This node renders a strip (connected series) of slices. The strip is defined by a set of 2D vertices that form a polyline. Each segment of the polyline is extruded along the axis specified in the OIV.VolumeViz.Nodes.SoFenceSlice.axis field to form (in effect) an oblique slice. The default axis is Z, so the 2D points are treated as (X,Y) values. The points may be outside the 3D extent of the volume, but only the portion of the slice inside the volume will be drawn (subject to region of interest and other clipping nodes).

The OIV.VolumeViz.Nodes.SoVolumeData node on which this shape is applied can be specified with OIV.VolumeViz.Nodes.SoFenceSlice.dataSetId. When this field is set to 0, the last OIV.VolumeViz.Nodes.SoVolumeData node on state is used.

A similar effect could be obtained using volume geometry (e.g. OIV.VolumeViz.Nodes.SoVolumeFaceSet), but OIV.VolumeViz.Nodes.SoFenceSlice is more convenient and is optimized for this specific case.

The 2D coordinates are interpreted according to the following table. See the code example below.

Fence axis	Coordinate axes
X	Y , Z
Y	Z , X
Z	X , Y

The interpolation field controls how the texture is interpolated.

The alphaUse field (OIV.VolumeViz.Nodes.SoSlice) controls how the voxel's alpha component is used when drawing the fence slice.

Optionally a bump mapping effect may be applied. Normal vectors are automatically computed from the data value gradient. The enableBumpMapping and bumpScale fields (OIV.VolumeViz.Nodes.SoSlice) control whether bump mapping is active and the intensity of the effect.

Notes:

• Transformation matrices: The volume size and orientation (like geometry) can be modified by transformation nodes in the scene graph and this in turn modifies the appearance of volume visualization nodes. However the same transformation must be applied to the volume data node and all volume visualization nodes associated with that volume. So effectively any transformation nodes that affect the volume must be placed before the volume data node.

• Picking: The entire slice is pickable, even where it is transparent as a result of the current transfer function. Currently OIV.VolumeViz.Nodes.SoFenceSlice does not provide an OIV.Inventor.Details.SoDetail object.

• Interpolation: Interpolation is specified using the interpolation field. The default (LINEAR) does bi-linear interpolation between voxel values. The NEAREST value can be used to display individual voxels. For best image quality we recommend using the MULTISAMPLE_12 value.

• Data range: By default VolumeViz maps the entire range of the voxel's data type (e.g. 0..65535 for unsigned short) into the colormap. This is often correct for byte (8 bit) voxels, but seldom correct for 16 bit voxels and never correct for floating point voxels. Use an OIV.LDM.Nodes.SoDataRange node to specify the actual (or desired) range of data values to be mapped. Also use an OIV.LDM.Nodes.SoDataRange node to implement brightness/contrast control like the Window/Level setting commonly used with medical images.

• Clipping: Volume primitives can be clipped using a region of interest (OIV.LDM.Nodes.SoROI), geometry (OIV.VolumeViz.Nodes.SoVolumeClippingGroup) and/or height fields (OIV.VolumeViz.Nodes.SoUniformGridClipping). They are also clipped by OpenGL clipping planes (OIV.Inventor.Nodes.SoClipPlane), but we recommend using the VolumeViz clipping nodes instead.

• Material: The color of each voxel is modulated by the current diffuse color in the traversal state. The default diffuse color is 0.8,0.8,0.8. This results in full intensity values in the color map being displayed as 80% intensity. Therefore we recommend adding an OIV.Inventor.Nodes.SoMaterial node before the slice and setting its diffuseColor field to full white (1,1,1).

• Transparency:

  • Typically the color map (OIV.LDM.Nodes.SoTransferFunction) used for volume rendering (OIV.VolumeViz.Nodes.SoVolumeRender) assigns transparency (alpha < 1) to some voxel values. If you want to use the same color map for slice rendering, but render the slice completely opaque, set the alphaUse field to ALPHA_OPAQUE. This overrides the alpha values in the color map (or an RGBA volume). However it does not affect transparency assigned using an OIV.Inventor.Nodes.SoMaterial node.

  • If you want to adjust the overall transparency of the slice, add an OIV.Inventor.Nodes.SoMaterial node and set its transparency field (keeping alphaUse set to ALPHA_AS_IS). Effectively a scale factor 1-transparency is applied to each voxel's alpha value.

  • Intersecting transparent slices cannot be rendered correctly by the basic blending transparency algorithms. To render this case correctly, set the transparency algorithm to SORTED_PIXEL using the viewer class or OIV.Inventor.Actions.SoGLRenderAction.

• Voxel edges: The edges of the voxels can also be rendered. See options in the OIV.VolumeViz.Nodes.SoVolumeRenderingQuality node.

