feat: render spinning 3D cube with glm

- Add glm dependency for matrix transformations
- Replace triangle with 6-face colored cube
- Add MVP shader uniforms (model, view, projection)
- Enable depth testing for proper 3D rendering
- Spin cube around (1, 0.5, 0.3) axis

style: fix trailing return type for move assignment operators

- buffer/texture/shader/vao: use auto fn() -> T& style
- Document trailing return type convention in AGENTS.md

AGENTS.md

@@ -38,11 +38,12 @@ to the custom scripts instead of system-installed packages.
 ## Coding Conventions
 
 - **Language:** C++23
-- **Trailing return type** for function signatures
+- **Trailing return type** for function signatures (e.g. `auto fn() -> void`)
 - **4-space indentation**
 - **No semicolons after closing braces** for namespaces/classes
 - `auto` for obvious types (e.g. `auto main(...) -> int`)
 - **East const** (e.g. `char const*` not `const char*`)
+- **Trailing return type** for all function definitions, including operators (e.g. `auto operator=(T&&) noexcept -> T&`)
 - `<>` includes only for system headers (std, OS, etc.)
 - `""` includes for third-party dependencies (e.g. `fmt`, `nlohmann/json`)
 - **Naming:** `snake_case` for variables, functions, and classes

CMakeLists.txt

@@ -16,6 +16,7 @@ find_package(fmt REQUIRED)
 find_package(glfw3 REQUIRED)
 find_package(glad REQUIRED)
 find_package(asio REQUIRED)
+find_package(glm REQUIRED)
 
 # Target setup
 add_executable(cuber "cuber.cpp"
@@ -30,4 +31,4 @@ target_include_directories(cuber PRIVATE ".")
 target_compile_features(cuber PRIVATE cxx_std_23)
 target_compile_options(cuber PRIVATE ${BASE_OPTIONS})
 target_compile_definitions(cuber PRIVATE ${BASE_DEFINITIONS})
-target_link_libraries(cuber PRIVATE fmt::fmt glfw::glfw glad::glad asio::asio ${BASE_LIBRARIES})
+target_link_libraries(cuber PRIVATE fmt::fmt glfw::glfw glad::glad asio::asio glm::glm ${BASE_LIBRARIES})

cbt/opengl/buffer.cpp

@@ -25,7 +25,7 @@ buffer::buffer(buffer&& other) noexcept
     other.m_id = 0;
 }
 
-buffer& buffer::operator=(buffer&& other) noexcept {
+auto buffer::operator=(buffer&& other) noexcept -> buffer& {
     if (this != &other) {
         if (m_id) {
             glDeleteBuffers(1, &m_id);

cbt/opengl/shader.cpp

@@ -20,7 +20,7 @@ shader::shader(shader&& other) noexcept
     other.m_id = 0;
 }
 
-shader& shader::operator=(shader&& other) noexcept {
+auto shader::operator=(shader&& other) noexcept -> shader& {
     if (this != &other) {
         if (m_id) {
             glDeleteProgram(m_id);

cbt/opengl/texture.cpp

@@ -25,7 +25,7 @@ texture::texture(texture&& other) noexcept
     other.m_id = 0;
 }
 
-texture& texture::operator=(texture&& other) noexcept {
+auto texture::operator=(texture&& other) noexcept -> texture& {
     if (this != &other) {
         if (m_id) {
             glDeleteTextures(1, &m_id);

cbt/opengl/vao.cpp

@@ -17,7 +17,7 @@ vao::vao(vao&& other) noexcept
     other.m_id = 0;
 }
 
-vao& vao::operator=(vao&& other) noexcept {
+auto vao::operator=(vao&& other) noexcept -> vao& {
     if (this != &other) {
         if (m_id) {
             glDeleteVertexArrays(1, &m_id);

cuber.cpp

@@ -6,10 +6,16 @@
 #include "cbt/opengl/buffer.hpp"
 #include "cbt/opengl/vao.hpp"
 
-#include "asio.hpp"
+#include <chrono>
+
+#include <asio.hpp>
+#include <glm/glm.hpp>
+#include <glm/gtc/matrix_transform.hpp>
+#include <glm/gtc/type_ptr.hpp>
+
 #include "glad/glad.h"
 
-auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
+auto main(int, char const*[]) -> int {
     auto ctx = cbt::opengl::context("cuber", 1280, 720);
 
     if (!ctx.valid()) {
@@ -23,9 +29,12 @@ auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
         #version 410 core
         layout(location = 0) in vec3 a_pos;
         layout(location = 1) in vec3 a_color;
+        uniform mat4 u_model;
+        uniform mat4 u_view;
+        uniform mat4 u_proj;
         out vec3 v_color;
         void main() {
-            gl_Position = vec4(a_pos, 1.0);
+            gl_Position = u_proj * u_view * u_model * vec4(a_pos, 1.0);
             v_color = a_color;
         }
     )glsl";
@@ -43,12 +52,56 @@ auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
         return 1;
     }
 
