I've tried to clear surface and post to a window with below implementation using Khononos OpenVG Sample Reference Implementation.

Code :
VGfloat clearColor[4] = { 1.0f, 1.0f, 1.0f, 0.7f };
vgSetfv( VG_CLEAR_COLOR, 4, blackcolor );

I think it shows gray color (R = 0.7f, G = 0.7f, B = 0.7f). But the window was filled the white color. It looks like alpha value (0.7f) is ignored.

Please let me wheather alpha value is ignored posting to a window.