Put the UV orientation tests beside the projection tests

They were appended at the end of test_texture_displacement.cpp, where every
other open texture-displacement branch also appends, so the four of them
cannot be merged in any order without a conflict in a file none of them
really disagree about. Same tests, moved next to the patch-frame test they
belong with.

resources/shaders/110/texture_displacement_shaded.fs

@@ -72,17 +72,21 @@ varying float weight;
 varying float island_active;
 varying vec2  vertex_uv;
 
-// The cylinder's own frame, built exactly as libslic3r's project_cylindrical() builds it - including
+// The cylinder's own frame, built exactly as libslic3r's project_cylindrical() builds it - right
-// the handedness, which comes out left-handed for an axis of +Z. Copied rather than "corrected", so
+// handed, so the angle taken from it grows anticlockwise about the axis, which seen from outside the
-// the preview wraps the texture the same way round as the bake.
+// cylinder is to the viewer's right rather than mirrored.
 void cylinder_frame(out vec3 up, out vec3 right, out vec3 fwd)
 {
     up = (length(patch_axis) > 1e-8) ? normalize(patch_axis) : vec3(0.0, 0.0, 1.0);
     vec3 arbitrary = (abs(up.z) < 0.9) ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
     right = normalize(cross(up, arbitrary));
-    fwd   = normalize(cross(right, up));
+    fwd   = normalize(cross(up, right));
 }
 
+// Which side of a world plane a face is on, with zero counting as the positive side. Not GLSL's own
+// sign(), which returns 0 there and would collapse the coordinate onto a line.
+float box_face_sign(float n) { return n >= 0.0 ? 1.0 : -1.0; }
+
 // The raw, millimetre-valued projection of `p` (a position in the texture frame), before the layer's
 // tiling/rotation/aspect/offset - a term-for-term transcription of libslic3r's project_planar(),
 // project_cylindrical() and project_spherical(). `n` is read by the planar mode only.
@@ -94,7 +98,7 @@ vec2 projection_raw(vec3 p, vec3 n)
         vec3  rel = p - patch_center;
         float x   = dot(rel, right);
         float y   = dot(rel, fwd);
-        return vec2(atan(y, x) * sqrt(x * x + y * y), dot(rel, up));
+        return vec2(atan(y, x) * sqrt(x * x + y * y), -dot(rel, up));
     }
     if (projection_mode == 2) {                 // Spherical: (longitude, latitude) * radius
         vec3  rel    = p - patch_center;
@@ -102,10 +106,18 @@ vec2 projection_raw(vec3 p, vec3 n)
         if (radius < 1e-8)
             return vec2(0.0);
         vec3 dir = rel / radius;
-        return vec2(atan(dir.y, dir.x), asin(clamp(dir.z, -1.0, 1.0))) * radius;
+        return vec2(atan(dir.y, dir.x), -asin(clamp(dir.z, -1.0, 1.0))) * radius;
     }
-    vec3 an = abs(n);                           // Triplanar: drop the dominant normal axis
+    // Triplanar: drop the dominant normal axis. v is negated so the image's first pixel row (v = 0)
-    return (an.x >= an.y && an.x >= an.z) ? p.yz : ((an.y >= an.x && an.y >= an.z) ? p.xz : p.xy);
+    // sits at the top of the model instead of upside down, and u is flipped on the far side of each
+    // plane so the two faces sharing it are not mirror images. libslic3r's box_plane_yz() and its two
+    // siblings, term for term.
+    vec3 an = abs(n);
+    if (an.x >= an.y && an.x >= an.z)
+        return vec2(box_face_sign(n.x) * p.y, -p.z);
+    if (an.y >= an.x && an.y >= an.z)
+        return vec2(-box_face_sign(n.y) * p.x, -p.z);
+    return vec2(box_face_sign(n.z) * p.x, -p.y);
 }
 
 // The two surface directions projection_raw()'s u and v run along at `p`, plus how many raw units one
@@ -124,7 +136,7 @@ void projection_axes(vec3 p, vec3 n, out vec3 t, out vec3 b, out vec2 units_per_
         vec2  xy  = vec2(dot(rel, right), dot(rel, fwd));
         float r   = length(xy);
         t = (r > 1e-6) ? (fwd * xy.x - right * xy.y) / r : right; // circumferential: u runs along it
-        b = up;                                                   // v is the distance along the axis
+        b = -up;                                                  // v is the distance *against* the axis
         return;
     }
     if (projection_mode == 2) {
@@ -133,20 +145,20 @@ void projection_axes(vec3 p, vec3 n, out vec3 t, out vec3 b, out vec2 units_per_
         vec3  dir = (r > 1e-8) ? rel / r : vec3(0.0, 0.0, 1.0);
         float c   = length(dir.xy);                               // cos(latitude)
         t = (c > 1e-6) ? vec3(-dir.y, dir.x, 0.0) / c : vec3(1.0, 0.0, 0.0);
-        b = cross(dir, t);                                        // increasing latitude, unit length
+        b = -cross(dir, t);                                       // v grows away from the pole, unit length
         units_per_mm = vec2(1.0 / max(c, 1e-3), 1.0);
         return;
     }
-    vec3 an = abs(n);
+    vec3 an = abs(n);                          // the same signed basis projection_raw() uses
-    if (an.x >= an.y && an.x >= an.z) {        // planar = p.yz
+    if (an.x >= an.y && an.x >= an.z) {        // planar = (+-y, -z)
-        t = vec3(0.0, 1.0, 0.0);
+        t = vec3(0.0, box_face_sign(n.x), 0.0);
-        b = vec3(0.0, 0.0, 1.0);
+        b = vec3(0.0, 0.0, -1.0);
-    } else if (an.y >= an.x && an.y >= an.z) { // planar = p.xz
+    } else if (an.y >= an.x && an.y >= an.z) { // planar = (+-x, -z)
-        t = vec3(1.0, 0.0, 0.0);
+        t = vec3(-box_face_sign(n.y), 0.0, 0.0);
-        b = vec3(0.0, 0.0, 1.0);
+        b = vec3(0.0, 0.0, -1.0);
-    } else {                                   // planar = p.xy
+    } else {                                   // planar = (+-x, -y)
-        t = vec3(1.0, 0.0, 0.0);
+        t = vec3(box_face_sign(n.z), 0.0, 0.0);
-        b = vec3(0.0, 1.0, 0.0);
+        b = vec3(0.0, -1.0, 0.0);
     }
 }
 

resources/shaders/110/texture_displacement_uvcheck.fs

@@ -36,14 +36,20 @@ varying vec4  world_pos;
 varying float distortion;
 varying vec2  vertex_uv;
 
