From 5339d9154e788ee32dd41e15e911768734e96adb Mon Sep 17 00:00:00 2001
From: Haroon Qureshi <haroonq@google.com>
Date: Tue, 12 May 2026 01:26:50 -0700
Subject: [PATCH] Ensure faces for meshes are correctly oriented.

This is in preparation for a future CL that will disable backface culling.

PiperOrigin-RevId: 914136370
Change-Id: Ie8db1268faaa306c3f3356efc5e1305d791cf525
---
 .../filament/compat/model_objects.cc             | 12 ++++++------
 src/experimental/filament/filament/builtins.cc   | 16 ++++++++++------
 2 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/src/experimental/filament/compat/model_objects.cc b/src/experimental/filament/compat/model_objects.cc
index a619e73d..dfd7d644 100644
--- a/src/experimental/filament/compat/model_objects.cc
+++ b/src/experimental/filament/compat/model_objects.cc
@@ -254,17 +254,17 @@ static void FillHeightFieldBuffer(MeshBuilder& builder, const mjModel* model,
   // Build the front edge.
   for (int col = 0; col < ncol - 1; ++col) {
     const float3 a = get_pos(0, col);
-    const float3 b = get_pos(0, col + 1);
-    const float3 c = {b.x, b.y, -sz[3]};
-    const float3 d = {a.x, a.y, -sz[3]};
+    const float3 b = {a.x, a.y, -sz[3]};
+    const float3 d = get_pos(0, col + 1);
+    const float3 c = {d.x, d.y, -sz[3]};
     append_quad(a, b, c, d);
   }
   // Build the back edge.
   for (int col = 0; col < ncol - 1; ++col) {
     const float3 a = get_pos(nrow - 1, col + 1);
-    const float3 b = get_pos(nrow - 1, col);
-    const float3 c = {b.x, b.y, -sz[3]};
-    const float3 d = {a.x, a.y, -sz[3]};
+    const float3 b = {a.x, a.y, -sz[3]};
+    const float3 d = get_pos(nrow - 1, col);
+    const float3 c = {d.x, d.y, -sz[3]};
     append_quad(a, b, c, d);
   }
   // Build the base. We use the visualization quality as the size rather than
diff --git a/src/experimental/filament/filament/builtins.cc b/src/experimental/filament/filament/builtins.cc
index b18e28e5..98a14296 100644
--- a/src/experimental/filament/filament/builtins.cc
+++ b/src/experimental/filament/filament/builtins.cc
@@ -153,7 +153,7 @@ class PlaneBuilder : public BuiltinBuilder {
         const int i1 = base_idx + 1;
         const int i2 = base_idx + num_quads_per_axis + 2;
         const int i3 = base_idx + num_quads_per_axis + 1;
-        AppendQuadIndices(indices_, i0, i1, i2, i3);
+        AppendQuadIndices(indices_, i0, i3, i2, i1);
       }
     }
 
@@ -268,7 +268,11 @@ class BoxBuilder : public BuiltinBuilder {
           const int i1 = base_idx + 1;
           const int i2 = base_idx + num_quads_per_axis_ + 2;
           const int i3 = base_idx + num_quads_per_axis_ + 1;
-          AppendQuadIndices(indices_, i0, i1, i2, i3);
+          if (i == 2 || i == 1 || i == 4) {
+            AppendQuadIndices(indices_, i0, i3, i2, i1);
+          } else {
+            AppendQuadIndices(indices_, i0, i1, i2, i3);
+          }
         }
       }
     }
@@ -326,7 +330,7 @@ class TubeBuilder : public BuiltinBuilder {
         const int i1 = base_idx + 1;
         const int i2 = (base_idx + num_stacks + 2) % num_vertices;
         const int i3 = (base_idx + num_stacks + 1) % num_vertices;
-        AppendQuadIndices(indices_, i0, i1, i2, i3);
+        AppendQuadIndices(indices_, i0, i3, i2, i1);
       }
     }
 
@@ -494,8 +498,8 @@ class SphereBuilder : public BuiltinBuilder {
     for (int lon = 0; lon < num_slices; ++lon) {
       const int next = lon < (num_slices - 1) ? lon + 1 : 0;
       indices_.push_back(kNorthPoleIndex);
-      indices_.push_back(row_start + next);
       indices_.push_back(row_start + lon);
+      indices_.push_back(row_start + next);
     }
 
     // Latitudinal triangle strips.
@@ -519,8 +523,8 @@ class SphereBuilder : public BuiltinBuilder {
     for (int lon = 0; lon < num_slices; ++lon) {
       const int adjacent = lon < (num_slices - 1) ? lon + 1 : 0;
       indices_.push_back(kSouthPoleIndex);
-      indices_.push_back(row_start + lon);
       indices_.push_back(row_start + adjacent);
+      indices_.push_back(row_start + lon);
     }
 
     SetBounds({-1, -1, -1}, {1, 1, 1});
@@ -585,8 +589,8 @@ class DomeBuilder : public BuiltinBuilder {
     for (int lon = 0; lon < num_slices; ++lon) {
       const int next = lon < (num_slices - 1) ? lon + 1 : 0;
       indices_.push_back(kPoleIndex);
-      indices_.push_back(row_start + next);
       indices_.push_back(row_start + lon);
+      indices_.push_back(row_start + next);
     }
 
     // Latitudinal quad strips.  The first "stack" was handled above, so we