An overloaded version of voAppearanceActions::clip() clips a polygon to a convex polyhedron determined by set of positive half-spaces:

static int clip(voVertexData* vertexData, voInterleavedArrayType
interleavedArrayFormat,float* planes,int planeCount, voIndexedFaceSet* cSet);

The convex polyhedron is specified by a set of positive half planes given by their equations {A, B, C, D]. The input array specifies the coordinates of the polygon's vertices and possibly other per-vertex attributes. The input format is described by voVertexData. The clipped polygon ends up as a voIndexedFaceSet.

This action allows applications to implement alternative ways of sampling and clipping. For example, applications may want to implement a box volumetric primitive by directly sampling a cube without tessellating it into tetrahedra. In this case, the default instance of voAppearanceActions::sample() can be replaced with a call to clip().

Similarly, for primitives that are convex polyhedra (for example, tetrahedra) it is possible to collapse sample() and clip() into a single routine. After all, “slicing” a tetrahedron with a plane followed by clipping the resulting polygon to a box, is equivalent to clipping the plane to the convex region determined by the ten positive half-planes defining the region of intersection between the tetrahedron and the box.

clip() is a general purpose utility and may be used by applications to accomplish general-purpose clipping to a clip box. (For more information about clip boxes, see “Clip Boxes”.)

For example, it is common in many diagnostic applications to display an arbitrarily oriented plane through the data set that may not necessarily coincide with the original acquisition direction (this task is referred to as Multi-Planar Reformation (MPR). For more information about MPR, see “Polygonizing Arbitrarily Oriented Cross Sections”.

Similarly, in seismic data interpretation it is desired to visualize data along an arbitrarily shaped surface (e.g., geological horizon). In both of these situations, the planes/polygons need to be clipped to the brick boundaries so that the resulting polygons can be texture mapped correctly.

Consider a situation where an MPR along a plane described by [A, B, C, D] is required. One can, apply the sample() in order to clip the plane to each individual tetrahedron and feed the results into clip() to clip them to each brick.

float planeEquation[4] = { A, B, C, D,};
float tetraVerts[4][3] = { ... };  // vertices of the tetrahedron
float brickLowerLeft[3]  = { 0.5, 0.5, 0.5 };
float brickUpperRight[3] = { 254.5, 254.5, 254.5 };
voVertexData vData(4,valuesPerVertex); // intermediate polygon verts

sample(tetraVerts,planeEquation,&polygonVerts);
clip(&polygonVerts,brickLowerLeft, brickUpperRight, aFaceSet);

Given a brick, one should clip to the clip box rather than the brick boundary to avoid seams:

voBrick *aBrick; 

aBrick->getClipBrickSizes(brickLowerLeft, brickUpperRight);

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Note: For applications that deal exclusively with canonical volumes, where the geometry coincides with appearance, it is not necessary to clip to tetrahedra boundaries. For example, the polygons that comprise the geological horizon can be fed directly to clip().

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Example 8-0 shows how to use findClosestAxisIndex().

if( aVolume->interpolationType == voInterpolationTypeScope::_2D)
    aVolume->setCurrentBrickSet(
        findClosestAxisIndex(
            modelMatrix,projMatrix,
            voSamplingModeScope::AXIS_ALIGNED) );

draw()
// voGeometryActions::draw() can be applied to a voFaceSet to render it into the // frame buffer:  

void
draw(
    voIndexedFaceSet *aFaceSet,
    GLenum mode,  // GL_LINE or GL_FILL
    voInterleavedArrayType interleavedArrayFormat);

interleavedArrayFormat is one of the interleaved array types described for voVertexData in “Read Brick Data from Disk”.