style: color-code sphere faces to visualize seams

Each of the 6 cube-to-sphere faces now has a distinct color
(red, green, blue, yellow, magenta, cyan) passed via a
vertex color attribute. The fragment shader multiplies
lighting by this color instead of a uniform blue.

This makes the cube-face seams clearly visible when
inspecting the mapped sphere.

scenes/sphere.cpp

@@ -53,15 +53,26 @@ auto sphere::build_mesh() -> void {
         glm::vec3 position;
         glm::vec3 normal;
         glm::vec2 uv;
+        glm::vec3 color;
     };
 
     std::uint32_t const div = 32;
     std::vector<vertex> vertices;
     std::vector<std::uint32_t> indices;
 
+    // Distinct colors for each cube face so seams are visible
+    glm::vec3 const face_colors[6] = {
+        {1.0f, 0.2f, 0.2f}, // +X red
+        {0.2f, 1.0f, 0.2f}, // -X green
+        {0.2f, 0.2f, 1.0f}, // +Y blue
+        {1.0f, 1.0f, 0.2f}, // -Y yellow
+        {1.0f, 0.2f, 1.0f}, // +Z magenta
+        {0.2f, 1.0f, 1.0f}, // -Z cyan
+    };
+
     // Generate 6 cube faces, each with div x div vertices
     auto add_face = [&](glm::vec3 const& center, glm::vec3 const& u_axis,
-                        glm::vec3 const& v_axis) -> void {
+                        glm::vec3 const& v_axis, std::uint32_t face_idx) -> void {
         for (std::uint32_t i = 0; i < div; ++i) {
             for (std::uint32_t j = 0; j < div; ++j) {
                 float const s = float(i) / float(div - 1) * 2.0f - 1.0f;
@@ -75,23 +86,23 @@ auto sphere::build_mesh() -> void {
                 float const len = glm::length(pos);
                 glm::vec3 normal = pos / len;
 
-                vertices.push_back({normal, normal, {float(i) / float(div - 1), float(j) / float(div - 1)}});
+                vertices.push_back({normal, normal, {float(i) / float(div - 1), float(j) / float(div - 1)}, face_colors[face_idx]});
             }
         }
     };
 
     // +X face (right)
-    add_face(glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f}, glm::vec3{0.0f, 0.0f, 1.0f});
+    add_face(glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f}, glm::vec3{0.0f, 0.0f, 1.0f}, 0);
     // -X face (left)
-    add_face(glm::vec3{-1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f}, glm::vec3{0.0f, 0.0f, -1.0f});
+    add_face(glm::vec3{-1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f}, glm::vec3{0.0f, 0.0f, -1.0f}, 1);
     // +Y face (top)
-    add_face(glm::vec3{0.0f, 1.0f, 0.0f}, glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 0.0f, -1.0f});
+    add_face(glm::vec3{0.0f, 1.0f, 0.0f}, glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 0.0f, -1.0f}, 2);
     // -Y face (bottom)
-    add_face(glm::vec3{0.0f, -1.0f, 0.0f}, glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 0.0f, 1.0f});
+    add_face(glm::vec3{0.0f, -1.0f, 0.0f}, glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 0.0f, 1.0f}, 3);
     // +Z face (front)
-    add_face(glm::vec3{0.0f, 0.0f, 1.0f}, glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f});
+    add_face(glm::vec3{0.0f, 0.0f, 1.0f}, glm::vec3{1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f}, 4);
     // -Z face (back)
-    add_face(glm::vec3{0.0f, 0.0f, -1.0f}, glm::vec3{-1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f});
+    add_face(glm::vec3{0.0f, 0.0f, -1.0f}, glm::vec3{-1.0f, 0.0f, 0.0f}, glm::vec3{0.0f, 1.0f, 0.0f}, 5);
 
     // Generate indices for each face
     std::uint32_t offset = 0;
@@ -132,16 +143,21 @@ auto sphere::build_mesh() -> void {
     // location 1: normal (vec3)
     glEnableVertexAttribArray(1);
     glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(vertex),
-                         reinterpret_cast<void*>(offsetof(vertex, normal)));
+                         reinterpret_cast<void*>(3 * sizeof(float)));
 
     // location 2: uv (vec2)
     glEnableVertexAttribArray(2);
     glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(vertex),
-                         reinterpret_cast<void*>(3 * sizeof(float) + 3 * sizeof(float)));
+                         reinterpret_cast<void*>(6 * sizeof(float)));
+
+    // location 3: color (vec3)
+    glEnableVertexAttribArray(3);
+    glVertexAttribPointer(3, 3, GL_FLOAT, GL_FALSE, sizeof(vertex),
+                         reinterpret_cast<void*>(8 * sizeof(float)));
 
-    m_ebo.unbind();
-    m_vbo.unbind();
     m_vao.unbind();
+    m_vbo.unbind();
+    m_ebo.unbind();
 }
 
 auto sphere::build_shader() -> bool {
@@ -150,17 +166,20 @@ auto sphere::build_shader() -> bool {
         layout(location = 0) in vec3 a_pos;
         layout(location = 1) in vec3 a_normal;
         layout(location = 2) in vec2 a_uv;
+        layout(location = 3) in vec3 a_color;
         uniform mat4 u_model;
         uniform mat4 u_view;
         uniform mat4 u_proj;
         out vec3 v_normal;
         out vec3 v_world_pos;
         out vec2 v_uv;
+        out vec3 v_color;
         void main() {
             gl_Position = u_proj * u_view * u_model * vec4(a_pos, 1.0);
             v_normal = mat3(u_model) * a_normal;
             v_world_pos = (u_model * vec4(a_pos, 1.0)).xyz;
             v_uv = a_uv;
+            v_color = a_color;
         }
     )glsl";
 
@@ -169,14 +188,14 @@ auto sphere::build_shader() -> bool {
         in vec3 v_normal;
         in vec3 v_world_pos;
         in vec2 v_uv;
+        in vec3 v_color;
         out vec4 frag_color;
         void main() {
             vec3 light_dir = normalize(vec3(1.0, 1.0, 2.0));
             vec3 normal = normalize(v_normal);
             float diff = max(dot(normal, light_dir), 0.0);
             vec3 ambient = vec3(0.2);
-            vec3 color = vec3(0.4, 0.6, 1.0);
-            vec3 result = (ambient + diff * 0.8) * color;
+            vec3 result = (ambient + diff * 0.8) * v_color;
             frag_color = vec4(result, 1.0);
         }
     )glsl";