Follow-up of 6194e67e68 - Separated the part that computed triangles normals and lighting inside the fragment shader into a separate shader mm_gouraud, which is only used for the multi-material painting gizmo.

resources/shaders/mm_gouraud.fs

@@ -0,0 +1,55 @@
+#version 110
+
+#define INTENSITY_CORRECTION 0.6
+
+// normalized values for (-0.6/1.31, 0.6/1.31, 1./1.31)
+const vec3 LIGHT_TOP_DIR = vec3(-0.4574957, 0.4574957, 0.7624929);
+#define LIGHT_TOP_DIFFUSE    (0.8 * INTENSITY_CORRECTION)
+#define LIGHT_TOP_SPECULAR   (0.125 * INTENSITY_CORRECTION)
+#define LIGHT_TOP_SHININESS  20.0
+
+// normalized values for (1./1.43, 0.2/1.43, 1./1.43)
+const vec3 LIGHT_FRONT_DIR = vec3(0.6985074, 0.1397015, 0.6985074);
+#define LIGHT_FRONT_DIFFUSE  (0.3 * INTENSITY_CORRECTION)
+
+#define INTENSITY_AMBIENT    0.3
+
+const vec3  ZERO    = vec3(0.0, 0.0, 0.0);
+const float EPSILON = 0.0001;
+
+uniform vec4 uniform_color;
+
+varying vec3 clipping_planes_dots;
+varying vec4 model_pos;
+
+void main()
+{
+    if (any(lessThan(clipping_planes_dots, ZERO)))
+        discard;
+    vec3  color = uniform_color.rgb;
+    float alpha = uniform_color.a;
+
+    vec3 triangle_normal = normalize(cross(dFdx(model_pos.xyz), dFdy(model_pos.xyz)));
+#ifdef FLIP_TRIANGLE_NORMALS
+    triangle_normal = -triangle_normal;
+#endif
+
+    // First transform the normal into camera space and normalize the result.
+    vec3 eye_normal = normalize(gl_NormalMatrix * triangle_normal);
+
+    // Compute the cos of the angle between the normal and lights direction. The light is directional so the direction is constant for every vertex.
+    // Since these two are normalized the cosine is the dot product. We also need to clamp the result to the [0,1] range.
+    float NdotL = max(dot(eye_normal, LIGHT_TOP_DIR), 0.0);
+
+    // x = diffuse, y = specular;
+    vec2 intensity = vec2(0.0, 0.0);
+    intensity.x = INTENSITY_AMBIENT + NdotL * LIGHT_TOP_DIFFUSE;
+    vec3 position = (gl_ModelViewMatrix * model_pos).xyz;
+    intensity.y = LIGHT_TOP_SPECULAR * pow(max(dot(-normalize(position), reflect(-LIGHT_TOP_DIR, eye_normal)), 0.0), LIGHT_TOP_SHININESS);
+
+    // Perform the same lighting calculation for the 2nd light source (no specular applied).
+    NdotL = max(dot(eye_normal, LIGHT_FRONT_DIR), 0.0);
+    intensity.x += NdotL * LIGHT_FRONT_DIFFUSE;
+
+    gl_FragColor = vec4(vec3(intensity.y) + color * intensity.x, alpha);
+}