Identify an object currently selected in the game

Question

I am making a 2D isometric game that uses the mouse. When I left click on an object, this object should be selected, but how do I determine what object that's currently selected and show a halo around it?

My objects are stored in a list of game objects as std::vector<Game_Object *> Object_Game. I am using the SDL library with C++. I did some research on Google but only found articles about Unity. Can someone help me with this?

Answer 1

Your use-case is two-fold:

1. Projecting your 2d mouse coordinates to 3d world coordinates, to create a ray.
2. Performing collision detection on the new coordinates, compared to your game objects.

NDC to 3D world coordinates

To do this, you need your view and projection transform matrices:

mat4 view = lookat(camPos, camPos+camDir, vec3(0.0f,1.0f,0.0f);
// For 3D
mat4 projection = perspective(fovAngle, aspectRatio, znear, zfar);
//for 2D
mat4 projection = ortho(0, width, height, 0);

combine these matrices using affine transformations:

mat4 worldToScreen = projection * view;

then invert them:

mat4 screenToWorld = inverse(worldToScreen);

Now, we take our mouse coordinates, and transform into world coordinates:

vec3 worldMouse = screenToWorld * vec4(mouseX,mouseY,-1.0f,1.0f);

Then finally we create our ray:

vec3 ray = normalize(worldMouse - camPos);

Collision Detection

Now, we can iterate through a vector of game objects, to determine which one has been clicked on:

gameObject *selectedObject; = nullptr;
for (auto &gameobject: gameObjects)
{
    if (gameobject.collide(ray))
    {
        selectedObject = gameobject;
        break;
    }
}

Then you can have some rendering code that draws a halo on the ground at the targeted object position:

void render()
{
    if (selectedObject != nullptr)
    {
        selectedHalo.drawAt(selectedObject->getPosition());
    }
    // then draw your game objects as normal.
}

This technique is known as "3D picking". There are plenty of more detailed tutorials available, but I covered the basic gist of it.