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docs/lib3x-sf_splines.md

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+# sf_splines
+
+Given a set of control points and spline functions, the `sf_splines` function returns surface points.
+
+**Since:** 3.1
+
+## Parameters
+
+- `ctrl_pts` : A set of control points. See examples below.
+- `row_spline` : The spline function for row points.
+- `column_spline` : The spline function for columns points. Default to `row_spline`.
+
+## Examples
+
+If you want to make a bezier surface:
+
+	use <bezier_curve.scad>;
+	use <function_grapher.scad>;
+	use <surface/sf_splines.scad>;
+
+	ctrl_pts = [
+		[[0, 0, 20],  [60, 0, -35],   [90, 0, 60],    [200, 0, 5]],
+		[[0, 50, 30], [100, 60, -25], [120, 50, 120], [200, 50, 5]],
+		[[0, 100, 0], [60, 120, 35],  [90, 100, 60],  [200, 100, 45]],
+		[[0, 150, 0], [60, 150, -35], [90, 180, 60],  [200, 150, 45]]
+	];
+
+	thickness = 2;
+	t_step = 0.05;
+
+	bezier = function(points) bezier_curve(t_step, points);
+
+	function_grapher(sf_splines(ctrl_pts, bezier), thickness);
+
+![sf_splines](images/lib3x-sf_splines-1.JPG)
+
+The following figure shows controll points and bazier curves around the surface.
+
+![sf_splines](images/lib3x-sf_splines-2.JPG)
+
+If you want to make a bspline surface:
+
+	use <bspline_curve.scad>;
+	use <function_grapher.scad>;
+	use <surface/sf_splines.scad>;
+
+	ctrl_pts = [
+		[[0, 0, 20],  [60, 0, -35],   [90, 0, 60],    [200, 0, 5]],
+		[[0, 50, 30], [100, 60, -25], [120, 50, 120], [200, 50, 5]],
+		[[0, 100, 0], [60, 120, 35],  [90, 100, 60],  [200, 100, 45]],
+		[[0, 150, 0], [60, 150, -35], [90, 180, 60],  [200, 150, 45]]
+	];
+
+	thickness = 2;
+	t_step = 0.05;
+	degrees = 2;
+
+	bspline = function(points) bspline_curve(t_step, degrees, points);
+
+	function_grapher(sf_splines(ctrl_pts, bspline), thickness);
+
+![sf_splines](images/lib3x-sf_splines-3.JPG)