+// Right handed (up x right), like libslic3r's project_cylindrical(), so the angle taken from it grows
+// anticlockwise about the axis - to the viewer's right when seen from outside the cylinder.
 void cylinder_frame(out vec3 up, out vec3 right, out vec3 fwd)
 {
     up = (length(patch_axis) > 1e-8) ? normalize(patch_axis) : vec3(0.0, 0.0, 1.0);
     vec3 arbitrary = (abs(up.z) < 0.9) ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
     right = normalize(cross(up, arbitrary));
-    fwd   = normalize(cross(right, up));
+    fwd   = normalize(cross(up, right));
 }
 
+// Which side of a world plane a face is on, with zero counting as the positive side. Not GLSL's own
+// sign(), which returns 0 there and would collapse the coordinate onto a line.
+float box_face_sign(float n) { return n >= 0.0 ? 1.0 : -1.0; }
+
 // Term for term libslic3r's project_planar() / project_cylindrical() / project_spherical(), in mm.
 vec2 projection_raw(vec3 p, vec3 n)
 {
@@ -53,7 +59,7 @@ vec2 projection_raw(vec3 p, vec3 n)
         vec3  rel = p - patch_center;
         float x   = dot(rel, right);
         float y   = dot(rel, fwd);
-        return vec2(atan(y, x) * sqrt(x * x + y * y), dot(rel, up));
+        return vec2(atan(y, x) * sqrt(x * x + y * y), -dot(rel, up));
     }
     if (projection_mode == 2) {
         vec3  rel    = p - patch_center;
@@ -61,10 +67,16 @@ vec2 projection_raw(vec3 p, vec3 n)
         if (radius < 1e-8)
             return vec2(0.0);
         vec3 dir = rel / radius;
-        return vec2(atan(dir.y, dir.x), asin(clamp(dir.z, -1.0, 1.0))) * radius;
+        return vec2(atan(dir.y, dir.x), -asin(clamp(dir.z, -1.0, 1.0))) * radius;
     }
+    // v negated so the image's first pixel row sits at the top of the model, u flipped on the far side
+    // of each plane so the two faces sharing it are not mirror images - see libslic3r's box_plane_yz().
     vec3 an = abs(n);
-    return (an.x >= an.y && an.x >= an.z) ? p.yz : ((an.y >= an.x && an.y >= an.z) ? p.xz : p.xy);
+    if (an.x >= an.y && an.x >= an.z)
+        return vec2(box_face_sign(n.x) * p.y, -p.z);
+    if (an.y >= an.x && an.y >= an.z)
+        return vec2(-box_face_sign(n.y) * p.x, -p.z);
+    return vec2(box_face_sign(n.z) * p.x, -p.y);
 }
 
 vec2 project_uv(vec3 p, vec3 n)

resources/shaders/140/texture_displacement_shaded.fs

@@ -138,17 +138,21 @@ in vec2  vertex_uv;
 
 out vec4 out_color;
 
-// The cylinder's own frame, built exactly as libslic3r's project_cylindrical() builds it - including
+// The cylinder's own frame, built exactly as libslic3r's project_cylindrical() builds it - right
-// the handedness, which comes out left-handed for an axis of +Z. Copied rather than "corrected", so
+// handed, so the angle taken from it grows anticlockwise about the axis, which seen from outside the
-// the preview wraps the texture the same way round as the bake.
+// cylinder is to the viewer's right rather than mirrored.
 void cylinder_frame(out vec3 up, out vec3 right, out vec3 fwd)
 {
     up = (length(patch_axis) > 1e-8) ? normalize(patch_axis) : vec3(0.0, 0.0, 1.0);
     vec3 arbitrary = (abs(up.z) < 0.9) ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
     right = normalize(cross(up, arbitrary));
-    fwd   = normalize(cross(right, up));
+    fwd   = normalize(cross(up, right));
 }
 
+// Which side of a world plane a face is on, with zero counting as the positive side. Not GLSL's own
+// sign(), which returns 0 there and would collapse the coordinate onto a line.
+float box_face_sign(float n) { return n >= 0.0 ? 1.0 : -1.0; }
+
 // The raw, millimetre-valued projection of `p` (a position in the texture frame), before the layer's
 // tiling/rotation/aspect/offset - a term-for-term transcription of libslic3r's project_planar(),
 // project_cylindrical() and project_spherical(). `n` is read by the planar mode only.
@@ -160,7 +164,7 @@ vec2 projection_raw(vec3 p, vec3 n)
         vec3  rel = p - patch_center;
         float x   = dot(rel, right);
         float y   = dot(rel, fwd);
-        return vec2(atan(y, x) * sqrt(x * x + y * y), dot(rel, up));
+        return vec2(atan(y, x) * sqrt(x * x + y * y), -dot(rel, up));
     }
     if (projection_mode == 2) {                 // Spherical: (longitude, latitude) * radius
         vec3  rel    = p - patch_center;
@@ -168,10 +172,18 @@ vec2 projection_raw(vec3 p, vec3 n)
         if (radius < 1e-8)
             return vec2(0.0);
         vec3 dir = rel / radius;
-        return vec2(atan(dir.y, dir.x), asin(clamp(dir.z, -1.0, 1.0))) * radius;
+        return vec2(atan(dir.y, dir.x), -asin(clamp(dir.z, -1.0, 1.0))) * radius;
     }
-    vec3 an = abs(n);                           // Triplanar: drop the dominant normal axis
+    // Triplanar: drop the dominant normal axis. v is negated so the image's first pixel row (v = 0)
-    return (an.x >= an.y && an.x >= an.z) ? p.yz : ((an.y >= an.x && an.y >= an.z) ? p.xz : p.xy);
+    // sits at the top of the model instead of upside down, and u is flipped on the far side of each
+    // plane so the two faces sharing it are not mirror images. libslic3r's box_plane_yz() and its two
+    // siblings, term for term.
+    vec3 an = abs(n);
+    if (an.x >= an.y && an.x >= an.z)
+        return vec2(box_face_sign(n.x) * p.y, -p.z);
+    if (an.y >= an.x && an.y >= an.z)
+        return vec2(-box_face_sign(n.y) * p.x, -p.z);
+    return vec2(box_face_sign(n.z) * p.x, -p.y);
 }
 
