Get the brightness (gamma multiplier) for a given window's display.
float SDL_GetWindowBrightness(SDL_Window * window);
the window used to select the display whose brightness will be queried
Returns the brightness for the display where 0.0 is completely dark and 1.0 is normal brightness.
Despite the name and signature, this method retrieves the brightness of the entire display, not an individual window. A window is considered to be owned by the display that contains the window's center pixel. (The index of this display can be retrieved using SDL_GetWindowDisplayIndex().)