Opacity and Fullscreen Windows

When a window is in fullscreen mode, opacity handling depends on the platform and the graphics driver. Many platforms do not support transparency in true fullscreen mode because the window is rendered directly to the screen, bypassing the compositor responsible for blending transparent surfaces.

Behavior in Fullscreen Modes

1. True Fullscreen (SDL_WINDOW_FULLSCREEN): Transparency is typically ignored, and the window appears fully opaque. This is because the application takes over the entire screen, and there's no compositor to blend with the background.
2. Fullscreen Desktop (SDL_WINDOW_FULLSCREEN_DESKTOP): Transparency might work because the window is treated as a borderless window covering the screen, allowing the compositor to manage blending.

Practical Notes

• If you need transparency in fullscreen, prefer SDL_WINDOW_FULLSCREEN_DESKTOP.
• Keep in mind that performance might differ between true fullscreen and fullscreen desktop, especially for games or graphics-intensive applications.

Example: Testing Opacity in Fullscreen

Below, we attempt to set the transparency of a fullscreen window:

1#include <SDL.h>
2#include <iostream>
3
4int main() {
5  if (SDL_Init(SDL_INIT_VIDEO) < 0) {
6    std::cerr << "SDL Init Error: "
7      << SDL_GetError() << "\n";
8    return -1;
9  }
10
11  SDL_Window* window = SDL_CreateWindow(
12    "Fullscreen Opacity Test",
13    SDL_WINDOWPOS_CENTERED,
14    SDL_WINDOWPOS_CENTERED,
15    800, 600,
16    SDL_WINDOW_SHOWN
17  );
18
19  // Set transparency
20  SDL_SetWindowOpacity(window, 0.5f); 
21
22  SDL_SetWindowFullscreen(window, 
23    SDL_WINDOW_FULLSCREEN_DESKTOP); 
24  SDL_Delay(3000);
25
26  SDL_DestroyWindow(window);
27  SDL_Quit();
28  return 0;
29}

1In fullscreen desktop mode, transparency works if supported by the platform.

Test fullscreen opacity behavior on your target platform to avoid unexpected results.