 // The two surface directions projection_raw()'s u and v run along at `p`, plus how many raw units one
@@ -190,7 +202,7 @@ void projection_axes(vec3 p, vec3 n, out vec3 t, out vec3 b, out vec2 units_per_
         vec2  xy  = vec2(dot(rel, right), dot(rel, fwd));
         float r   = length(xy);
         t = (r > 1e-6) ? (fwd * xy.x - right * xy.y) / r : right; // circumferential: u runs along it
-        b = up;                                                   // v is the distance along the axis
+        b = -up;                                                  // v is the distance *against* the axis
         return;
     }
     if (projection_mode == 2) {
@@ -199,20 +211,20 @@ void projection_axes(vec3 p, vec3 n, out vec3 t, out vec3 b, out vec2 units_per_
         vec3  dir = (r > 1e-8) ? rel / r : vec3(0.0, 0.0, 1.0);
         float c   = length(dir.xy);                               // cos(latitude)
         t = (c > 1e-6) ? vec3(-dir.y, dir.x, 0.0) / c : vec3(1.0, 0.0, 0.0);
-        b = cross(dir, t);                                        // increasing latitude, unit length
+        b = -cross(dir, t);                                       // v grows away from the pole, unit length
         units_per_mm = vec2(1.0 / max(c, 1e-3), 1.0);
         return;
     }
-    vec3 an = abs(n);
+    vec3 an = abs(n);                          // the same signed basis projection_raw() uses
-    if (an.x >= an.y && an.x >= an.z) {        // planar = p.yz
+    if (an.x >= an.y && an.x >= an.z) {        // planar = (+-y, -z)
-        t = vec3(0.0, 1.0, 0.0);
+        t = vec3(0.0, box_face_sign(n.x), 0.0);
-        b = vec3(0.0, 0.0, 1.0);
+        b = vec3(0.0, 0.0, -1.0);
-    } else if (an.y >= an.x && an.y >= an.z) { // planar = p.xz
+    } else if (an.y >= an.x && an.y >= an.z) { // planar = (+-x, -z)
-        t = vec3(1.0, 0.0, 0.0);
+        t = vec3(-box_face_sign(n.y), 0.0, 0.0);
-        b = vec3(0.0, 0.0, 1.0);
+        b = vec3(0.0, 0.0, -1.0);
-    } else {                                   // planar = p.xy
+    } else {                                   // planar = (+-x, -y)
-        t = vec3(1.0, 0.0, 0.0);
+        t = vec3(box_face_sign(n.z), 0.0, 0.0);
-        b = vec3(0.0, 1.0, 0.0);
+        b = vec3(0.0, -1.0, 0.0);
     }
 }
 

resources/shaders/140/texture_displacement_uvcheck.fs

@@ -47,14 +47,20 @@ in vec2  vertex_uv;
 
 out vec4 out_color;
 
+// Right handed (up x right), like libslic3r's project_cylindrical(), so the angle taken from it grows
+// anticlockwise about the axis - to the viewer's right when seen from outside the cylinder.
 void cylinder_frame(out vec3 up, out vec3 right, out vec3 fwd)
 {
     up = (length(patch_axis) > 1e-8) ? normalize(patch_axis) : vec3(0.0, 0.0, 1.0);
     vec3 arbitrary = (abs(up.z) < 0.9) ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
     right = normalize(cross(up, arbitrary));
-    fwd   = normalize(cross(right, up));
+    fwd   = normalize(cross(up, right));
 }
 
+// Which side of a world plane a face is on, with zero counting as the positive side. Not GLSL's own
+// sign(), which returns 0 there and would collapse the coordinate onto a line.
+float box_face_sign(float n) { return n >= 0.0 ? 1.0 : -1.0; }
+
 // Term for term libslic3r's project_planar() / project_cylindrical() / project_spherical(), in mm.
 vec2 projection_raw(vec3 p, vec3 n)
 {
@@ -64,7 +70,7 @@ vec2 projection_raw(vec3 p, vec3 n)
         vec3  rel = p - patch_center;
         float x   = dot(rel, right);
         float y   = dot(rel, fwd);
-        return vec2(atan(y, x) * sqrt(x * x + y * y), dot(rel, up));
+        return vec2(atan(y, x) * sqrt(x * x + y * y), -dot(rel, up));
     }
     if (projection_mode == 2) {
         vec3  rel    = p - patch_center;
@@ -72,10 +78,16 @@ vec2 projection_raw(vec3 p, vec3 n)
         if (radius < 1e-8)
             return vec2(0.0);
         vec3 dir = rel / radius;
-        return vec2(atan(dir.y, dir.x), asin(clamp(dir.z, -1.0, 1.0))) * radius;
+        return vec2(atan(dir.y, dir.x), -asin(clamp(dir.z, -1.0, 1.0))) * radius;
     }
+    // v negated so the image's first pixel row sits at the top of the model, u flipped on the far side
+    // of each plane so the two faces sharing it are not mirror images - see libslic3r's box_plane_yz().
     vec3 an = abs(n);
-    return (an.x >= an.y && an.x >= an.z) ? p.yz : ((an.y >= an.x && an.y >= an.z) ? p.xz : p.xy);
+    if (an.x >= an.y && an.x >= an.z)
+        return vec2(box_face_sign(n.x) * p.y, -p.z);
+    if (an.y >= an.x && an.y >= an.z)
+        return vec2(-box_face_sign(n.y) * p.x, -p.z);
+    return vec2(box_face_sign(n.z) * p.x, -p.y);
 }
 
 vec2 project_uv(vec3 p, vec3 n)

src/libslic3r/TextureDisplacement.cpp

@@ -491,6 +491,57 @@ V2Resolution recommend_v2_resolution(const indexed_triangle_set
     return out;
 }
 
+namespace {
+// Which side of a world plane a face is on, with an exactly zero component counted as the positive
+// side. The tie never shows: a normal component of zero carries zero tri-planar blend weight (see
+// triplanar_weights()), so the plane it orients contributes nothing at that point.
+inline float box_face_sign(float n) { return n >= 0.f ? 1.f : -1.f; }
+
+// The box projection's 2D basis for one of the three world planes, oriented the way a viewer
+// standing outside the model sees that face.
+//
+// v runs *down* the image, because that is what v means to the sampler: v = 0 is the texture's first
+// pixel row, i.e. its top (DecodedHeightTexture::sample(); the UV editor negates Y on screen for the
+// same reason, see UVEditorCanvas::render()). So v = -z on the four upright planes and v = -y on the
+// horizontal one, which is what puts the top of an image at the top of the model rather than
+// upside down.
+//
+// u then has to depend on which *side* of the plane the face is on, or one of the two faces sharing
+// a plane comes out left-right mirrored: with a single u per plane, a portrait image reads upright on
+// +X and 180 degrees round on -X. Fixing that is the sign below, chosen so that u x v == -normal,
+// i.e. (right, down, into the surface) is right-handed for a viewer outside:
+//
+//   face  u     v       face  u     v       face  u     v
+//   +X    +y    -z      +Y    -x    -z      +Z    +x    -y
+//   -X    -y    -z      -Y    +x    -z      -Z    -x    -y
+//
+// The sign flips where its own normal component crosses zero, and the blend weight of the plane it
+// belongs to is exactly zero there, so the blended tri-planar sum stays continuous across the flip -
+// it is not a return of the hard axis switch described on TextureProjectionMethod::Triplanar.
+inline Vec2f box_plane_yz(const Vec3f &p, float nx) { return Vec2f(box_face_sign(nx) * p.y(), -p.z()); }
+inline Vec2f box_plane_xz(const Vec3f &p, float ny) { return Vec2f(-box_face_sign(ny) * p.x(), -p.z()); }
+inline Vec2f box_plane_xy(const Vec3f &p, float nz) { return Vec2f(box_face_sign(nz) * p.x(), -p.y()); }
+} // namespace
+
+void box_projection_axes(const Vec3f &normal, Vec3f &u_axis, Vec3f &v_axis)
+{
+    // The same table as box_plane_yz()/box_plane_xz()/box_plane_xy(), handed out as a pair of world
+    // directions for the plane the normal is closest to. Two shapes of one table, because the bake's
+    // hot path wants the closed form and the GUI wants the axes; the test suite checks them against
+    // each other so they cannot drift apart.
+    const Vec3f n = normal.cwiseAbs();
+    if (n.x() >= n.y() && n.x() >= n.z()) {
+        u_axis = box_face_sign(normal.x()) * Vec3f::UnitY();
+        v_axis = -Vec3f::UnitZ();
+    } else if (n.y() >= n.x() && n.y() >= n.z()) {
+        u_axis = -box_face_sign(normal.y()) * Vec3f::UnitX();
+        v_axis = -Vec3f::UnitZ();
+    } else {
+        u_axis = box_face_sign(normal.z()) * Vec3f::UnitX();
+        v_axis = -Vec3f::UnitY();
+    }
+}
+
 Vec2f project_planar(const Vec3f &position, const Vec3f &normal)
 {
     // Planar-project onto the two axes orthogonal to the dominant component of `normal`. Called
@@ -501,23 +552,26 @@ Vec2f project_planar(const Vec3f &position, const Vec3f &normal)
     // other face in the same patch - exactly the bug an earlier version of this feature had).
     const Vec3f n = normal.cwiseAbs();
     if (n.x() >= n.y() && n.x() >= n.z())
-        return Vec2f(position.y(), position.z());
+        return box_plane_yz(position, normal.x());
     if (n.y() >= n.x() && n.y() >= n.z())
-        return Vec2f(position.x(), position.z());
+        return box_plane_xz(position, normal.y());
-    return Vec2f(position.x(), position.y());
+    return box_plane_xy(position, normal.z());
 }
 
