Added initial Fractal Tree demo tutorial. Still need to gen images.

tutorials/Fractal_Tree.md

@@ -0,0 +1,139 @@
+# Fractal Tree Tutorial
+
+### Start with a Tree Trunk
+
+Firstoff, include the BOSL2 library, then add a tapered cylinder for the tree trunk.
+
+```openscad-example
+include <BOSL2/std.scad>
+cylinder(l=1500, d1=300, d2=210);
+```
+
+### Parameterize It
+
+It's easier to adjust a model if you split out the defining parameters.
+
+```openscad-example
+include <BOSL2/std.scad>
+l = 1500;
+sc = 0.7;
+cylinder(l=l, d1=l/5, d2=l/5*sc);
+```
+
+### Attaching Branches
+
+You can attach branches to the top of the trunk by using `attach()` as a child of the trunk cylinder.
+
+```openscad-example
+include <BOSL2/std.scad>
+l = 1500;
+sc = 0.7;
+cylinder(l=l, d1=l/5, d2=l/5*sc) {
+    attach(TOP) yrot( 30) cylinder(l=l*sc, d1=l/5*sc, d2=l/5*sc*sc);
+    attach(TOP) yrot(-30) cylinder(l=l*sc, d1=l/5*sc, d2=l/5*sc*sc);
+}
+```
+
+### Replicate Branches
+
+Instead of attaching each branch individually, you can attach multiple branch copies at once.
+
+```openscad-example
+include <BOSL2/std.scad>
+l = 1500;
+sc = 0.7;
+cylinder(l=l, d1=l/5, d2=l/5*sc)
+    attach(TOP)
+        zrot_copies(n=2)  // Make multiple rotated copies
+        yrot(30) cylinder(l=l*sc, d1=l/5*sc, d2=l/5*sc*sc);
+```
+
+### Make it a Module
+
+Lets make this into a module, for convenience.
+
+```openscad-example
+include <BOSL2/std.scad>
+module tree(l=1500, sc=0.7)
+    cylinder(l=l, d1=l/5, d2=l/5*sc)
+        attach(TOP)
+            zrot_copies(n=2)
+            yrot(30) cylinder(l=l*sc, d1=l/5*sc, d2=l/5*sc*sc);
+tree();
+```
+
+### Use Recursion
+
+Since branches look much like the main trunk, we can make it recursive. Don't forget the termination clause, or else it'll try to recurse forever!
+
+```openscad-example
+include <BOSL2/std.scad>
+module tree(l=1500, sc=0.7, depth=10)
+    cylinder(l=l, d1=l/5, d2=l/5*sc)
+        attach(TOP)
+            if (depth>0)  // Important!
+                zrot_copies(n=2)
+                yrot(30) tree(depth=depth-1, l=l*sc, sc=sc);
+tree();
+```
+
+### Make it Not Flat
+
+A flat planar tree isn't what we want, so lets bush it out a bit by rotating each level 90 degrees.
+
+```openscad-example
+include <BOSL2/std.scad>
+module tree(l=1500, sc=0.7, depth=10)
+    cylinder(l=l, d1=l/5, d2=l/5*sc)
+        attach(TOP)
+            if (depth>0)
+                zrot(90)  // Bush it out
+                zrot_copies(n=2)
+                yrot(30) tree(depth=depth-1, l=l*sc, sc=sc);
+tree();
+```
+
+### Adding Leaves
+
+Let's add leaves. They look much like squashed versions of the standard teardrop() module, so lets use that.
+
+```openscad-example
+include <BOSL2/std.scad>
+module tree(l=1500, sc=0.7, depth=10)
+    cylinder(l=l, d1=l/5, d2=l/5*sc)
+        attach(TOP)
+            if (depth>0)
+                zrot(90)
+                zrot_copies(n=2)
+                yrot(30) tree(depth=depth-1, l=l*sc, sc=sc);
+            else
+                yscale(0.67)
+                teardrop(d=l*3, l=1, anchor=BOT, spin=90);
+tree();
+```
+
+### Adding Color
+
+We can finish this off with some color. The `color()` module will force all it's children and
+their descendants to the new color, even if they were colored before. The `recolor()` module,
+however, will only color children and decendants that don't already have a color set by a more
+nested `recolor()`.
+
+```openscad-example
+include <BOSL2/std.scad>
+module tree(l=1500, sc=0.7, depth=10)
+    recolor("lightgray")
+    cylinder(l=l, d1=l/5, d2=l/5*sc)
+        attach(TOP)
+            if (depth>0)
+                zrot(90)
+                zrot_copies(n=2)
+                yrot(30)
+                tree(depth=depth-1, l=l*sc, sc=sc);
+            else
+                recolor("springgreen")
+                yscale(0.67)
+                teardrop(d=l*3, l=1, anchor=BOT, spin=90);
+tree();
+```
+