Objects on the fly

This tutorial creates two OkItem quads at runtime, parents one to the other with attachTo, and rotates the parent every frame from the step callback. Because the second quad is a child, it inherits the parent's rotation: spin the parent and the child orbits with it. See Items for the full transform API.

Build the meshes

An OkItem takes interleaved vertex data (position plus UV) and an index buffer. Here a simple quad (two triangles) is the whole mesh:

std::vector<float> vertices = {
  // positions          // texture coords
   0.5f,  0.5f, 0.0f,    1.0f, 1.0f,  // top right
   0.5f, -0.5f, 0.0f,    1.0f, 0.0f,  // bottom right
  -0.5f, -0.5f, 0.0f,    0.0f, 0.0f,  // bottom left
  -0.5f,  0.5f, 0.0f,    0.0f, 1.0f,  // top left
};

std::vector<unsigned int> indices = {
  0, 1, 3,  // first triangle
  1, 2, 3   // second triangle
};

Create, configure and parent

Create both items, draw them as wireframe, add the first to the scene and attach the second to the first. Positions set after attachTo are relative to the parent.

OkItem *item = new OkItem("quad", vertices.data(),
                          static_cast<int>(vertices.size()),
                          indices.data(),
                          static_cast<int>(indices.size()));
item->setWireframe(true);

OkItem *item2 = new OkItem("quad2", vertices.data(),
                           static_cast<int>(vertices.size()),
                           indices.data(),
                           static_cast<int>(indices.size()));
item2->setWireframe(true);
item2->rotate(0.0f, glm::radians(90.0f), 0.0f);

scene->addObject(item);
item2->attachTo(item);                 // item2 is now a child of item

item->setPosition(-2.0f, 0.0f, -10.0f);  // left quad, in front of the camera
item2->setPosition(2.0f, 0.0f, 0.0f);    // relative to the parent

The scene owns item (added with addObject), and item owns item2 (its child), so the scene destructor cleans both up. Do not delete them yourself.

Rotate the parent each frame

In the step callback, rotate the parent around its Y axis. deltaTime keeps the motion frame-rate independent. The child orbits along because the transforms compose.

void stepCallback(float deltaTime) {
  if (OkCore::getInput()->getState().exit) {
    OkCore::askForExit();
    return;
  }

  // Spin the parent; the attached child follows.
  item->rotate(0.0f, glm::radians(0.1f * deltaTime), 0.0f);
}

Run the app and you will see the parent quad turning in place with the child quad swinging around it.