Use this function to get the opacity of a window.
int SDL_GetWindowOpacity(SDL_Window* window, float* opacity)
the window to get the current opacity value from
the float filled in (0.0f - transparent, 1.0f - opaque)
Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.
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If transparency isn't supported on this platform, opacity will be reported as 1.0f without error.
The parameter opacity is ignored if it is NULL.
This function also returns -1 if an invalid window was provided.
This function is only supported on DirectFB, X11, Cocoa (Apple Mac OS X) and Microsoft Windows.
This function is available since SDL 2.0.5.