DirectFB Reference Manual
IDirectFBWindow   SetOpaqueRegion

Disable alpha channel blending for one region of the window.

SetOpaqueRegion (
  IDirectFBWindow *thiz
  int x1
  int y1
  int x2
  int y2
);

If DWOP_ALPHACHANNEL and DWOP_OPAQUE_REGION are set but not DWOP_COLORKEYING and the opacity of the window is 255 the window gets rendered without alpha blending within the specified region.

This is extremely useful for alpha blended window decorations while the main content stays opaque and gets rendered faster.

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