-    // vertex data: triangle with per-vertex colors
+    // cube vertices: 6 faces, 2 triangles per face, 3 verts each = 36 vertices
-    // red, green, blue corners
+    // each vertex: x, y, z, r, g, b
-    std::array<float, 18> vertices = {
+    std::array<float, 36 * 6> vertices = {
-        -0.5f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,
+        // front face (cyan)
-         0.5f, -0.5f, 0.0f,  0.0f, 1.0f, 0.0f,
+         0.5f, -0.5f,  0.5f,  0.0f, 1.0f, 1.0f,
-         0.0f,  0.5f, 0.0f,  0.0f, 0.0f, 1.0f,
+        -0.5f, -0.5f,  0.5f,  0.0f, 1.0f, 1.0f,
+        -0.5f,  0.5f,  0.5f,  0.0f, 1.0f, 1.0f,
+         0.5f, -0.5f,  0.5f,  0.0f, 1.0f, 1.0f,
+        -0.5f,  0.5f,  0.5f,  0.0f, 1.0f, 1.0f,
+         0.5f,  0.5f,  0.5f,  0.0f, 1.0f, 1.0f,
+
+        // back face (magenta)
+         0.5f, -0.5f, -0.5f,  1.0f, 0.0f, 1.0f,
+        -0.5f, -0.5f, -0.5f,  1.0f, 0.0f, 1.0f,
+        -0.5f,  0.5f, -0.5f,  1.0f, 0.0f, 1.0f,
+         0.5f, -0.5f, -0.5f,  1.0f, 0.0f, 1.0f,
+        -0.5f,  0.5f, -0.5f,  1.0f, 0.0f, 1.0f,
+         0.5f,  0.5f, -0.5f,  1.0f, 0.0f, 1.0f,
+
+        // top face (yellow)
+         0.5f,  0.5f,  0.5f,  1.0f, 1.0f, 0.0f,
+        -0.5f,  0.5f,  0.5f,  1.0f, 1.0f, 0.0f,
+        -0.5f,  0.5f, -0.5f,  1.0f, 1.0f, 0.0f,
+         0.5f,  0.5f,  0.5f,  1.0f, 1.0f, 0.0f,
+        -0.5f,  0.5f, -0.5f,  1.0f, 1.0f, 0.0f,
+         0.5f,  0.5f, -0.5f,  1.0f, 1.0f, 0.0f,
+
+        // bottom face (white)
+         0.5f, -0.5f, -0.5f,  1.0f, 1.0f, 1.0f,
+        -0.5f, -0.5f, -0.5f,  1.0f, 1.0f, 1.0f,
+        -0.5f, -0.5f,  0.5f,  1.0f, 1.0f, 1.0f,
+         0.5f, -0.5f, -0.5f,  1.0f, 1.0f, 1.0f,
+        -0.5f, -0.5f,  0.5f,  1.0f, 1.0f, 1.0f,
+         0.5f, -0.5f,  0.5f,  1.0f, 1.0f, 1.0f,
+
+        // right face (lime)
+         0.5f, -0.5f, -0.5f,  0.0f, 1.0f, 0.0f,
+         0.5f, -0.5f,  0.5f,  0.0f, 1.0f, 0.0f,
+         0.5f,  0.5f,  0.5f,  0.0f, 1.0f, 0.0f,
+         0.5f, -0.5f, -0.5f,  0.0f, 1.0f, 0.0f,
+         0.5f,  0.5f,  0.5f,  0.0f, 1.0f, 0.0f,
+         0.5f,  0.5f, -0.5f,  0.0f, 1.0f, 0.0f,
+
+        // left face (orange)
+        -0.5f, -0.5f, -0.5f,  1.0f, 0.5f, 0.0f,
+        -0.5f, -0.5f,  0.5f,  1.0f, 0.5f, 0.0f,
+        -0.5f,  0.5f,  0.5f,  1.0f, 0.5f, 0.0f,
+        -0.5f, -0.5f, -0.5f,  1.0f, 0.5f, 0.0f,
+        -0.5f,  0.5f,  0.5f,  1.0f, 0.5f, 0.0f,
+        -0.5f,  0.5f, -0.5f,  1.0f, 0.5f, 0.0f,
     };
 
     auto vbo = cbt::opengl::buffer(cbt::opengl::buffer_type::vertex);
@@ -66,6 +119,9 @@ auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
     vbo.unbind();
     vao.unbind();
 
+    // enable depth testing
+    glEnable(GL_DEPTH_TEST);
+
     // signal handling
     asio::io_context io;
     asio::signal_set signals(io, SIGINT, SIGTERM);
@@ -81,6 +137,8 @@ auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
     };
 
     // render loop
+    auto start = std::chrono::steady_clock::now();
+
     while (!ctx.should_close()) {
         process_signals();
 
@@ -88,12 +146,25 @@ auto main([[maybe_unused]]int argc, [[maybe_unused]]char const* argv[]) -> int {
             break;
         }
 
-        glClearColor(0.6f, 0.8f, 1.0f, 1.0f);
+        auto now = std::chrono::steady_clock::now();
-        glClear(GL_COLOR_BUFFER_BIT);
+        auto elapsed = std::chrono::duration<float>(now - start).count();
+
+        glClearColor(0.15f, 0.15f, 0.2f, 1.0f);
+        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+
+        // compute MVP matrices
+        auto aspect = 1280.0f / 720.0f;
+        auto proj = glm::perspective(glm::radians(45.0f), aspect, 0.1f, 100.0f);
+        auto view = glm::translate(glm::mat4{1.0f}, glm::vec3{0.0f, 0.0f, -3.0f});
+        auto model = glm::rotate(glm::mat4{1.0f}, elapsed, glm::vec3{1.0f, 0.5f, 0.3f});
 
         prog.use();
+        glUniformMatrix4fv(glGetUniformLocation(prog.id(), "u_proj"), 1, GL_FALSE, glm::value_ptr(proj));
+        glUniformMatrix4fv(glGetUniformLocation(prog.id(), "u_view"), 1, GL_FALSE, glm::value_ptr(view));
+        glUniformMatrix4fv(glGetUniformLocation(prog.id(), "u_model"), 1, GL_FALSE, glm::value_ptr(model));
+
         vao.bind();
-        glDrawArrays(GL_TRIANGLES, 0, 3);
+        glDrawArrays(GL_TRIANGLES, 0, 36);
         vao.unbind();
         prog.unuse();