SoObliqueSlice Class Reference
[Nodes]

Oblique slice shape node. More...

#include <VolumeViz/nodes/SoObliqueSlice.h>

Inheritance diagram for SoObliqueSlice:

Public Member Functions
virtual SoType	getTypeId () const
	SoObliqueSlice ()
Static Public Member Functions
static SoType	getClassTypeId ()
Public Attributes
SoSFPlane	plane

Detailed Description

Oblique slice shape node.

This node defines an oblique (arbitrarily oriented) slice which is the intersection of the volume data defined by an SoVolumeData node and the plane defined by the plane field.

For a non-RGBA (scalar valued) volume, each voxel's RGBA value is determined by the current SoDataRange and SoTransferFunction. The current diffuse color and transparency (set, for example, with an SoMaterial node) modify the appearance of the slice. This means that, for example, the current transparency can be used as a global alpha value to modulate the overall opacity of the slice. For an RGBA volume each voxel's RGBA value comes directly from the volume data.

The interpolation field controls how the texture is interpolated.

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

Optionally a bump mapping effect may be applied. Normal vectors are automatically computed from the data value gradient. The enableBumpMapping and bumpScale fields (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.

Plane normal:
The "polarity" of the plane normal (for example whether it is +Z or -Z) doesn't affect rendering the oblique slice. However it's very important if you also use that plane to define a clipping plane. The clipping plane will clip away everything on the side of the plane *opposite* the normal vector direction. So to clip away the "front" (+Z) of the volume set the normal to 0,0,-1.

Picking:
The entire slice is pickable, even where it is transparent as a result of the current transfer function. The SoObliqueSliceDetail class allows you to get the voxel position and value after picking.

Dragging:
It is possible to interactively translate and rotate slices using an Open Inventor dragger, e.g. SoTranslate1Dragger. However the dragger's field values must be converted to an SbPlane. For a dragger that has a convenient "plane" field that can be directly connected, see the ClipPlaneDragger article in the Resources section of the Open Inventor Forum: http://www.openinventor.net/showthread.php?t=733 (registration is required to view this section of the forum).

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 TRILINEAR or 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 SoDataRange node to specify the actual (or desired) range of data values to be mapped. Also use an 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 (SoROI), geometry (SoVolumeClippingGroup) and/or height fields (SoUniformGridClipping). They are also clipped by OpenGL clipping planes (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 SoMaterial node before the slice and setting its diffuseColor field to full white (1,1,1).

Transparency:
- Typically the color map (SoTransferFunction) used for volume rendering (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 SoMaterial node.
- If you want to adjust the overall transparency of the slice, add an 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_PIXELS_BLEND (or similar value) using the viewer class or SoGLRenderAction.

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

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

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 (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 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 SoConverter and the "-t" option). For other data data formats the tile size can be specified using the 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 (SoDataRange). We recommend setting this variable to false (see SoPreferences) unless saving CPU memory is critical.

EXAMPLE

SoVolumeData

Note that some of the property nodes (data, material, color map, etc) will typically be shared by multiple rendering nodes. In other words the volume usually only needs to be loaded once, using a single SoVolumeData node, then multiple slices and/or regions can be rendered using that data node.

Also note that this example is for a data volume, where each voxel can be considered a discrete sample from a continuous data field and interpolation should be used to compute values between voxel centers. If you are rendering a label volume, then each voxel is an "id" assigning that voxel to a specific material, object, etc. In this case, set the interpolation field to NEAREST to disable interpolation.

   // Default setting can be a performance bottleneck
   SoPreferences::setValue( "LDM_USE_IN_MEM_COMPRESSION", "0" );

   // Keep volume viz separate from geometry
   SoSeparator* volSep = new SoSeparator();
     root->addChild( volSep );

   // Load volume data
   SoVolumeData* volData = new SoVolumeData();
     volData->fileName = "$OIVHOME/examples/source/VolumeViz/Data/3DHead.vol";
     volSep->addChild( volData );

   // Set range of data values to visualize.
   // Not required for 8-bit voxels, critical for larger data types.
   // The getMinMax() call may be expensive for non-LDM file formats.
   SoDataRange* volRange = new SoDataRange();
     if (volData->getDatumSize() > 1) {
       double minVal, maxVal;
       volData->getMinMax( minVal, maxVal );
       volRange->min = minVal;
       volRange->max = maxVal;
     }
     volSep->addChild( volRange );

   // Load opaque intensity ramp
   SoTransferFunction* volTF = new SoTransferFunction();
     volTF->predefColorMap = SoTransferFunction::INTENSITY;
     volSep->addChild( volTF );

   // Display slice at full intensity
   SoMaterial* volMat = new SoMaterial();
     volMat->diffuseColor.setValue( 1, 1, 1 );
     volSep->addChild( volMat );

   // Remove tile boundary artifacts while moving. 
   SoVolumeShader* volShader = new SoVolumeShader();
     volShader->interpolateOnMove = TRUE;
     volSep->addChild( volShader );

   // Display a Z axis slice at center of volume
   SoObliqueSlice* slice = new SoObliqueSlice();
     SbBox3f volext = volData->extent.getValue();
     SbVec3f center = volext.getCenter();
     slice->plane         = SbPlane( SbVec3f(0,0,1), center[2] );
     slice->interpolation = SoObliqueSlice::MULTISAMPLE_12;
     volSep->addChild( slice );