feat: add sphere scene with fixed cube-to-sphere mapping

Added scenes/sphere.{hpp,cpp} using the cube-to-sphere
approach from nrz.cpp, but with corrected math: vertices
are simply normalized to project onto the unit sphere
(the original used a broken formula with p=50.0 as an
exponent).

The sphere uses indexed geometry with position, normal,
and UV attributes, plus a simple diffuse lighting shader.

Press 1/2 to switch between cube and sphere scenes.
Updated .gitignore to exclude generated PNG screenshots.

scenes/sphere.cpp

@@ -0,0 +1,195 @@
+#include <cstddef>
+#include <cmath>
+#include <vector>
+
+#include "glad/glad.h"
+#include "glm/gtc/matrix_transform.hpp"
+#include "glm/gtc/type_ptr.hpp"
+
+#include "scenes/sphere.hpp"
+
+namespace cbt::scenes {
+
+sphere::sphere() {
+    m_start = std::chrono::steady_clock::now();
+}
+
+auto sphere::init() -> bool {
+    if (!build_shader()) {
+        return false;
+    }
+    build_mesh();
+    glEnable(GL_DEPTH_TEST);
+    return true;
+}
+
+auto sphere::update(float) -> void {}
+
+auto sphere::render() -> void {
+    auto now = std::chrono::steady_clock::now();
+    auto elapsed = std::chrono::duration<float>(now - m_start).count();
+
+    glClearColor(0.15f, 0.15f, 0.2f, 1.0f);
+    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+
+    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, -4.0f});
+    auto model = glm::rotate(glm::mat4{1.0f}, elapsed, glm::vec3{1.0f, 0.3f, 0.2f});
+
+    m_prog.use();
+    glUniformMatrix4fv(m_loc_proj, 1, GL_FALSE, glm::value_ptr(proj));
+    glUniformMatrix4fv(m_loc_view, 1, GL_FALSE, glm::value_ptr(view));
+    glUniformMatrix4fv(m_loc_model, 1, GL_FALSE, glm::value_ptr(model));
+
+    m_vao.bind();
+    glDrawElements(GL_TRIANGLES, m_index_count, GL_UNSIGNED_INT, nullptr);
+    m_vao.unbind();
+    m_prog.unuse();
+}
+
+auto sphere::build_mesh() -> void {
+    struct vertex {
+        glm::vec3 position;
+        glm::vec3 normal;
+        glm::vec2 uv;
+    };
+
+    std::uint32_t const div = 32;
+    std::vector<vertex> vertices;
+    std::vector<std::uint32_t> indices;
+
+    // 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 {
+        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;
+                float const t = float(j) / float(div - 1) * 2.0f - 1.0f;
+
+                // Position on cube face
+                glm::vec3 pos = center + u_axis * s + v_axis * t;
+
+                // FIX: normalize to project onto unit sphere
+                // (the original nrz.cpp used a broken formula with p=50.0)
+                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)}});
+            }
+        }
+    };
+
+    // +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});
+    // -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});
+    // +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});
+    // -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});
+    // +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});
+    // -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});
+
+    // Generate indices for each face
+    std::uint32_t offset = 0;
+    for (std::uint32_t face = 0; face < 6; ++face) {
+        for (std::uint32_t i = 0; i < div - 1; ++i) {
+            for (std::uint32_t j = 0; j < div - 1; ++j) {
+                std::uint32_t const a = offset + i * div + j;
+                std::uint32_t const b = offset + (i + 1) * div + j;
+                std::uint32_t const c = offset + (i + 1) * div + j + 1;
+                std::uint32_t const d = offset + i * div + j + 1;
+
+                // Two triangles per quad (consistent winding)
+                indices.push_back(a);
+                indices.push_back(b);
+                indices.push_back(d);
+
+                indices.push_back(b);
+                indices.push_back(c);
+                indices.push_back(d);
+            }
+        }
+        offset += div * div;
+    }
+
+    m_index_count = static_cast<GLsizei>(indices.size());
+
+    m_vbo.upload(vertices.data(), vertices.size() * sizeof(vertex));
+    m_ebo.upload(indices.data(), indices.size() * sizeof(std::uint32_t));
+
+    m_vao.bind();
+    m_vbo.bind();
+    m_ebo.bind();
+
+    // location 0: position (vec3)
+    glEnableVertexAttribArray(0);
+    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(vertex), nullptr);
+
+    // location 1: normal (vec3)
+    glEnableVertexAttribArray(1);
+    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(vertex),
+                         reinterpret_cast<void*>(offsetof(vertex, normal)));
+
+    // location 2: uv (vec2)
+    glEnableVertexAttribArray(2);
+    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, sizeof(vertex),
+                         reinterpret_cast<void*>(3 * sizeof(float) + 3 * sizeof(float)));
+
+    m_ebo.unbind();
+    m_vbo.unbind();
+    m_vao.unbind();
+}
+
+auto sphere::build_shader() -> bool {
+    char const* vert_src = R"glsl(
+        #version 410 core
+        layout(location = 0) in vec3 a_pos;
+        layout(location = 1) in vec3 a_normal;
+        layout(location = 2) in vec2 a_uv;
+        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;
+        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;
+        }
+    )glsl";
+
+    char const* frag_src = R"glsl(
+        #version 410 core
+        in vec3 v_normal;
+        in vec3 v_world_pos;
+        in vec2 v_uv;
+        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;
+            frag_color = vec4(result, 1.0);
+        }
+    )glsl";
+
+    if (!m_prog.compile_vertex(vert_src) || !m_prog.compile_fragment(frag_src) || !m_prog.link()) {
+        return false;
+    }
+
+    m_loc_proj = glGetUniformLocation(m_prog.id(), "u_proj");
+    m_loc_view = glGetUniformLocation(m_prog.id(), "u_view");
+    m_loc_model = glGetUniformLocation(m_prog.id(), "u_model");
+
+    return true;
+}
+
+}

scenes/sphere.hpp

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <chrono>
+
+#include "glm/glm.hpp"
+
+#include "cbt/scene.hpp"
+#include "cbt/opengl/buffer.hpp"
+#include "cbt/opengl/shader.hpp"
+#include "cbt/opengl/vao.hpp"
+
+namespace cbt::scenes {
+
+class sphere final : public scene {
+public:
+    sphere();
+    auto init() -> bool override;
+    auto update(float delta_time) -> void override;
+    auto render() -> void override;
+
+private:
+    opengl::shader m_prog;
+    opengl::buffer m_vbo;
+    opengl::buffer m_ebo{opengl::buffer_type::index};
+    opengl::vao m_vao;
+
+    GLint m_loc_proj = -1;
+    GLint m_loc_view = -1;
+    GLint m_loc_model = -1;
+
+    GLsizei m_index_count = 0;
+
+    std::chrono::steady_clock::time_point m_start;
+
+    auto build_mesh() -> void;
+    auto build_shader() -> bool;
+};
+
+}