The grid worked out for each pixel (as in Blender): one square on its plane, every set of lines showing everywhere their lines are far enough apart on the screen (no patch round the middle, no pieces smearing or fading as you move), a pixel or so wide and smooth, the axes in the colour of the world's axis they run along; in every renderer (drawGrid: a sprite marked by its colour's alpha)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

engine/render/renderer.cpp

@@ -211,6 +211,12 @@ void Renderer::drawSprite(const glm::vec3& at, float size, const glm::vec4& colo
     list.push_back({glm::vec4(at, size * 0.5f), color, glm::vec4(0, 0, 0, rotation), glm::vec4(0, 0, 0, tex)});
 }
 
+void Renderer::drawGrid(const glm::vec3& centre, const glm::vec3& u, const glm::vec3& v, float first, const glm::vec3& look, float reach) {
+    if (packet_.sprites.size() >= 8192) return;
+    // (colour.a < 0 marks it; sprite.vert / sprite.frag)
+    packet_.sprites.push_back({glm::vec4(centre, reach), glm::vec4(look, -1.0f), glm::vec4(u, first), glm::vec4(v, 0.0f)});
+}
+
 void Renderer::drawStreak(const glm::vec3& from, const glm::vec3& to, float width, const glm::vec4& color, TextureId texture, bool additive, float widthTo) {
     auto& list = additive ? packet_.glowSprites : packet_.sprites;
     if (list.size() >= 8192) return;
@@ -757,6 +763,7 @@ void Renderer::buildPacket(uint32_t width, uint32_t height) {
                         up = glm::cross(right, f);
         for (auto* list : {&p.sprites, &p.glowSprites})
             for (gpu::Sprite& s : *list) {
+                if (s.color.a < 0.0f) continue; // (a grid: on its own plane)
                 if (const glm::vec3 half(s.up); glm::dot(half, half) > 0.0f) { // a streak: each end's side faces the eye (sprite.vert)
                     s.right = glm::vec4(camera_.origin, s.right.w);
                     s.posSize.w = -std::max(s.posSize.w, 1e-9f); // (below 0: a streak; its half width at `from`)
@@ -765,6 +772,7 @@ void Renderer::buildPacket(uint32_t width, uint32_t height) {
                 s.right = glm::vec4(right, s.right.w), s.up = glm::vec4(up, s.up.w);
             }
         std::sort(p.sprites.begin(), p.sprites.end(), [&](const gpu::Sprite& a, const gpu::Sprite& b) {
+            if ((a.color.a < 0.0f) != (b.color.a < 0.0f)) return a.color.a < 0.0f; // (grids first: they're under everything)
             return glm::dot(glm::vec3(a.posSize) - camera_.origin, f) > glm::dot(glm::vec3(b.posSize) - camera_.origin, f);
         });
     }