Difference Between SDL_Window and SDL_Surface in SDL2

Think of an SDL_Window and an SDL_Surface like a picture frame and the picture (or canvas) inside it.

SDL_Window

An SDL_Window represents the operating system window itself. This includes things like:

• The window's border or frame.
• The title bar at the top.
• Minimize, maximize, and close buttons.
• Its position and size on the desktop.

It's the container that holds our application's visual output. We create it using SDL_CreateWindow(). It handles interactions with the operating system's window manager, like moving, resizing, or iconifying the window. It doesn't directly contain pixel data for drawing.

1// Creates the OS window container
2SDL_Window* MyWindow = SDL_CreateWindow(
3  "My Application Window", // Title
4  SDL_WINDOWPOS_CENTERED, // Initial x position
5  SDL_WINDOWPOS_CENTERED, // Initial y position
6  640, // Width
7  480, // Height
8  SDL_WINDOW_SHOWN // Flags (e.g., make it visible)
9);

SDL_Surface

An SDL_Surface represents a collection of pixels in CPU memory. It's essentially a 2D array of color data that you can draw onto or manipulate directly. Key aspects include:

• A pointer to the actual pixel data.
• The dimensions (width and height) of the pixel grid.
• Information about how the colors are stored (the SDL_PixelFormat).

Surfaces are used for software rendering (drawing using the CPU). You can load images into surfaces, create blank surfaces to draw on, or get the special surface associated with a window for direct drawing.

1// Get the surface associated with the
2// window (for drawing)
3SDL_Surface* WindowSurface{
4  SDL_GetWindowSurface(MyWindow)};
5
6// Or, create a new, blank surface
7// (e.g., for off-screen drawing)
8SDL_Surface* MySurface{SDL_CreateRGBSurface(
9  0, // Flags (must be 0)
10  100, // Width
11  50, // Height
12  32, // Bits per pixel (e.g., 32 for RGBA8888)
13  0, 0, 0, 0 // Masks (can often be 0 for standard formats)
14)};
15
16// Remember to free surfaces you create manually
17if (MySurface) {
18  SDL_FreeSurface(MySurface);
19}

The Connection

The function SDL_GetWindowSurface() provides the link: it returns the SDL_Surface that acts as the drawing canvas for that specific SDL_Window. When you draw on this particular surface and call SDL_UpdateWindowSurface(), SDL copies the pixel data from that surface onto the actual window displayed by the OS.

So, SDL_Window is the frame, and SDL_Surface is the canvas inside (or any other canvas you might create in memory). You need the window to show things on the screen, and you need a surface to define what pixels to show.