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diff --git a/r/forceFieldTransform.R b/r/forceFieldTransform.R
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+# -*- coding: utf -*-
+#
+# Calculate the "force field transform" of the image, using the
+# specified *cell* size.
+#
+# The image is padded with the mirrored boundary condition.
+#
+# NOTE:TODO:
+# The transformation output strengths image is NOT normalized!
+#
+#
+# Credit:
+# [1] TODO:
+#     Hurley et al., 2002, 2005
+#
+#
+# Aaron LI
+# 2015/08/28
+#
+
+
+# The attractive force between two points on the image.
+# NOTE: the coefficient is ignored
+#
+# Arguments:
+#   p0, p1 - (r, c, value), the row and column number of the point position,
+#            and the value of that point
+#
+# Return:
+#   the force vector (f_r, f_c):
+#     'f_r': force along the row direction, positive goes downside
+#     'f_c': force along the column direction, positive goes to the right
+#   Note that this is the force that 'p1' act on 'p0', and is directed
+#   to point 'p1'.
+p2p.force <- function(p0, p1) {
+    r0 = p0[1]
+    c0 = p0[2]
+    r1 = p1[1]
+    c1 = p1[2]
+    f_r = p0[3]*p1[3] * (r1-r0) / ((r1-r0)^2 + (c1-c0)^2)^1.5
+    f_c = p0[3]*p1[3] * (c1-c0) / ((r1-r0)^2 + (c1-c0)^2)^1.5
+    return(c(f_r, f_c))
+}
+
+
+#  vim: set ts=8 sw=4 tw=0 fenc=utf-8 ft=r: #