Draw quad

src/main.cc

@@ -37,6 +37,10 @@ struct App {
   //  -----------------------------------------------------------------------------------
   GLuint        vbo;
 
+  // Holds the indices of the vertices that draw each triangle. Each triangle constitutes what
+  // OpenGL refers to as element, hence the name Element Buffer Object
+  GLuint        ebo;
+
   GLuint        shader_program;
   bool          running;
 };
@@ -114,18 +118,32 @@ int init(App &app) {
 
 void create_vertex_spec(App &app) {
   const std::vector<GLfloat> vertices = {
-    -0.8f, -0.8f, 0.0f, // position
+    // vert0
+    -0.5f, -0.5f, 0.0f, // position
      1.0f,  0.0f, 0.0f, // colour
-     0.8f, -0.8f, 0.0f, // position
+    // vert1
+     0.5f, -0.5f, 0.0f, // position
      0.0f,  1.0f, 0.0f, // colour
-     0.0f,  0.8f, 0.0f, // position
+    // vert2
+    -0.5f,  0.5f, 0.0f, // position
      0.0f,  0.0f, 1.0f, // colour
+    // vert3
+     0.5f,  0.5f, 0.0f, // position
+     1.0f,  1.0f, 0.0f, // colour
+  };
+
+  const std::vector<GLuint> indices = {
+    // first triangle
+    0, 1, 2,
+    // second triangle
+    2, 1, 3,
   };
 
   // Create and activate the VAO
   glGenVertexArrays(1, &app.vao);
   glBindVertexArray(app.vao);
 
+  // Create and set up the VBO
   glGenBuffers(1, &app.vbo);
   glBindBuffer(GL_ARRAY_BUFFER, app.vbo);
   glBufferData(GL_ARRAY_BUFFER,
@@ -134,6 +152,15 @@ void create_vertex_spec(App &app) {
                GL_STATIC_DRAW
   );
 
+  // Create and set up the EBO
+  glGenBuffers(1, &app.ebo);
+  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, app.ebo);
+  glBufferData(GL_ELEMENT_ARRAY_BUFFER,
+               indices.size() * sizeof(GLuint),
+               (void *)indices.data(),
+               GL_STATIC_DRAW
+  );
+
   GLsizei vertex_stride          = 6 * sizeof(GLfloat);
   GLvoid  *position_start_offset = (void *)0;
   GLvoid  *colour_start_offset   = (void *)(3 * sizeof(GLfloat));
@@ -154,6 +181,7 @@ void create_vertex_spec(App &app) {
   // drawn during the rendering stage
   glBindVertexArray(0);
   glBindBuffer(GL_ARRAY_BUFFER, 0);
+  glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
   glDisableVertexAttribArray(0);
   glDisableVertexAttribArray(1);
 }
@@ -190,8 +218,7 @@ void run_main_loop(App &app) {
 
     // Draw call
     glBindVertexArray(app.vao);
-    glBindBuffer(GL_ARRAY_BUFFER, app.vbo); // Is this actually needed?
-    glDrawArrays(GL_TRIANGLES, 0, 3);
+    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, (void *)0);
     // End draw call
 
     // Not necessary if we only have one shader program