Improve lighting

- No specular on surfaces facing away from the light source
- Transform the light direction to view space to ensure that all
  lighting calculations happen in a consistent coordinate space.
- Other lighting tweaks

assets/shaders/gl_textured.frag

@@ -6,28 +6,37 @@ in  vec3 v_position;
 
 out vec4 frag_color;
 
-uniform vec3 u_light;
+uniform vec3 u_light;      // direction TO the light (normalized)
 uniform sampler2D tex;
-uniform vec3 color; // base colour factor (acts as solid colour when no texture)
+uniform vec3 color;        // base colour factor (acts as solid colour when no texture)
 
 void main() {
     // Combine base texture (or constant white) with colour factor supplied by CPU.
     vec3 base_col = texture(tex, v_tex).rgb * color;
 
-    vec3 ambient_color  = base_col * 0.2;
-    vec3 diffuse_color  = base_col * 0.6;
-    vec3 specular_color = vec3(1.0);
+    vec3 N = normalize(v_normal);
+    vec3 L = normalize(u_light);
 
-    // u_light is the direction **from the light towards the fragment**.
-    float diffuse = max(dot(normalize(v_normal), normalize(u_light)), 0.0);
+    // Classic Blinn-Phong lighting
+    // Ambient: always present
+    vec3 ambient = base_col * 0.15;
 
-    vec3 camera_dir = normalize(-v_position);
-    vec3 half_dir   = normalize(normalize(u_light) + camera_dir);
-    float specular  = pow(max(dot(half_dir, normalize(v_normal)), 0.0), 16.0);
+    // Diffuse: N dot L, clamped
+    float NdotL = max(dot(N, L), 0.0);
+    vec3 diffuse = base_col * NdotL * 0.7;
 
-    vec3 result = ambient_color + diffuse * diffuse_color + specular * specular_color;
+    // Specular: only on surfaces facing the light (NdotL > 0)
+    float specular = 0.0;
+    if (NdotL > 0.0) {
+        vec3 V = normalize(-v_position);         // view direction (camera at origin in view space)
+        vec3 H = normalize(L + V);               // half-vector
+        float NdotH = max(dot(N, H), 0.0);
+        specular = pow(NdotH, 32.0) * 0.5;       // tighter highlight, moderated intensity
+    }
 
-    // Convert from linear to sRGB for display (approximate γ-correction)
+    vec3 result = ambient + diffuse + vec3(specular);
+
+    // Convert from linear to sRGB for display (approximate gamma correction)
     result = pow(result, vec3(1.0 / 2.2));
 
     frag_color = vec4(result, 1.0);