OOF2: The Manual

Details

Description

SnapRefine is a SkeletonModifier that attempts to combine the most desirable features of Refine and SnapNodes into a single method. The method operates like Refine, except that the Element Segments are split by points located at the interface of two different pixel categories. These points become the corners of new elements that subdivide the original elements. Figure 6.106 illustrates the result of applying SnapRefine to the starting Skeleton depicted in Figure 6.105.

Figure 6.105. SnapRefining a Skeleton

A skeleton in need of refinement.

Figure 6.106. 

The skeleton after applying SnapRefine to elements with homogeneity less than 0.9.

SnapRefine can yield a Skeleton with a comparable homogeneity to a Skeleton produced by Refine, but often with significantly fewer elements. SnapRefine can also be more flexible than SnapNodes in fitting a Skeleton to a microstructure, because SnapRefine introduces new nodes and additional edges that can be made to follow the boundaries of pixel categories. For example, Figure 6.107 shows the result of using the standard Refine method and the result of using the stand-alone SnapNodes method on the original Skeleton.

Figure 6.107. Refine and Snap Separately

The result of Refineing the Skeleton from Figure 6.105.

The result of appling SnapNodes directly to the Skeleton in Figure 6.105. Most borders are resolved well, but some aren't resolved at all.

However, SnapRefine can lead to the creation of very thin elements (see Figure 6.106). The creation of such badly-shaped elements may be prevented in many cases by a judicious choice of the parameter min_distance. SnapRefine will not place a new Node within a distance min_distance of one of the corners of an Element. min_distance is given in units of the pixel size.^[36]

In general, iterating SnapRefine produces very bad results, with a lot of very thin elements. A good strategy is often to use the regular Refine method first, and to apply SnapRefine just once at the end to smooth out jagged edges.

Currently, SnapRefine finds at most two transition points along an edge. The method also generates both triangular and quadrilateral elements liberally (corresponding to the liberal rule set in Refine). Note also that SnapRefine does not use an alpha parameter. Instead, it takes into account the pixel categories underneath the element edges, as well as the bisection of the edges, when deciding among several ways of subdividing an element. The subdivision of an element may also introduce new nodes within the element. These nodes may also be located at the interface of pixel categories.