1. Callbacks

Definition: A callback is a function or routine that is passed as an argument to another function, to be invoked automatically in response to an event.

Purpose in HCI/Graphics:

• Enable event-driven programming
• Handle user input such as mouse clicks, keyboard presses, or window resizing

How it works:

1. The programmer defines a callback function (e.g., onMouseClick).
2. The function is registered with the system or graphics library (like OpenGL/GLUT).
3. When the specific event occurs, the system automatically calls the callback.

Example in OpenGL/GLUT (C):

void onMouseClick(int button, int state, int x, int y) {
    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) {
        printf("Mouse clicked at (%d, %d)\n", x, y);
    }
}

int main(int argc, char** argv) {
    glutInit(&argc, argv);
    glutCreateWindow("Callback Example");
    glutMouseFunc(onMouseClick); // Register callback
    glutMainLoop();
}

Advantages:

• Makes programs responsive and interactive
• Separates event handling logic from main program flow

2. Picking

Definition: Picking is the process of selecting graphical objects (points, lines, polygons) using an input device like a mouse, stylus, or touch.

Purpose:

• Allows users to interact directly with objects on the screen
• Essential for object manipulation, selection, and editing in 2D/3D graphics

How Picking Works:

1. User clicks or points at the screen.
2. System converts the 2D screen coordinates to the 3D world coordinates.
3. Objects under the pointer are detected (via bounding boxes or ray intersection).
4. The selected object is highlighted, moved, or otherwise manipulated.

Picking Methods:

• Color-based Picking:

  • Render each object in a unique color to an off-screen buffer.
  • Detect which color is under the cursor → identify the object.

• Geometry-based Picking:

  • Perform mathematical tests (ray-object intersection) in 3D space.

• OpenGL Selection Mode (Deprecated but still relevant conceptually):

  • OpenGL can switch to selection mode, render objects, and detect hits using name stacks.

Example Scenario:

• In a 3D modeling application, clicking on a cube selects it for rotation or scaling.

3. Relationship Between Callbacks and Picking

• Picking often relies on callbacks.

• For example:

  1. Register a mouse callback (onMouseClick).
  2. When the mouse is clicked, the callback triggers a picking routine.
  3. The system determines which object was selected and performs the desired action.

Workflow Diagram (Conceptual):

User clicks → Mouse callback invoked → Picking routine checks objects → Object selected → Action performed

4. Key Takeaways

• Callbacks allow programs to respond to user input events asynchronously.
• Picking enables direct interaction with graphical objects.
• Together, they form the basis of interactive 2D/3D graphics applications, CAD systems, and HCI interfaces.