From 4473ffa5dfa5d2afb02bf9a9095085cab2b03b08 Mon Sep 17 00:00:00 2001
From: Simon Brooke <simon@journeyman.cc>
Date: Sat, 6 Sep 2025 08:04:41 +0100
Subject: [PATCH] Forgot the 'skew' library!

---
 model/fourbar.scad      |  2 +-
 model/library/skew.scad | 43 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+), 1 deletion(-)
 create mode 100644 model/library/skew.scad

diff --git a/model/fourbar.scad b/model/fourbar.scad
index f94f0bc..7ab7953 100644
--- a/model/fourbar.scad
+++ b/model/fourbar.scad
@@ -3,7 +3,7 @@
 // (c) Simon Brooke 2025; CC-BY-SA
 
 
-include <BentSim/Bike_accessories.scad>
+include <BentSim/Library/Bike_accessories.scad>
 include <NACAAirfoils/files/Naca4.scad>
 include <library/skew.scad>
 
diff --git a/model/library/skew.scad b/model/library/skew.scad
new file mode 100644
index 0000000..ec76370
--- /dev/null
+++ b/model/library/skew.scad
@@ -0,0 +1,43 @@
+/* Taken from https://gist.github.com/boredzo/fde487c724a40a26fa9c?permalink_comment_id=4258747 */
+
+/* skew takes an array of six angles:
+ * x along y
+ * x along z
+ * y along x
+ * y along z
+ * z along x
+ * z along y
+ */
+module skew(dims) {
+matrix = [
+	[ 1, dims[0]/45, dims[1]/45, 0 ],
+	[ dims[2]/45, 1, dims[4]/45, 0 ],
+	[ dims[5]/45, dims[3]/45, 1, 0 ],
+	[ 0, 0, 0, 1 ]
+];
+multmatrix(matrix)
+children();
+}
+
+/* example:
+ * skew([45, 0, 0, 0, 0, 0])
+ * cube([10,10,10]);
+ */
+
+// Skews the child geometry.
+// xy: Angle towards X along Y axis.
+// xz: Angle towards X along Z axis.
+// yx: Angle towards Y along X axis.
+// yz: Angle towards Y along Z axis.
+// zx: Angle towards Z along X axis.
+// zy: Angle towards Z along Y axis.
+module skew2(xy = 0, xz = 0, yx = 0, yz = 0, zx = 0, zy = 0) {
+	matrix = [
+		[ 1, tan(xy), tan(xz), 0 ],
+		[ tan(yx), 1, tan(yz), 0 ],
+		[ tan(zx), tan(zy), 1, 0 ],
+		[ 0, 0, 0, 1 ]
+	];
+	multmatrix(matrix)
+	children();
+}