added boundary rag construction

doc/examples/plot_rag_boundary.py

@@ -0,0 +1,28 @@
+"""
+==========================
+Region Boundary based RAGs
+==========================
+
+This example demonstrates construction of region boundary based RAGs with the
+`rag_boundary` function.
+"""
+from skimage.future import graph
+from skimage import data, segmentation, color, filters, io
+from matplotlib import pyplot as plt, colors
+
+img = data.coffee()
+gimg = color.rgb2gray(img)
+
+labels = segmentation.slic(img, compactness=30, n_segments=400)
+edges = filters.sobel(gimg)
+edges_rgb = color.gray2rgb(edges)
+
+mimg = segmentation.mark_boundaries(img, labels, (0,0,0))
+
+g = graph.rag_boundary(labels, edges)
+
+cmap = colors.ListedColormap(['#0000ff', '#ff0000'])
+out = graph.draw_rag(labels, g, edges_rgb, node_color="#ffff00", colormap=cmap)
+
+io.imshow(out)
+io.show()