From d3f13ab0dc8da566802e6b06721b8a81c0f24dd8 Mon Sep 17 00:00:00 2001
From: Geoff deRosenroll <geoffderosenroll@gmail.com>
Date: Sat, 6 Aug 2022 17:09:20 -0700
Subject: [PATCH] Match affine3d skews up to affine2d_skew behaviour

Currently, affine2d_skew and affine3d_skew_xy have different behaviour.
Similarly, affine3d_skew_xz and affine3d_skew_yz do not skew the same
was as affine2d_skew does (if you were to look down the third axis at
the relevant plane). This commit brings them into agreement.
---
 affine.scad            | 24 ++++++++++++------------
 tests/test_affine.scad |  6 +++---
 2 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/affine.scad b/affine.scad
index 3bf47c8..94091c9 100644
--- a/affine.scad
+++ b/affine.scad
@@ -519,10 +519,10 @@ function affine3d_skew_xy(xa=0, ya=0) =
     assert(is_finite(xa))
     assert(is_finite(ya))
     [
-        [1, 0, tan(xa), 0],
-        [0, 1, tan(ya), 0],
-        [0, 0,       1, 0],
-        [0, 0,       0, 1]
+        [      1, tan(xa), 0, 0],
+        [tan(ya),       1, 0, 0],
+        [      0,       0, 1, 0],
+        [      0,       0, 0, 1]
     ];
 
 
@@ -551,10 +551,10 @@ function affine3d_skew_xz(xa=0, za=0) =
     assert(is_finite(xa))
     assert(is_finite(za))
     [
-        [1, tan(xa), 0, 0],
-        [0,       1, 0, 0],
-        [0, tan(za), 1, 0],
-        [0,       0, 0, 1]
+        [      1, 0, tan(xa), 0],
+        [      0, 1,       0, 0],
+        [tan(za), 0,       1, 0],
+        [      0, 0,       0, 1]
     ];
 
 
@@ -583,10 +583,10 @@ function affine3d_skew_yz(ya=0, za=0) =
     assert(is_finite(ya))
     assert(is_finite(za))
     [
-        [      1, 0, 0, 0],
-        [tan(ya), 1, 0, 0],
-        [tan(za), 0, 1, 0],
-        [      0, 0, 0, 1]
+        [1,       0,       0, 0],
+        [0,       1, tan(ya), 0],
+        [0, tan(za),       1, 0],
+        [0,       0,       0, 1]
     ];
 
 
diff --git a/tests/test_affine.scad b/tests/test_affine.scad
index 7e90ebd..3668bab 100644
--- a/tests/test_affine.scad
+++ b/tests/test_affine.scad
@@ -137,7 +137,7 @@ test_affine3d_skew();
 module test_affine3d_skew_xy() {
     for(ya = [-89:3:89]) {
         for(xa = [-89:3:89]) {
-            assert(affine3d_skew_xy(xa=xa, ya=ya) == [[1,0,tan(xa),0],[0,1,tan(ya),0],[0,0,1,0],[0,0,0,1]]);
+            assert(affine3d_skew_xy(xa=xa, ya=ya) == [[1,tan(xa),0,0],[tan(ya),1,0,0],[0,0,1,0],[0,0,0,1]]);
         }
     }
 }
@@ -147,7 +147,7 @@ test_affine3d_skew_xy();
 module test_affine3d_skew_xz() {
     for(za = [-89:3:89]) {
         for(xa = [-89:3:89]) {
-            assert(affine3d_skew_xz(xa=xa, za=za) == [[1,tan(xa),0,0],[0,1,0,0],[0,tan(za),1,0],[0,0,0,1]]);
+            assert(affine3d_skew_xz(xa=xa, za=za) == [[1,0,tan(xa),0],[0,1,0,0],[tan(za),0,1,0],[0,0,0,1]]);
         }
     }
 }
@@ -157,7 +157,7 @@ test_affine3d_skew_xz();
 module test_affine3d_skew_yz() {
     for(za = [-89:3:89]) {
         for(ya = [-89:3:89]) {
-            assert(affine3d_skew_yz(ya=ya, za=za) == [[1,0,0,0],[tan(ya),1,0,0],[tan(za),0,1,0],[0,0,0,1]]);
+            assert(affine3d_skew_yz(ya=ya, za=za) == [[1,0,0,0],[0,1,tan(ya),0],[0,tan(za),1,0],[0,0,0,1]]);
         }
     }
 }