unavailable, the ShadowVolumeManager divides the shadow volume into smaller chunks and updates exactly one chunk per frame.

Direct Volume Rendering maps density (and optionally gradient magnitude) to color and opacity. You can modify these via the menu bar: Volume Rendering > 1D Transfer Function or 2D Transfer Function.

1D Transfer Function

• X-axis: Density.
• Y-axis: Alpha (opacity).
• Interaction: Move grey alpha knots to create an opacity curve. Right-click to add new alpha knots. Use the bottom gradient panel to map color to density (right-click to add knots; click existing knots to modify color).

2D Transfer Function

• X-axis: Density.
• Y-axis: Gradient magnitude.
• Interaction: Click "add rectangle" to create a new shape. Use the four sliders (bottom left) to adjust size/position, and the two sliders on the right to adjust min/max alpha/opacity. Each rectangle can be assigned a specific color via a color picker.

The raymarching shader can be configured to use Early ray termination. This optimization allows the shader to exit early once enough samples have been accumulated, avoiding the enumeration of invisible voxels.

It is recommended to keep this setting enabled, as it provides significant performance improvements with typically no noticeable visual changes.

Enabling lighting provides a more realistic visualization by calculating the gradient at each voxel to determine a normal, which is then used for Phong lighting.

Note: This is computationally expensive and may impact both memory usage and rendering speed.

To reduce "staircase artifacts" in the volume, you can enable Cubic interpolation.

Note: This can be quite expensive in terms of performance, particularly when lighting is also enabled.

The Sphere cutout tool spawns a sphere that can be moved and rotated to perform a cutout on a dataset. Like the box tool, it features two modes:

• Exclusive: Cuts out everything that overlaps the sphere.
• Inclusive: Cuts out everything that is outside the sphere.

To quickly preview the renderer with sample data:

1. Install the package via UPM.
2. In the Package Manager, find "Unity Volume Rendering", go to the Samples tab, and import both Basic Demo and Sample Data.
3. Open the imported TestScene.unity.
4. In the Unity menu bar, click Volume Rendering.
5. Select Load Raw dataset.
6. Choose the VisMale.raw file from the imported Sample Data.
7. Click the import button.