 namespace {
 // Wrapped around patch_axis, centered at patch_center. u is the arc length (mm) around the axis at
-// this point's own radius, v is the signed distance along the axis - a reasonable approximation
+// this point's own radius, v is the distance along the axis, measured *against* it - a reasonable
-// for roughly cylindrical selections, not an exact fit for arbitrary geometry.
+// approximation for roughly cylindrical selections, not an exact fit for arbitrary geometry.
 Vec2f project_cylindrical(const Vec3f &position, const Vec3f &patch_center, const Vec3f &patch_axis)
 {
     Vec3f up = patch_axis;
     up       = (up.norm() > 1e-8f) ? Vec3f(up.normalized()) : Vec3f::UnitZ();
     const Vec3f arbitrary = (std::abs(up.dot(Vec3f::UnitZ())) < 0.9f) ? Vec3f::UnitZ() : Vec3f::UnitX();
     const Vec3f right     = Vec3f(up.cross(arbitrary).normalized());
-    const Vec3f fwd       = Vec3f(right.cross(up).normalized());
+    // up x right, not right x up: that makes (right, fwd, up) right-handed, so the angle below grows
+    // anticlockwise about the axis - which, seen from outside the cylinder, is to the viewer's right.
+    // The other way round the texture wraps on mirrored, as if seen through the model from behind.
+    const Vec3f fwd       = Vec3f(up.cross(right).normalized());
 
     const Vec3f rel        = position - patch_center;
     const float along_axis = rel.dot(up);
@@ -526,7 +580,9 @@ Vec2f project_cylindrical(const Vec3f &position, const Vec3f &patch_center, cons
     const float radius     = std::sqrt(x * x + y * y);
     const float angle      = std::atan2(y, x);
 
-    return Vec2f(angle * radius, along_axis);
+    // Negated, like every other projection's v: v = 0 is the texture's top row, so v has to grow
+    // *down* the axis for the top of the image to sit at the top of the model.
+    return Vec2f(angle * radius, -along_axis);
 }
 
 // Longitude/latitude around patch_center. u/v are scaled by this point's own distance from the
@@ -542,7 +598,9 @@ Vec2f project_spherical(const Vec3f &position, const Vec3f &patch_center)
     const Vec3f dir       = rel / radius;
     const float longitude = std::atan2(dir.y(), dir.x());
     const float latitude  = std::asin(std::clamp(dir.z(), -1.f, 1.f));
-    return Vec2f(longitude, latitude) * radius;
+    // Longitude already grows to the right as seen from outside; latitude grows toward the north
+    // pole, so it is negated to put the image's top row (v = 0) at the top of the model.
+    return Vec2f(longitude, -latitude) * radius;
 }
 
 // CGAL's LSCM parameterizer expects a clean mesh with no isolated (unreferenced) vertices - but
@@ -837,9 +895,26 @@ std::vector<int> segment_into_charts(const indexed_triangle_set &mesh, const std
 // solve at all, and why this also serves as the fallback for a chart LSCM cannot handle.
 std::vector<Vec2f> project_to_tangent_plane(const indexed_triangle_set &chart, const Vec3f &normal)
 {
+    // Oriented like the box projection (see box_projection_axes()), so a chart reads the right way up
+    // and unmirrored instead of being flattened to whichever basis a cross product happened to give.
+    // For a chart whose normal *is* a world axis - a cube face, which is the common case once the
+    // seam angle has cut the patch along its sharp edges - this is exactly the box basis, so such a
+    // face unwraps to the same orientation Triplanar would have given it.
+    Vec3f u, v;
+    box_projection_axes(normal, u, v);
+    // The box axes lie in the tangent plane already when the normal is a world axis; for any other
+    // normal, u is squared up against it and v follows. u x v == -normal keeps the chart the way
+    // round a viewer outside the model sees it (a plain normal x u would mirror it).
+    u = u - normal * normal.dot(u);
+    if (u.norm() <= 1e-6f) {
+        // Out of reach for a unit normal - the box basis puts u perpendicular to the normal's own
+        // dominant axis, so the two can never be within 45 degrees of each other - but a caller
+        // handing over an unnormalized normal should still get a usable basis rather than a NaN.
         const Vec3f seed = (std::abs(normal.z()) < 0.9f) ? Vec3f::UnitZ() : Vec3f::UnitX();
-    const Vec3f u    = Vec3f(seed.cross(normal).normalized());
+        u = seed.cross(normal);
-    const Vec3f v    = Vec3f(normal.cross(u).normalized());
+    }
+    u = Vec3f(u.normalized());
+    v = Vec3f(u.cross(normal).normalized());
 
