Filling Only Part of an SDL_Surface with SDL_FillRect()

The SDL_FillRect() function is designed to fill a rectangular area on a surface. While passing nullptr for the rectangle argument fills the entire surface, you can specify a particular area by providing a pointer to an SDL_Rect structure.

The SDL_Rect Structure

An SDL_Rect defines a rectangular area using four integer members:

• x: The x-coordinate of the top-left corner of the rectangle.
• y: The y-coordinate of the top-left corner of the rectangle.
• w: The width of the rectangle (in pixels).
• h: The height of the rectangle (in pixels).

Remember that the coordinate system for surfaces usually starts with (0,0) at the top-left corner, with X increasing to the right and Y increasing downwards.

Using SDL_Rect with SDL_FillRect()

To fill only a specific part of a surface, you need to:

1. Create an SDL_Rect variable.
2. Set its x, y, w, and h members to define the target area.
3. Pass the address (a pointer) of this SDL_Rect variable as the second argument to SDL_FillRect().

1// Assume 'WindowSurface' is a valid SDL_Surface*
2// (e.g., from SDL_GetWindowSurface)
3// Assume 'format' is the SDL_PixelFormat*
4// from WindowSurface->format
5// Assume 'MyWindow' is the SDL_Window*
6
7// 1. Define the rectangle area
8SDL_Rect FillArea; 
9FillArea.x = 50;   // 50 pixels from the left edge 
10FillArea.y = 100;  // 100 pixels from the top edge 
11FillArea.w = 200;  // 200 pixels wide 
12FillArea.h = 150;  // 150 pixels tall 
13
14// 2. Define the color (Bright Green)
15Uint32 FillColor = SDL_MapRGB(format, 0, 255, 0);
16
17// 3. Call SDL_FillRect with the address of the SDL_Rect
18SDL_FillRect(
19  WindowSurface, // The surface to draw on
20  &FillArea,     // Pointer to the rectangle to fill 
21  FillColor      // The color to use
22);
23
24// 4. Update the window to see the change
25SDL_UpdateWindowSurface(MyWindow);

This code would fill only a green rectangle within the specified coordinates (from x=50, y=100 to x=250, y=250) on the WindowSurface, leaving the rest of the surface unchanged from its previous state.

We'll explore SDL_Rect and coordinate systems in much more detail in later lessons, as they are fundamental to positioning graphics, handling collisions, and managing camera views.