Draw first triangle

src/main.cc

@@ -6,6 +6,8 @@
 #include <cstdint>
 #include <cstdio>
 #include <stdbool.h>
+#include <string>
+#include <vector>
 
 #define WINDOW_WIDTH 1280
 #define WINDOW_HEIGHT 720
@@ -22,11 +24,29 @@ struct App {
   SDL_Window    *window;
   SDL_GLContext context;
   SDL_Event     event;
+
+  // Defines the different attributes in the vertex data and how to access them.
+  // Think of it as a specification or a C struct that defines the types of data stored for each
+  // vertex.
+  GLuint        vao;
+
+  // The buffer that contains all the vertex data
+  // e.g. assume 3 vertices with position, colour and uv
+  //  -----------------------------------------------------------------------------------
+  // | position1 | colour1 | uv1 | position2 | colour2 | uv2 | position3 | colour3 | uv3 |
+  //  -----------------------------------------------------------------------------------
+  GLuint        vbo;
+
+  GLuint        shader_program;
 };
 
-int  init         (App &app);
-void run_main_loop(App &app);
-void cleanup      (App &app);
+int    init                    (App &app);
+void   create_vertex_spec      (App &app);
+void   create_graphics_pipeline(App &app);
+void   run_main_loop           (App &app);
+void   cleanup                 (App &app);
+GLuint create_shader_program   (const std::string &vertex_shader_source, const std::string &fragment_shader_source);
+GLuint compile_shader          (GLuint type, const std::string &source);
 
 int main() {
   App app    = {};
@@ -35,6 +55,10 @@ int main() {
     return result;
   }
 
+  create_vertex_spec(app);
+
+  create_graphics_pipeline(app);
+
   run_main_loop(app);
 
   cleanup(app);
@@ -49,7 +73,7 @@ int init(App &app) {
   }
 
   uint32_t flags = SDL_WINDOW_SHOWN | SDL_WINDOW_OPENGL | SDL_WINDOW_ALLOW_HIGHDPI;
-  app.window        = SDL_CreateWindow("Window", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
+  app.window     = SDL_CreateWindow("Window", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
                                     WINDOW_WIDTH, WINDOW_HEIGHT, flags);
   if (!app.window) {
     fprintf(stderr, "Failed to create window: %s", SDL_GetError());
@@ -84,6 +108,57 @@ int init(App &app) {
   return EXIT_CODE_SUCCESS;
 }
 
+void create_vertex_spec(App &app) {
+  const std::vector<GLfloat> vertices = {
+    -0.8f, -0.8f, 0.0f,
+     0.8f, -0.8f, 0.0f,
+     0.0f,  0.8f, 0.0f,
+  };
+
+  glGenVertexArrays(1, &app.vao);
+  glBindVertexArray(app.vao);
+
+  glGenBuffers(1, &app.vbo);
+  glBindBuffer(GL_ARRAY_BUFFER, app.vbo);
+  glBufferData(GL_ARRAY_BUFFER,
+               vertices.size() * sizeof(GLfloat),
+               (void *)vertices.data(),
+               GL_STATIC_DRAW
+  );
+
+  // Defines the vertex attributes and their indices. This defines the attribute for the currently
+  // bound VAO
+  glEnableVertexAttribArray(0);
+
+  // Defines the number of components for the attribute, its type, the stride between the component
+  // for each vertex and the offset of the first instance of the component in the buffer
+  glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void *)0);
+
+  // Cleanup. The order matters. Unbind VAO first then disable the attributes. If you do it the
+  // other way around, the VAO will store that the attributes are disabled and nothing will be
+  // drawn during the rendering stage
+  glBindVertexArray(0);
+  glBindBuffer(GL_ARRAY_BUFFER, 0);
+  glDisableVertexAttribArray(0);
+}
+
+void create_graphics_pipeline(App &app) {
+  const std::string vs_source =
+    "#version 460 core\n"
+    "in vec4 position;\n"
+    "void main() {\n"
+    "  gl_Position = vec4(position.x, position.y, position.z, position.w);\n"
+    "}";
+  const std::string fs_source =
+    "#version 460 core\n"
+    "out vec4 color;\n"
+    "void main() {\n"
+    "  color = vec4(0.93f, 0.42f, 0.35f, 1.0f);\n"
+    "}";
+
+  app.shader_program = create_shader_program(vs_source, fs_source);
+}
+
 void run_main_loop(App &app) {
   bool running = true;
 
@@ -96,6 +171,23 @@ void run_main_loop(App &app) {
       }
     }
 
+    // Pre draw setup
+    glDisable(GL_DEPTH_TEST);
+    glDisable(GL_CULL_FACE);
+
+    glViewport  (0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
+    glClearColor(0.36f, 0.34f, 0.42f, 1.0f);
+    glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
+
+    glUseProgram(app.shader_program);
+    // End pre draw setup
+
+    // Draw call
+    glBindVertexArray(app.vao);
+    glBindBuffer(GL_ARRAY_BUFFER, app.vbo); // Is this actually needed?
+    glDrawArrays(GL_TRIANGLES, 0, 3);
+    // End draw call
+
     SDL_GL_SwapWindow(app.window);
   }
 }
@@ -105,3 +197,37 @@ void cleanup(App &app) {
   SDL_DestroyWindow(app.window);
   SDL_Quit();
 }
+
+GLuint create_shader_program(const std::string &vertex_shader_source, const std::string &fragment_shader_source) {
+  GLuint shader_program  = glCreateProgram();
+  GLuint vertex_shader   = compile_shader(GL_VERTEX_SHADER, vertex_shader_source);
+  GLuint fragment_shader = compile_shader(GL_FRAGMENT_SHADER, fragment_shader_source);
+
+  glAttachShader(shader_program, vertex_shader);
+  glAttachShader(shader_program, fragment_shader);
+  glLinkProgram (shader_program);
+
+  // Validate the shader_program
+  glValidateProgram(shader_program);
+
+  return shader_program;
+}
+
+GLuint compile_shader(GLuint type, const std::string &source) {
+  GLuint shader             = glCreateShader(type);
+  const char *shader_source = source.c_str();
+
+  glShaderSource (shader, 1, &shader_source, NULL);
+  glCompileShader(shader);
+
+  GLint compile_status;
+  glGetShaderiv(shader, GL_COMPILE_STATUS, &compile_status);
+  if (compile_status != GL_TRUE) {
+    GLsizei log_length = 0;
+    GLchar message[1024];
+    glGetShaderInfoLog(shader, 1024, &log_length, message);
+    fprintf(stderr, "%s\n", message);
+  }
+
+  return shader;
+}