     std::vector<Vec2f> uvs(chart.vertices.size());
     for (size_t i = 0; i < chart.vertices.size(); ++i)
@@ -1814,8 +1889,10 @@ float sample_layer_height(const DecodedHeightTexture &texture, const TextureDisp
             return texture.sample(uv, layer.tile_enabled, layer.tile_method);
         }
         // Flat projection onto the captured projector plane. Single-valued per point, so unlike
-        // blended triplanar it is one sample, and it is what "project from view" places.
+        // blended triplanar it is one sample, and it is what "project from view" places. The
-        return sample_at(Vec2f(position.dot(layer.view_project_right), position.dot(layer.view_project_up)));
+        // projector's up axis is negated because v runs *down* the image (v = 0 is its first pixel
+        // row), the same reason the projective branch above measures window y downward.
+        return sample_at(Vec2f(position.dot(layer.view_project_right), -position.dot(layer.view_project_up)));
     case TextureProjectionMethod::LSCM: // no usable unwrap for this patch - fall back to Triplanar
     case TextureProjectionMethod::Triplanar:
     default: break;
@@ -1830,14 +1907,15 @@ float sample_layer_height(const DecodedHeightTexture &texture, const TextureDisp
     const float w_sum = w.x() + w.y() + w.z();
     if (w_sum < 1e-8f)
         // Degenerate normal: no axis is meaningfully dominant, so no blend is meaningful either.
-        return sample_at(Vec2f(position.x(), position.y()));
+        return sample_at(box_plane_xy(position, normal.z()));
     w /= w_sum;
 
-    // Each plane drops the axis it is named for, matching project_planar()'s own convention (which
+    // Each plane drops the axis it is named for and is oriented for the side of that plane the normal
-    // the GUI's on-canvas placement gizmo also relies on).
+    // points to, matching project_planar()'s own convention (which the GUI's on-canvas placement
-    return w.x() * sample_at(Vec2f(position.y(), position.z())) +
+    // gizmo also relies on). See box_plane_yz() for why the orientation is not simply (y, z).
-           w.y() * sample_at(Vec2f(position.x(), position.z())) +
+    return w.x() * sample_at(box_plane_yz(position, normal.x())) +
-           w.z() * sample_at(Vec2f(position.x(), position.y()));
+           w.y() * sample_at(box_plane_xz(position, normal.y())) +
+           w.z() * sample_at(box_plane_xy(position, normal.z()));
 }
 
 bool sample_layer_color(const DecodedHeightTexture &texture, const TextureDisplacementLayer &layer,
@@ -1899,7 +1977,8 @@ bool sample_layer_color(const DecodedHeightTexture &texture, const TextureDispla
             out = texture.sample_color(uv, layer.tile_enabled, layer.tile_method);
             return true;
         } else {
-            const Vec2f p(position.dot(layer.view_project_right), position.dot(layer.view_project_up));
+            // Negated up, exactly as sample_layer_height() does it.
+            const Vec2f p(position.dot(layer.view_project_right), -position.dot(layer.view_project_up));
             if (!covered(p))
                 return false;
             out = sample_at(p);
@@ -1915,7 +1994,7 @@ bool sample_layer_color(const DecodedHeightTexture &texture, const TextureDispla
     Vec3f w = triplanar_weights(normal);
     const float w_sum = w.x() + w.y() + w.z();
     if (w_sum < 1e-8f) {
-        const Vec2f p(position.x(), position.y());
+        const Vec2f p = box_plane_xy(position, normal.z());
         if (!covered(p))
             return false;
         out = sample_at(p);
@@ -1926,8 +2005,8 @@ bool sample_layer_color(const DecodedHeightTexture &texture, const TextureDispla
     // A blend of three planes is only "not covered" where *every* contributing plane is outside the
     // placement; where some are, the covered ones are renormalised so the colour does not fade toward
     // black at the edge of an untiled tri-planar layer.
-    const std::array<Vec2f, 3> planes = { Vec2f(position.y(), position.z()), Vec2f(position.x(), position.z()),
+    const std::array<Vec2f, 3> planes = { box_plane_yz(position, normal.x()), box_plane_xz(position, normal.y()),
-                                          Vec2f(position.x(), position.y()) };
+                                          box_plane_xy(position, normal.z()) };
     Vec3f acc     = Vec3f::Zero();
     float acc_w   = 0.f;
     for (int i = 0; i < 3; ++i)

src/libslic3r/TextureDisplacement.hpp

@@ -524,11 +524,24 @@ struct TextureColorSettings
     bool empty() const { return palette.empty(); }
 };
 
+// The two world directions the box projection takes its u and v along on a face whose outward
+// normal is `normal`: the plane is the one that normal is closest to, and the pair is oriented as a
+// viewer standing outside the model sees it - u to their right, v *down* the image, which is the
+// direction v runs in the texture (v = 0 is the image's first pixel row, see
+// DecodedHeightTexture::sample()). So an image comes out the way it looks in a file manager rather
+// than upside down, and it is not mirrored on the three faces facing the other way.
+//
+// Exposed because the GUI needs the same basis for the on-canvas "adjust texture placement" gizmo
+// (its ring's zero angle and its offset handle's plane), and it must not be a second, drifting copy
+// of the table.
+void box_projection_axes(const Vec3f &normal, Vec3f &u_axis, Vec3f &v_axis);
+
 // Raw dominant-axis planar projection of `position` (in mm, not yet scaled/rotated/offset by any
-// layer), dropping the axis position that best aligns with `normal`. Exposed on its own (rather
+// layer), dropping the axis position that best aligns with `normal`. Equivalent to projecting onto
-// than only inline inside project_texture_displacement_uv()) so GUI code - the on-canvas
+// box_projection_axes()' pair. Exposed on its own (rather than only inline inside
-// "adjust texture placement" gizmo - can map a dragged 3D point into the exact same 2D space
+// project_texture_displacement_uv()) so GUI code - the on-canvas "adjust texture placement" gizmo -
-// tiling_scale/rotation_deg/offset operate in, without duplicating the axis-selection logic.
+// can map a dragged 3D point into the exact same 2D space tiling_scale/rotation_deg/offset operate
+// in, without duplicating the axis-selection logic.
 Vec2f project_planar(const Vec3f &position, const Vec3f &normal);
 
 // Applies a layer's tiling_scale/rotation_deg/offset to an already-projected planar coordinate

src/slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.cpp

@@ -1252,8 +1252,9 @@ std::vector<Vec2f> GLGizmoTextureDisplacement::compute_layer_vertex_uvs(const in
                     uv[vi] = Vec2f(-1e6f, -1e6f);
                 continue;
             }
+            // Negated up, as sample_layer_height()'s affine branch has it: v runs down the image.
             const Vec2f planar(patch.vertices[vi].dot(layer.view_project_right),
-                               patch.vertices[vi].dot(layer.view_project_up));
+                               -patch.vertices[vi].dot(layer.view_project_up));
             uv[vi] = apply_uv_transform(planar, layer, aspect);
         }
         return uv;
@@ -3292,19 +3293,10 @@ Vec3f GLGizmoTextureDisplacement::adjust_handle_center(const TextureDisplacement
 
