From 8813a338cd265d3a58fd14993891420767596a34 Mon Sep 17 00:00:00 2001
From: Baplar <me@baplar.fr>
Date: Mon, 1 Jun 2026 23:33:56 +0200
Subject: [PATCH] Fix particles facing the wrong direction

- On the CPU side, change the order of vertices to form counterclockwise
  triangles
- On the GPU side, compensate this change by reversing relevant cross
  products
---
 .../shaders/materials/billboardunlit.wgsl     | 33 ++++++++++---------
 crates/renderide/src/particles/point.rs       |  4 +--
 crates/renderide/src/particles/tests.rs       |  2 +-
 3 files changed, 21 insertions(+), 18 deletions(-)

diff --git a/crates/renderide/shaders/materials/billboardunlit.wgsl b/crates/renderide/shaders/materials/billboardunlit.wgsl
index f89d5999d..782272baf 100644
--- a/crates/renderide/shaders/materials/billboardunlit.wgsl
+++ b/crates/renderide/shaders/materials/billboardunlit.wgsl
@@ -187,28 +187,31 @@ fn facing_basis(center_world: vec3<f32>, view_layer: u32, roll: f32, allow_roll:
     return RenderBufferBillboardBasis(right, up);
 }
 
-fn local_particle_basis(
+fn direction_stretch_particle_basis(
     d: dt::PerDrawUniforms,
     center_world: vec3<f32>,
-    pointdata: vec3<f32>,
     point_forward_upz: vec4<f32>,
     point_up_xy: vec2<f32>,
     view_layer: u32,
-    direction_stretch: bool,
+) -> RenderBufferBillboardBasis {
+    let up = rmath::safe_normalize(mv::model_vector(d, point_forward_upz.xyz), vec3<f32>(0.0, 0.0, 1.0));
+    let to_camera = rg::view_dir_for_world_pos(center_world, view_layer);
+    let right = rmath::safe_normalize(cross(up, to_camera), vec3<f32>(1.0, 0.0, 0.0));
+    return RenderBufferBillboardBasis(right, up);
+}
+
+fn local_particle_basis(
+    d: dt::PerDrawUniforms,
+    pointdata: vec3<f32>,
+    point_forward_upz: vec4<f32>,
+    point_up_xy: vec2<f32>,
 ) -> RenderBufferBillboardBasis {
     let raw_forward = rmath::safe_normalize(point_forward_upz.xyz, vec3<f32>(0.0, 0.0, 1.0));
     let raw_up = rmath::safe_normalize(vec3<f32>(point_up_xy, point_forward_upz.w), vec3<f32>(0.0, 1.0, 0.0));
     let world_forward = rmath::safe_normalize(mv::model_vector(d, raw_forward), vec3<f32>(0.0, 0.0, 1.0));
     let world_up = rmath::safe_normalize(mv::model_vector(d, raw_up), vec3<f32>(0.0, 1.0, 0.0));
-    if (direction_stretch) {
-        let to_camera = rg::view_dir_for_world_pos(center_world, view_layer);
-        let up = world_forward;
-        let right = rmath::safe_normalize(cross(to_camera, up), vec3<f32>(1.0, 0.0, 0.0));
-        return RenderBufferBillboardBasis(right, up);
-    }
-
-    var right = rmath::safe_normalize(cross(world_up, world_forward), vec3<f32>(1.0, 0.0, 0.0));
-    var up = rmath::safe_normalize(cross(world_forward, right), world_up);
+    var right = rmath::safe_normalize(cross(world_forward, world_up), vec3<f32>(1.0, 0.0, 0.0));
+    var up = rmath::safe_normalize(cross(right, world_forward), world_up);
     if (abs(pointdata.z) > 1e-4) {
         let rotated = rotate_render_buffer_axes(pointdata.z, right, up);
         right = rotated.right;
@@ -227,13 +230,13 @@ fn render_buffer_billboard_basis(
 ) -> RenderBufferBillboardBasis {
     let alignment = pd::particle_alignment(d);
     if (alignment == 1u) {
-        return facing_basis(center_world, view_layer, pointdata.z, false);
+        return facing_basis(center_world, view_layer, pointdata.z, true);
     }
     if (alignment == 2u || alignment == 3u) {
-        return local_particle_basis(d, center_world, pointdata, point_forward_upz, point_up_xy, view_layer, false);
+        return local_particle_basis(d, pointdata, point_forward_upz, point_up_xy);
     }
     if (alignment == 4u) {
-        return local_particle_basis(d, center_world, pointdata, point_forward_upz, point_up_xy, view_layer, true);
+        return direction_stretch_particle_basis(d, center_world, point_forward_upz, point_up_xy, view_layer);
     }
     return view_plane_basis(view_layer, pointdata.z, true);
 }
diff --git a/crates/renderide/src/particles/point.rs b/crates/renderide/src/particles/point.rs
index 39dc31731..0a8411a28 100644
--- a/crates/renderide/src/particles/point.rs
+++ b/crates/renderide/src/particles/point.rs
@@ -310,11 +310,11 @@ fn fill_billboard_particle(
         indices,
         &[
             base_vertex,
-            base_vertex + 2,
             base_vertex + 1,
             base_vertex + 2,
-            base_vertex + 3,
+            base_vertex + 2,
             base_vertex + 1,
+            base_vertex + 3,
         ],
     );
 }
diff --git a/crates/renderide/src/particles/tests.rs b/crates/renderide/src/particles/tests.rs
index 16b8190e3..d2e0f112e 100644
--- a/crates/renderide/src/particles/tests.rs
+++ b/crates/renderide/src/particles/tests.rs
@@ -118,7 +118,7 @@ fn billboard_fill_writes_stable_point_indices() {
     fill_billboard_buffers(&points, glam::IVec2::new(2, 1), &mut vertices, &mut indices);
 
     let index_words: &[u32] = bytemuck::cast_slice(&indices);
-    assert_eq!(index_words, &[0, 2, 1, 2, 3, 1, 4, 6, 5, 6, 7, 5]);
+    assert_eq!(index_words, &[0, 1, 2, 2, 1, 3, 4, 5, 6, 6, 5, 7]);
     let first_vertex: &[f32] = bytemuck::cast_slice(&vertices[..generated_vertex_stride()]);
     assert_eq!(&first_vertex[..3], &[1.0, 2.0, 3.0]);
     assert_eq!(&first_vertex[3..5], &[0.5, 0.5]);