Use this function to set the brightness (gamma multiplier) for the display that owns a given window.
int SDL_SetWindowBrightness(SDL_Window* window, float brightness)
the window used to select the display whose brightness will be changed
the brightness (gamma multiplier) value to set where 0.0 is completely dark and 1.0 is normal brightness
Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.
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Despite the name and signature, this method sets 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().) The brightness set will not follow the window if it is moved to another display.