From 2820240980664462aa32ef7c90659c0180d3f5c3 Mon Sep 17 00:00:00 2001
From: Adrian G L <adrian@lauterer.it>
Date: Thu, 2 Oct 2025 12:15:14 +0200
Subject: [PATCH] fix: compute and use normal matrix for correct normal
 transformations in shader

---
 shaders/simple.vert |  3 ++-
 src/main.rs         | 13 +++++++++++--
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/shaders/simple.vert b/shaders/simple.vert
index 568b9d2..bcf427c 100644
--- a/shaders/simple.vert
+++ b/shaders/simple.vert
@@ -6,9 +6,10 @@ in layout(location=2) vec3 aNormal;
 out vec4 vColor;
 out vec3 vNormal;
 layout(location = 0) uniform mat4 transform;
+layout(location = 1) uniform mat3 normalMatrix;
 
 void main() {
     gl_Position = transform * vec4(position, 1.0f);
     vColor = aColor;
-    vNormal = aNormal;
+    vNormal = normalize(normalMatrix * aNormal);
 }
diff --git a/src/main.rs b/src/main.rs
index d38d7f4..ef1df74 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -130,7 +130,7 @@ unsafe fn create_vao(vertices: &Vec<f32>, normals: &Vec<f32>, colors: &Vec<f32>,
 }
 
 // Scene graph drawing function
-unsafe fn draw_scene(node: &scene_graph::SceneNode, transform_loc: i32, view_projection: &glm::Mat4, transform_so_far: &glm::Mat4) {
+unsafe fn draw_scene(node: &scene_graph::SceneNode, transform_loc: i32, normal_loc: i32, view_projection: &glm::Mat4, transform_so_far: &glm::Mat4) {
     // Build model matrix from node transformations
     let translation = glm::translation(&node.position);
     let rot_x = glm::rotation(node.rotation.x, &glm::vec3(1.0, 0.0, 0.0));
@@ -148,6 +148,14 @@ unsafe fn draw_scene(node: &scene_graph::SceneNode, transform_loc: i32, view_pro
     if node.index_count > 0 {
         let mvp = view_projection * new_transform;
         gl::UniformMatrix4fv(transform_loc, 1, gl::FALSE, mvp.as_ptr());
+        // compute normal matrix (inverse‐transpose of model)
+        let nm4 = glm::transpose(&glm::inverse(&new_transform));
+        let normal_matrix = glm::mat3(
+            nm4[(0,0)], nm4[(0,1)], nm4[(0,2)],
+            nm4[(1,0)], nm4[(1,1)], nm4[(1,2)],
+            nm4[(2,0)], nm4[(2,1)], nm4[(2,2)],
+        );
+        gl::UniformMatrix3fv(normal_loc, 1, gl::FALSE, normal_matrix.as_ptr());
         gl::BindVertexArray(node.vao_id);
         gl::DrawElements(gl::TRIANGLES, node.index_count, gl::UNSIGNED_INT, ptr::null());
     }
@@ -349,6 +357,7 @@ fn main() {
         };
         // get uniform location for the matrix (named `transform` in your vertex shader)
         let transform_loc = unsafe { simple_shader.get_uniform_location("transform") };
+        let normal_loc    = unsafe { simple_shader.get_uniform_location("normalMatrix") };
 
         // Used to demonstrate keyboard handling for exercise 2.
         let mut _arbitrary_number = 0.0; // feel free to remove
@@ -460,7 +469,7 @@ fn main() {
 
                 // Draw scene via scene graph
                 let view_proj = projection * view;
-                draw_scene(&*root_node, transform_loc, &view_proj, &glm::identity());
+                draw_scene(&*root_node, transform_loc, normal_loc, &view_proj, &glm::identity());
             }
 
             // Display the new color buffer on the display