• Custom shaders: The current OIV.VolumeViz.Nodes.SoVolumeShader node, if any, allows custom shaders to be defined for special computation or rendering effects, including blending multiple volumes.

• Composition with Multiple Data: It is possible to compose datasets that have different dimensions, tile sizes and transformations. In order to help fetch the correct data values in custom shaders, texture coordinates conversion functions are provided in the VolumeViz/vvizStructure.h shader include. For instance,

  Copy

  vec3 VVizTextureToTextureVec(in VVizDataSetId datasetSrc, in VVizDataSetId datasetDst, in vec3 texCoord);

  can be used to convert texture coordinates related to one dataset to texture coordinates related to another dataset. The conversion is based solely on the transformations applied to each dataset, which are defined by their model matrix and their extent. Please note that the model matrix of a dataset is defined by to the OIV.Inventor.Nodes.SoTransformation nodes that are placed before the OIV.LDM.Nodes.SoDataSet node in the order of the traversal.

• Performance:

  • Tile size: For backward compatibility, the default tile size is still only 64. This is quite small for modern CPU/GPU hardware. The smaller the tile size, the larger the total number of tiles that must be managed by VolumeViz. This overhead can be significant, especially for operations that require reloading the data textures on the GPU, for example, changing the data range (OIV.LDM.Nodes.SoDataRange). For smaller volumes, like 512^3, it can be efficient to set the tile size large enough to contain the entire volume. For very large volumes, larger tile sizes are efficient for OIV.VolumeViz.Nodes.SoVolumeRender but somewhat inefficient for slice rendering because complete tiles must be loaded even though the slice only uses part of the data. Applications should experiment. For volumes stored in LDM file format, the tile size must be specified when the volume is converted to LDM (see OIV.LDM.Converters.SoConverter and the "-t" option). For other data data formats the tile size can be specified using the OIV.VolumeViz.Nodes.SoVolumeData node's ldmResourceParameters field, but only after setting the filename field or calling the SetReader()method.

  • LDM_USE_IN_MEM_COMPRESSION VolumeViz always manages data as "tiles", regardless of the data format. In many cases VolumeViz must create (or uncompress) the tiles at run time. These cases include in-memory volumes, any volume reader that does not implement the readTile() method (this includes all built-in formats except LDM, e.g. DICOM, SEGY, ...) and compressed LDM format files. If this variable is true (the default), VolumeViz only keeps a small cache of created/uncompressed tiles in CPU memory. If a tile's data is needed and that tile is not in the cache, the tile must be recreated. This overhead can be significant, especially for operations that require recreating all the data textures on the GPU, for example changing the data range (OIV.LDM.Nodes.SoDataRange). We recommend setting this variable to false (see OIV.Inventor.SoPreferences) unless saving CPU memory is critical.

• Hardware requirements: This node needs a graphic card with support for GLSL shader, vertex buffer objects (VBO) and framebuffer object (FBO). Use the OIV.VolumeViz.Nodes.SoFenceSlice.IsSupported(OIV.Inventor.Misc.SoState) method to check if the current graphics board can render a FenceSlice.

EXAMPLE

Please see OIV.VolumeViz.Nodes.SoObliqueSlice for a complete code example. The following shows how to set up the OIV.VolumeViz.Nodes.SoFenceSlice.axis and OIV.VolumeViz.Nodes.SoFenceSlice.points fields of OIV.VolumeViz.Nodes.SoFenceSlice.

SoFenceSlice fenceSlice   = new SoFenceSlice();
 fenceSlice.axis.Value = SoFenceSlice.AxisType.Y;
 fenceSlice.points[0] = new SbVec2f(-0.2f, -0.66f);
 fenceSlice.points[1] = new SbVec2f(0.2f, -0.4f);
 fenceSlice.points[2] = new SbVec2f(-0.2f, 0.4f);
 fenceSlice.points[3] = new SbVec2f(0.2f, 0.66f);

Fence slice on Y axis (Colt example data set):

Fence slice on X axis (Colt example data set):

Fence slice on Z axis (Colt example data set):

FILE FORMAT/DEFAULT

FenceSlice {

dataSetId	0
points	[ ]
axis	Z
interpolation	LINEAR
alphaUse	ALPHA_BINARY
useRGBA	false
alternateRep	NULL
enableBumpMapping	false
bumpScale	1.0

}

ACTION BEHAVIOR

OIV.Inventor.Actions.SoGLRenderAction Draws a textured shape based on current OIV.VolumeViz.Nodes.SoVolumeData, OIV.LDM.Nodes.SoTransferFunction, and OIV.LDM.Nodes.SoROI nodes.

OIV.Inventor.Actions.SoGetBoundingBoxAction Computes the bounding box that encloses the fence slice.

Reference