[openscad/en] Add OpenSCAD learnxinyminutes

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+---
+language: openscad
+filename: leanopenscad.scad
+contributors:
+    - ["Thomas Preston", "https://github.com/tompreston/"]
+---
+
+Draw 3D models with code using [OpenSCAD](https://openscad.org/).
+
+```openscad
+// Single-line comments start with //
+
+/*
+Multi-line comments look like this.
+*/
+
+/* 3D Primitives */
+cube(10);
+cube([5, 10, 20]);
+sphere(10);
+
+/* Transformations */
+translate([20, 0, 0]) cube(10);
+rotate([0, 20, 30]) cube(10);
+
+translate([20, 0, 0]) rotate([0, 20, 30]) cube(10);
+rotate([0, 20, 30]) translate([20, 0, 0]) cube(10);
+
+/* Modifiers
+ * * disable
+ * ! show only
+ * # highlight / debug
+ * % transparent / background
+ */
+/* For example, show only the rotated cube at the origin, before we translate it. */
+translate([20, 0, 0]) !rotate([0, 20, 30]) cube(10);
+
+/* Formatting
+ * The following models are the same. The official docs prefer the second.
+ */
+rotate([0, 20, 30]) translate([20, 0, 0]) cube(10);
+
+rotate([0, 20, 30])
+    translate([20, 0, 0])
+    cube(10);
+
+rotate([0, 20, 30]) {
+    translate([20, 0, 0]) {
+        cube(10);
+    }
+}
+
+/* Loops */
+num_cubes = 5;
+r = 20;
+cube_len = 5;
+
+for (i = [0:num_cubes]) {
+    echo(str("Plot cube ", i));
+    rotate([0, i * 360 / num_cubes, 0])
+        translate([r, 0, 0])
+        cube(cube_len, center=true);
+}
+
+/* Boolean operations.
+ * union() - the sum of both shapes
+ * difference() - the first shape, minus the second shape
+ * intersection() - only parts of both shapes which intersect
+ */
+cube_l = 20;
+cube_w = 10;
+cube_h = 10;
+
+hole_pos_l = 10;
+hole_pos_h = 5;
+hole_r = 3;
+
+difference() {
+    cube([cube_l, cube_w, cube_h]);
+    translate([hole_pos_l, 0, hole_pos_h])
+        rotate([-90, 0, 0])
+        cylinder(cube_w, r=hole_r);
+}
+
+/* Functions calculate values. */
+function inch2mm(i) = i * 25.4;
+
+cube(inch2mm(2));
+
+/* Modules create objects you want to use later. */
+module house(roof="flat", paint=[1,0,0]) {
+    color(paint)
+    if (roof=="flat") {
+        translate([0,-1,0]) cube();
+    } else if (roof=="pitched") {
+        rotate([90,0,0])
+            linear_extrude(height=1)
+            polygon(points=[[0,0],[0,1],[0.5,1.5],[1,1],[1,0]]);
+    }
+    else if (roof=="domical") {
+        translate([0,-1,0]) {
+            translate([0.5,0.5,1])
+                sphere(r=0.5,$fn=20);
+            cube(); 
+        }
+    }
+}
+
+house("pitched");
+translate([2, 0, 0]) house("domical");
+
+/* Import modules and function from other files. */
+include <filename> /* Import the content of the file as if they were written in this file. */
+use <filename> /* Import modules and functions, but do not execute any commands. */
+```
+
+## Further Reading
+* Official docs https://openscad.org/documentation.html
+* Cheat sheet https://openscad.org/cheatsheet/index.html
+* Vim bindings https://github.com/sirtaj/vim-openscad