added polysections

README.md

@@ -40,6 +40,7 @@ Some modules may depend on other modules. For example, the `polyline2d` module d
 	- [polyline3d](https://openhome.cc/eGossip/OpenSCAD/lib-polyline3d.html)
 	- [hull_polyline3d](https://openhome.cc/eGossip/OpenSCAD/lib-hull_polyline3d.html)
 	- [function_grapher](https://openhome.cc/eGossip/OpenSCAD/lib-function_grapher.html)
+	- [polysections](https://openhome.cc/eGossip/OpenSCAD/lib-polysections.html)
 	
 - Transformation
     - [along_with](https://openhome.cc/eGossip/OpenSCAD/lib-along_with.html)

docs/lib-polysections.md

@@ -0,0 +1,145 @@
+# polysections
+
+Crosscutting a tube-like shape at different points gets several cross-sections. This module can operate reversely. It uses cross-sections to construct a tube-like shape. For example, imagine that you have the following cross-sections:
+
+![polysections](images/lib-polysections-1.JPG)
+
+This module can use them to construct the following model:
+
+![polysections](images/lib-polysections-2.JPG)
+
+Looks like extruding along the path? Yes, it can perform the task; however, it's more flexible. 
+
+You can also view it as a better polyline3d module if you want. If you have only the points of a path, using `polyline3d` or `hull_polyline3d` is a simple solution. If you know the cross-sections along a path, you can use `polysections` to do more. 
+
+When using this module, you should use points to represent each cross-section. You need to provide indexes of triangles, too. This module provides two prepared triangles indexes. One is `"RADIAL"`. For example, if you have a cross-section such as:
+
+![polysections](images/lib-polysections-3.JPG)
+
+When using `"RADIAL"` (default), the module will cut them into triangles from the first point to each remaining point:
+
+![polysections](images/lib-polysections-4.JPG)
+
+If your cross-sections are hollow, you may use `"HOLLOW"`. For example:
+
+![polysections](images/lib-polysections-5.JPG)
+
+When using `"HOLLOW"`, the above shape will be cut into triangles such as:
+
+![polysections](images/lib-polysections-6.JPG)
+
+You can cut triangles by yourself, for example, the indexes of the above triangles is:
+
+    [
+        [0, 3, 4],
+        [0, 4, 1],
+        [1, 4, 5],
+        [1, 5, 2],
+        [2, 5, 3],
+        [2, 3, 0]
+    ]
+
+Triangles may be defined in any order.
+
+## Parameters
+
+- `sections` : A list of cross-sections. Each cross-section is represented by a list of points. See the example below.
+- `triangles` : `"RADIAL"` (default), `"HOLLOW"` or user-defined indexes. See example below.
+
+## Examples
+
+	include <rotate_p.scad>;
+	include <polysections.scad>;
+	
+	section1 = [
+	    [10, 0, 0],
+	    [15, 10, 0],
+	    [18, 9, 0],
+	    [20, 0, 0]
+	];
+	
+	// spin section1
+	sections = [
+	    for(i = [0:55]) 
+	        [
+	            for(p = section1)
+	                let(pt = rotate_p(p, [90, 0, -10 * i]))
+	                [pt[0], pt[1] , pt[2] + i]
+	        ]
+	];
+	    
+	polysections(sections);
+
+![polysections](images/lib-polysections-7.JPG)
+
+	include <rotate_p.scad>;
+	include <polysections.scad>;
+	
+	section1 = [
+	    // outer
+	    [10, 0, 0],
+	    [15, 10, 0],
+	    [18, 9, 0],
+	    [20, 0, 0],
+	    // inner
+	    [12, 2, 0],
+	    [15, 7, 0],
+	    [17, 7, 0],
+	    [18, 2, 0]
+	];
+	
+	// spin section1
+	sections = [
+	    for(i = [0:55]) 
+	        [
+	            for(p = section1)
+	                let(pt = rotate_p(p, [90, 0, -10 * i]))
+	                [pt[0], pt[1] , pt[2] + i]
+	        ]
+	];
+	    
+	polysections(sections, "HOLLOW");
+
+![polysections](images/lib-polysections-8.JPG)
+
+	include <rotate_p.scad>;
+	include <polysections.scad>;
+	
+	section1 = [
+	    // outer
+	    [10, 0, 0],
+	    [15, 10, 0],
+	    [30, 0, 0],
+	    // inner
+	    [12, 1, 0],
+	    [15, 8, 0],
+	    [26, 1, 0],        
+	];
+	
+	// spin section1
+	sections = [
+	    for(i = [0:55]) 
+	        [
+	            for(p = section1)
+	                let(pt = rotate_p(p, [90, 0, -10 * i]))
+	                [pt[0], pt[1] , pt[2] + i]
+	        ]
+	];
+	    
+	polysections(
+	   sections = sections, 
+	   triangles = [
+	        [0, 3, 4],
+	        [0, 4, 1],
+	        [1, 4, 5],
+	        [1, 5, 2],
+	        [2, 5, 3],
+	        [2, 3, 0]
+	    ]
+	);
+
+![polysections](images/lib-polysections-9.JPG)
+
+
+
+

src/polysections.scad

@@ -1,3 +1,16 @@
+/**
+* polysections.scad
+*
+* Crosscutting a tube-like shape at different points gets several cross-sections.
+* This module can operate reversely. It uses cross-sections to construct a tube-like shape.
+* 
+* @copyright Justin Lin, 2017
+* @license https://opensource.org/licenses/lgpl-3.0.html
+*
+* @see https://openhome.cc/eGossip/OpenSCAD/lib-polysections.html
+*
+**/
+
 module polysections(sections, triangles = "RADIAL") {
     module tri_sections(tri1, tri2) {
         polyhedron(
@@ -40,6 +53,7 @@ module polysections(sections, triangles = "RADIAL") {
     
     module two_sections(section1, section2) {
         for(idx = tris()) {
+            // hull is for preventing from WARNING: Object may not be a valid 2-manifold
             hull() tri_sections(
                 [
                     section1[idx[0]],