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+====== Background, straight line segments - and a version 4 bug ======
+Many users want to draw their background; here's a way to connect points together to form a background using fit_obj's. First define the macro's:
+<code topas>macro Bkg_Straight_Line_xy(n, x, y)
+{
+   prm !bkg_x_##n = x;
+   prm !bkg_y_##n = y;
+}
+macro Bkg_Straight_Line_First(n, x, y)
+{
+   Bkg_Straight_Line_xy(n, x, y)
+}
+macro Bkg_Straight_Line(n1, n2, x2, y2)
+{
+   fit_obj = (X - bkg_x_##n1) ((y2)-bkg_y_##n1)/((x2)-bkg_x_##n1) + bkg_y_##n1;
+      min_X >= bkg_x_##n1 + 1.0e-9;
+      #m_ifarg x2 "X2"
+         max_X >= x2 + 1.0e-10;
+      #m_else
+         max_X <= x2 + 1.0e-10;
+      #m_endif
+   Bkg_Straight_Line_xy(n2, x2, y2)
+}</code>
+Then graphically gather (x,y) points and form the straight line segments as follows:
+<code topas>DAT(y2o3a)
+	Bkg_Straight_Line_First(0, X1, 322)
+	Bkg_Straight_Line(0,  1,  22.0,  205.0)
+	Bkg_Straight_Line(1,  2,  25.6,  188.6)
+	Bkg_Straight_Line(2,  3,  36.9,  142.5)
+	Bkg_Straight_Line(3,  4,  45.5,  149.8)
+	Bkg_Straight_Line(4,  5,  67.5,  126.0)
+	Bkg_Straight_Line(5,  6,  88.6,  158.9)
+	Bkg_Straight_Line(6,  7,  100.7, 179.0)
+	Bkg_Straight_Line(7,  8,  108.1, 171.7)
+	Bkg_Straight_Line(8,  9,  112.0, 162.6)
+	Bkg_Straight_Line(9,  10, 130.6, 217.4)
+	Bkg_Straight_Line(10, 11, X2,    280.0)</code>
+This describes a background for the Y2O3a.INP file which can be found in the test_examples directory.
+Note, in doing the above macros a bug in the interpretation of the >= and <= signs for min_X/max_X have been uncovered. The macros as defined above will work in version 4. Version 5 will have a different combination of the >=,<= signs and the macros would be inside of TOPAS.INC.
+--- //[[alan.coelho@bigpond.com|Alan Coelho]] 2011/08/25 19:09//