Use this function to get the brightness (gamma multiplier) for the display that owns a given window.
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.
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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().)