Fix corner case in rotation matrix for 3D slice plots

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## [1.21.1] - 2026-09-30
+- Fix corner case in 3D slice plotting
+- Use special rotation for `x`-`y` rotations in 3D: keep `z` original, map it to `y`
+
 ## [1.21.0] - 2026-09-16
 - Replace plottertypes by Singleton
 

Project.toml

@@ -1,6 +1,6 @@
 name = "GridVisualize"
 uuid = "5eed8a63-0fb0-45eb-886d-8d5a387d12b8"
-version = "1.21"
+version = "1.21.1"
 authors = ["Juergen Fuhrmann <juergen.fuhrmann@wias-berlin.de>", "Patrick Jaap <patrick.jaap@wias-berlin.de>"]
 
 [deps]

src/slice_plots.jl

@@ -149,6 +149,11 @@ function compute_3d_z_rotation_matrix(target_vector)
         β = acos(t[3] / cos(α))
     end
 
+    # check t[1]:
+    if cos(α) * sin(β) ≉ t[1]
+        β = -β
+    end
+
     return R_y(β) * R_x(α)
 end
 
@@ -216,8 +221,8 @@ function slice_plot!(ctx, ::Type{Val{3}}, grid, values)
 
     a::Float64, b::Float64, c::Float64, _ = plane
 
-    # transformation matrix
-    if (a, b, c) == (1.0, 0.0, 0.0)
+
+    if (a, b, c) == (1.0, 0.0, 0.0) # simple axis directions
         rotation_matrix = @SArray [
             0.0 0.0 1.0
             1.0 0.0 0.0
@@ -235,6 +240,13 @@ function slice_plot!(ctx, ::Type{Val{3}}, grid, values)
             0.0 1.0 0.0
             0.0 0.0 1.0
         ]
+    elseif c == 0 # special case for c = 0: keep z-axis original
+        M = compute_2d_rotation_matrix([a, b])
+        rotation_matrix = @SArray [
+            M[1, 1] 0.0 M[1, 2]
+            M[2, 1] 0.0 M[2, 2]
+            0.0     1.0 0.0
+        ]
     else
         rotation_matrix = compute_3d_z_rotation_matrix([a, b, c])
     end

test/test_slice_plots.jl

@@ -62,14 +62,18 @@ end
     @test abs(η ⋅ t) < 1.0e-15
     @test abs(ξ ⋅ t) < 1.0e-15
 
+    # tricky case
+    t = [-1.0, 1.0, 0.0] / sqrt(2)
+    R = GridVisualize.compute_3d_z_rotation_matrix(t)
+    @test R[:, 3] ≈ t
+
     # corner case I: only one axis rotation necessary
     t = [1, 0, 0]
     @test GridVisualize.compute_3d_z_rotation_matrix(t) ≈ [0 0 1; 0 1 0; -1 0 0]
 
     # corner case II: only one axis rotation necessary
     t = [0, 1, 0]
     @test GridVisualize.compute_3d_z_rotation_matrix(t) ≈ [1 0 0; 0 0 1; 0 -1 0]
-
 end