 void GLGizmoTextureDisplacement::adjust_tangent_basis(Vec3f &u_axis, Vec3f &v_axis) const
 {
-    // Mirrors project_planar()'s own dominant-axis choice exactly, so the ring's "0 degrees" and
+    // libslic3r's own basis, asked for rather than restated here, so the ring's "0 degrees" and the
-    // the offset handle's plane always agree with what project_texture_displacement_uv() does.
+    // offset handle's plane always agree with what project_texture_displacement_uv() does - including
-    const Vec3f n = m_adjust_anchor_normal.cwiseAbs();
+    // v pointing *down* the texture and u flipping on the far side of each plane.
-    if (n.x() >= n.y() && n.x() >= n.z()) {
+    box_projection_axes(m_adjust_anchor_normal, u_axis, v_axis);
-        u_axis = Vec3f::UnitY();
-        v_axis = Vec3f::UnitZ();
-    } else if (n.y() >= n.x() && n.y() >= n.z()) {
-        u_axis = Vec3f::UnitX();
-        v_axis = Vec3f::UnitZ();
-    } else {
-        u_axis = Vec3f::UnitX();
-        v_axis = Vec3f::UnitY();
-    }
 }
 
 void GLGizmoTextureDisplacement::render_adjust_texture_gizmo()

src/slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.hpp

@@ -423,8 +423,8 @@ private:
     // obvious whether the next stroke adds paint (default) or erases it (Shift). Uses ImGui's
     // foreground draw list, so it must be called from inside the gizmo's ImGui frame.
     void render_paint_cursor_hint();
-    // Mesh-local tangent-plane basis at m_adjust_anchor_normal, matching project_planar()'s
+    // Mesh-local tangent-plane basis at m_adjust_anchor_normal: box_projection_axes(), so dragging
-    // dominant-axis convention so dragging on-canvas maps consistently onto offset.
+    // on-canvas maps consistently onto offset whichever face the anchor is on.
     void adjust_tangent_basis(Vec3f &u_axis, Vec3f &v_axis) const;
 
     // The plane a drag is measured against: the paint patch's anchor, lifted clear of the surface.

tests/libslic3r/test_texture_displacement.cpp

@@ -1652,6 +1652,321 @@ TEST_CASE("Per-corner LSCM UVs give each triangle its own island's placement", "
     CHECK(differs);
 }
 
