Correct for non uniform scale in lighting

shaders/flat.frag

@@ -1,7 +1,6 @@
 #version 330 core
 
 noperspective in vec2 UV;
-in vec3 NORMAL;
 
 in vec3 AMBIENT;
 in vec3 DIFFUSE;

shaders/flat.vert

@@ -7,9 +7,9 @@ layout (location = 2) in vec2 uv;
 uniform mat4 MODEL;
 uniform mat4 VIEW;
 uniform mat4 PROJECTION;
+uniform mat3 NORMAL;
 
 noperspective out vec2 UV; // PSX use affine texture mapping
-out vec3 NORMAL;
 
 out vec3 AMBIENT;
 out vec3 DIFFUSE;
@@ -46,18 +46,19 @@ void main() {
 
   UV = uv;
 
-  NORMAL = (VIEW * MODEL * vec4(normal, 0.0)).xyz;
+  vec3 my_normal = (VIEW * MODEL * vec4(normal, 0.0)).xyz;
+  my_normal *= NORMAL;
 
   // Flat shading, we compute light per vertex
   AMBIENT = directionalLight.ambient * directionalLight.color * material.ambient;
 
   vec3 direction = -(VIEW * vec4(directionalLight.direction, 0.0)).xyz;
-  float diff = dot(normalize(direction), normalize(NORMAL));
+  float diff = dot(normalize(direction), normalize(my_normal));
   diff = max(diff, 0.0);
   DIFFUSE = directionalLight.diffuse * diff * directionalLight.color * material.diffuse;
 
   vec3 viewDir = normalize((VIEW * MODEL * vec4(pos, 1.0)).xyz);
-  vec3 reflectionDir = reflect(normalize(direction), normalize(NORMAL));
+  vec3 reflectionDir = reflect(normalize(direction), normalize(my_normal));
   float spec = dot(viewDir, reflectionDir);
   spec = max(spec, 0.0);
   spec = pow(spec, material.shininess);