+// ---------------------------------------------------------------------------------------------
+// UV orientation: which way up a texture lands on the model.
+//
+// v = 0 is the height map's *first* pixel row, i.e. its top (DecodedHeightTexture::sample(), and
+// the UV editor negates Y on screen for the same reason). So "the image is the right way up" is the
+// statement that the row a point samples gets *lower* as the point moves up the face, and "it is
+// not left-right mirrored" is that the column gets higher as the point moves to the right of a
+// viewer standing outside the model. Both are measured below by sampling a ramp whose value says
+// which row (or column) it came from, which settles the question without having to reason about the
+// uv transform at all.
+
+static constexpr int   RAMP_N      = 64;   // the ramp texture's size, in texels
+static constexpr float RAMP_TILING = 40.f; // mm per tile
+static constexpr float CUBE_HALF   = 5.f;  // half extent of the cube the probes walk over
+// How far a probe steps off a face centre. Short on purpose: the cylindrical and spherical u is an
+// arc length (angle * radius), so a long step also changes the radius, and that term swamps the
+// angular one. It is symmetric about the face centre, so at a short step it cancels between the two
+// readings a check compares.
+static constexpr float RAMP_STEP   = 1.f;
+// What one step's worth of travel is in ramp rows: 2 * RAMP_STEP mm of separation, RAMP_N texels per
+// RAMP_TILING mm. Halved as the margin a check demands, so "flat" cannot pass as either direction.
+static constexpr double RAMP_MARGIN = RAMP_N * RAMP_STEP / RAMP_TILING;
+
+// A height map whose rows (or columns) carry a linear ramp: row r holds 3 + 4r, so the sampled
+// height identifies the row. Built directly rather than through a PNG because these tests are about
+// the projection, not the decoder - decode_height_texture() has its own tests.
+static DecodedHeightTexture ramp_texture(bool by_row)
+{
+    DecodedHeightTexture t;
+    t.width = t.height = RAMP_N;
+    t.pixels.resize(size_t(RAMP_N) * size_t(RAMP_N));
+    for (int y = 0; y < RAMP_N; ++y)
+        for (int x = 0; x < RAMP_N; ++x)
+            t.pixels[size_t(y) * size_t(RAMP_N) + size_t(x)] = uint8_t(3 + 4 * (by_row ? y : x));
+    return t;
+}
+
+// uv 0.5 at the cube's centre, and a tile wide enough that every point the probes visit stays well
+// inside the one tile - so no reading is a wrapped-around neighbour of the one it is compared with.
+static TextureDisplacementLayer ramp_layer(TextureProjectionMethod method)
+{
+    TextureDisplacementLayer layer;
+    layer.projection_method = method;
+    layer.tiling_scale      = RAMP_TILING;
+    layer.offset            = Vec2f(0.5f, 0.5f);
+    layer.depth_mm          = 1.f;
+    return layer;
+}
+
+// Which row (or column) of `ramp` the layer reads at `p`. Fractional, because a point between two
+// texels reads the ramp between them; only the direction it moves in matters here.
+static double ramp_index(const DecodedHeightTexture &ramp, const TextureDisplacementLayer &layer, const Vec3f &p,
+                         const Vec3f &n, const Vec3f &center = Vec3f(Vec3f::Zero()),
+                         const Vec3f &axis = Vec3f(Vec3f::UnitZ()))
+{
+    return (double(sample_layer_height(ramp, layer, p, n, center, axis, nullptr)) * 255.0 - 3.0) / 4.0;
+}
+
+// How a viewer standing outside the model sees a face: their right, and their up. Up is world +Z on
+// the four upright faces; on the top and bottom, where +Z is edge on, it is world +Y.
+static void outside_viewer_frame(const Vec3f &n, Vec3f &right, Vec3f &up)
+{
+    up    = (std::abs(n.z()) < 0.9f) ? Vec3f(0.f, 0.f, 1.f) : Vec3f(0.f, 1.f, 0.f);
+    right = Vec3f((-n).cross(up).normalized());
+}
+
+static const std::array<std::pair<const char *, Vec3f>, 6> CUBE_FACES = { {
+    { "+X", Vec3f(1.f, 0.f, 0.f) },
+    { "-X", Vec3f(-1.f, 0.f, 0.f) },
+    { "+Y", Vec3f(0.f, 1.f, 0.f) },
+    { "-Y", Vec3f(0.f, -1.f, 0.f) },
+    { "+Z", Vec3f(0.f, 0.f, 1.f) },
+    { "-Z", Vec3f(0.f, 0.f, -1.f) },
+} };
+
+TEST_CASE("the box projection lands a texture upright and unmirrored on all six faces of a cube",
+          "[TextureDisplacement]")
+{
+    const DecodedHeightTexture     rows  = ramp_texture(true);
+    const DecodedHeightTexture     cols  = ramp_texture(false);
+    const TextureDisplacementLayer layer = ramp_layer(TextureProjectionMethod::Triplanar);
+
+    for (const auto &[name, n] : CUBE_FACES) {
+        DYNAMIC_SECTION("face " << name)
+        {
+            Vec3f right, up;
+            outside_viewer_frame(n, right, up);
+            const Vec3f centre = n * CUBE_HALF;
+
+            // Upright: the row read near the top of the face is nearer the image's first row.
+            const double row_up   = ramp_index(rows, layer, centre + up * RAMP_STEP, n);
+            const double row_down = ramp_index(rows, layer, centre - up * RAMP_STEP, n);
+            INFO("row at the top of the face " << row_up << ", at the bottom " << row_down);
+            CHECK(row_down - row_up > RAMP_MARGIN);
+
+            // Unmirrored: the column read to the viewer's right is further into the image. Without a
+            // per-side sign this fails on exactly the three faces facing away - -X, +Y and -Z - which
+            // is the mirroring a bare v flip leaves behind.
+            const double col_left  = ramp_index(cols, layer, centre - right * RAMP_STEP, n);
+            const double col_right = ramp_index(cols, layer, centre + right * RAMP_STEP, n);
+            INFO("column at the left of the face " << col_left << ", at the right " << col_right);
+            CHECK(col_right - col_left > RAMP_MARGIN);
+        }
+    }
+}
+
+TEST_CASE("every projection method lands a texture upright on an upright face", "[TextureDisplacement]")
+{
+    const DecodedHeightTexture rows = ramp_texture(true);
+    // ViewProjected is one flat projection from the captured camera rather than a per-face map, so its
+    // "up" is the projector's own (view_project_up), which the default capture puts at +Y.
+    const auto method = GENERATE(TextureProjectionMethod::Triplanar, TextureProjectionMethod::Cylindrical,
+                                 TextureProjectionMethod::Spherical, TextureProjectionMethod::ViewProjected);
+    const TextureDisplacementLayer layer = ramp_layer(method);
+
+    for (const auto &[name, n] : CUBE_FACES) {
+        // Only the four upright faces: on the top and bottom a cylinder's v is constant (they are its
+        // caps) and a sphere's is at an extremum (they are its poles), so neither says anything about
+        // which way up the image is.
+        if (std::abs(n.z()) > 0.9f)
+            continue;
+        DYNAMIC_SECTION("method " << int(method) << ", face " << name)
+        {
+            const Vec3f up = (method == TextureProjectionMethod::ViewProjected) ? layer.view_project_up :
+                                                                                 Vec3f(0.f, 0.f, 1.f);
+            const Vec3f centre = n * CUBE_HALF;
+            const double row_up   = ramp_index(rows, layer, centre + up * RAMP_STEP, n);
+            const double row_down = ramp_index(rows, layer, centre - up * RAMP_STEP, n);
+            INFO("row at the top " << row_up << ", at the bottom " << row_down);
+            CHECK(row_down - row_up > RAMP_MARGIN);
+        }
+    }
+}
+
+TEST_CASE("the cylindrical and spherical wraps run left to right as seen from outside", "[TextureDisplacement]")
+{
+    const DecodedHeightTexture cols = ramp_texture(false);
+    const auto method = GENERATE(TextureProjectionMethod::Cylindrical, TextureProjectionMethod::Spherical);
+    const TextureDisplacementLayer layer = ramp_layer(method);
+
+    for (const auto &[name, n] : CUBE_FACES) {
+        // The caps and poles are out, as above - u is undefined on the axis. So is whichever face the
+        // atan2 branch cut runs through, where u jumps from +pi to -pi: that is a real seam in both
+        // projections, and a reading taken across it compares two opposite sides of the model.
+        if (std::abs(n.z()) > 0.9f)
+            continue;
+        const bool on_the_cut = (method == TextureProjectionMethod::Cylindrical) ? (n.y() < -0.9f) : (n.x() < -0.9f);
+        if (on_the_cut)
+            continue;
+        DYNAMIC_SECTION("method " << int(method) << ", face " << name)
+        {
+            Vec3f right, up;
+            outside_viewer_frame(n, right, up);
+            const Vec3f  centre    = n * CUBE_HALF;
+            const double col_left  = ramp_index(cols, layer, centre - right * RAMP_STEP, n);
+            const double col_right = ramp_index(cols, layer, centre + right * RAMP_STEP, n);
+            INFO("column at the left " << col_left << ", at the right " << col_right);
+            CHECK(col_right - col_left > RAMP_MARGIN);
+        }
+    }
+}
+
+TEST_CASE("an unwrapped cube's flat charts are laid out the right way up", "[TextureDisplacement]")
+{
+    indexed_triangle_set cube = its_make_cube(2. * CUBE_HALF, 2. * CUBE_HALF, 2. * CUBE_HALF);
+    for (Vec3f &v : cube.vertices)
+        v -= Vec3f(CUBE_HALF, CUBE_HALF, CUBE_HALF);
+
+    const PatchUnwrap unwrap = compute_patch_unwrap(cube, LSCM_DEFAULT_SEAM_ANGLE_DEG);
+    REQUIRE_FALSE(unwrap.empty());
+    REQUIRE(unwrap.chart_count == 6); // one flat chart per face, cut at the 90-degree edges
+
+    // Each chart carries one face. The four upright ones span world Z, and their v has to grow
+    // downwards like every other projection's - whatever the packing then did with the island.
+    int upright_charts = 0;
+    for (int c = 0; c < unwrap.chart_count; ++c) {
+        int top = -1, bottom = -1;
+        for (size_t i = 0; i < unwrap.uvs.size(); ++i) {
+            if (unwrap.vertex_chart[i] != c)
+                continue;
+            const float z = cube.vertices[size_t(unwrap.source_vertex[i])].z();
+            if (top < 0 || z > cube.vertices[size_t(unwrap.source_vertex[size_t(top)])].z())
+                top = int(i);
+            if (bottom < 0 || z < cube.vertices[size_t(unwrap.source_vertex[size_t(bottom)])].z())
+                bottom = int(i);
+        }
+        REQUIRE(top >= 0);
+        REQUIRE(bottom >= 0);
+        const float v_top = unwrap.uvs[size_t(top)].y(), v_bottom = unwrap.uvs[size_t(bottom)].y();
+        if (std::abs(v_top - v_bottom) < 1e-3f)
+            continue; // a cap: the whole chart is at one height, so it has no up
+        ++upright_charts;
+        DYNAMIC_SECTION("chart " << c)
+        {
+            INFO("v at the chart's topmost vertex " << v_top << ", at its bottommost " << v_bottom);
+            CHECK(v_bottom - v_top > 1.f);
+        }
+    }
+    CHECK(upright_charts == 4); // the four sides; the top and bottom faces are the caps skipped above
+}
+
+TEST_CASE("project_planar agrees with the box projection's own axes", "[TextureDisplacement]")
+{
+    // The two express the same table in different shapes - one a pair of world axes for the GUI, the
+    // other the closed form the bake's hot path uses - so they are checked against each other here
+    // rather than left to drift apart.
+    const Vec3f p(2.f, -3.f, 7.f);
+    for (const auto &[name, n] : CUBE_FACES) {
+        DYNAMIC_SECTION("face " << name)
+        {
+            Vec3f u_axis, v_axis;
+            box_projection_axes(n, u_axis, v_axis);
+            const Vec2f planar = project_planar(p, n);
+            CHECK_THAT(planar.x(), WithinAbs(p.dot(u_axis), 1e-5f));
+            CHECK_THAT(planar.y(), WithinAbs(p.dot(v_axis), 1e-5f));
+            // Right-handed for a viewer outside the model: u to their right, v down, so u x v points
+            // into the surface. Getting this wrong is exactly what mirrors a face.
+            CHECK_THAT(u_axis.cross(v_axis).dot(n), WithinAbs(-1.f, 1e-5f));
+        }
+    }
+}
+
+// An image with a different value in each quadrant, so one bake says both which way up it landed
+// and whether it came out mirrored. Distinct enough that a wrong quadrant cannot be mistaken for the
+// right one: top-left 255, top-right 170, bottom-left 85, bottom-right 0.
+static std::shared_ptr<std::vector<unsigned char>> make_quadrant_marker_png(size_t w = 32, size_t h = 32)
+{
+    std::vector<uint8_t> pixels(w * h);
+    for (size_t y = 0; y < h; ++y)
+        for (size_t x = 0; x < w; ++x)
+            pixels[y * w + x] = (y < h / 2) ? ((x < w / 2) ? 255 : 170) : ((x < w / 2) ? 85 : 0);
+    const boost::filesystem::path tmp_path = boost::filesystem::temp_directory_path()
+        / boost::filesystem::unique_path("texdisp_test_%%%%%%%%.png");
+    REQUIRE(Slic3r::png::write_gray_to_file(tmp_path.string(), w, h, pixels));
+
+    std::vector<unsigned char> bytes;
+    {
+        std::ifstream ifs(tmp_path.string(), std::ios::binary);
+        bytes.assign(std::istreambuf_iterator<char>(ifs), std::istreambuf_iterator<char>());
+    }
+    boost::system::error_code ec;
+    boost::filesystem::remove(tmp_path, ec);
+
+    REQUIRE_FALSE(bytes.empty());
+    return std::make_shared<std::vector<unsigned char>>(std::move(bytes));
+}
+
+TEST_CASE("baking an asymmetric image puts the top of the image at the top of the model", "[TextureDisplacement]")
+{
+    // An upright wall in the plane y = 0, facing -Y, tessellated so that each of the image's four
+    // quadrants gets a vertex at its centre. A flat sheet rather than a cube because every vertex
+    // normal is then exactly the face normal, which is what makes the expected depth below an exact
+    // number instead of a tri-planar blend of three of them.
+    constexpr int   STEPS = 4;    // cells per side, so 5x5 vertices
+    constexpr float SPAN  = 20.f; // mm, the wall's width and height
+    indexed_triangle_set wall;
+    for (int j = 0; j <= STEPS; ++j)
+        for (int i = 0; i <= STEPS; ++i)
+            wall.vertices.emplace_back(-SPAN / 2.f + SPAN * float(i) / float(STEPS), 0.f,
+                                       SPAN * float(j) / float(STEPS));
+    const auto at = [](int i, int j) { return j * (STEPS + 1) + i; };
+    for (int j = 0; j < STEPS; ++j)
+        for (int i = 0; i < STEPS; ++i) {
+            // Wound so that (+x) x (+z) = -y: the wall faces -Y, away from the origin.
+            wall.indices.emplace_back(at(i, j), at(i + 1, j), at(i + 1, j + 1));
+            wall.indices.emplace_back(at(i, j), at(i + 1, j + 1), at(i, j + 1));
+        }
+
+    TextureDisplacementFacetsData facets{};
+    facets[0] = paint_whole_mesh(wall);
+
+    TextureDisplacementLayer layer;
+    layer.slot              = 0;
+    layer.projection_method = TextureProjectionMethod::Triplanar;
+    layer.image_data        = make_quadrant_marker_png();
+    layer.depth_mm          = 2.f;
+    // The wall is 20 mm across and one tile is 40 mm, so it covers the middle half of the image in
+    // both axes: the quadrant boundaries land on x = 0 and z = 10, and the probed vertices sit a
+    // whole number of texels away from both.
+    layer.tiling_scale      = 40.f;
+    layer.offset            = Vec2f(0.5f, 0.75f);
+    // A single decal rather than a repeat, so that getting the orientation wrong cannot be masked by
+    // the wrap bringing the flipped coordinate back into the image at a plausible-looking row. With
+    // tiling off, uv stays inside [0.25, 0.75] in both axes only while v runs the right way.
+    layer.tile_enabled      = false;
+
+    const indexed_triangle_set result = build_texture_displacement(wall, { layer }, facets, classic_options());
+    REQUIRE(result.vertices.size() == wall.vertices.size());
+
+    // Seen from outside this face the viewer's right is +X and their up is +Z, so the image's
+    // top-left quadrant has to land at low x and high z. Each entry: the vertex, and the image value
+    // whose quadrant it must have read.
+    const std::array<std::tuple<const char *, int, int, int>, 4> expected = { {
+        { "top-left (255)", 1, 3, 255 },
+        { "top-right (170)", 3, 3, 170 },
+        { "bottom-left (85)", 1, 1, 85 },
+        { "bottom-right (0)", 3, 1, 0 },
+    } };
+
+    for (const auto &[name, i, j, value] : expected) {
+        DYNAMIC_SECTION("quadrant " << name)
+        {
+            const size_t vi = size_t(at(i, j));
+            // The whole displacement is along -Y, so this is the relief's height at that vertex.
+            const float raised = wall.vertices[vi].y() - result.vertices[vi].y();
+            INFO("vertex at " << wall.vertices[vi].x() << ", " << wall.vertices[vi].z());
+            CHECK_THAT(raised, WithinAbs(layer.depth_mm * float(value) / 255.f, 1e-3f));
+            // Nothing moved sideways or vertically: a mirrored or rotated basis would still have
+            // displaced along the normal, so this is here to catch the relief leaning instead.
+            CHECK_THAT(result.vertices[vi].x(), WithinAbs(wall.vertices[vi].x(), 1e-4f));
+            CHECK_THAT(result.vertices[vi].z(), WithinAbs(wall.vertices[vi].z(), 1e-4f));
+        }
+    }
+}
+
 TEST_CASE("The Cylindrical/Spherical patch frame is the bake's own, so a preview can share it", "[TextureDisplacement]")
 {
     // The fast preview reconstructs these two projections in the fragment shader and needs the very