Re: [AD] BUG(?): Page flipping and vsync

[Eric Botcazou <ebotcazou@xxxxxxxxxx>]

> From the docs::show_video_bitmap(..):
>
> "Allegro will handle any necessary vertical retrace synchronisation when
> page flipping, so you don't need to call vsync() before it. This means
> that show_video_bitmap() has the same time delay effects as vsync()."
>
> This is true for GFX_DIRECTX_FULLSCREEN, but not for GFX_DIRECTX_WINDOWED,
> though vsync is supported when windowed.

Well, this is also true for the GFX_DIRECTX_WINDOWED driver but not as 
clearly as for the GFX_DIRECTX_FULLSCREEN driver.

In fullscreen mode, page-flipping really means flipping two pages of video 
memory, i.e moving the point in video memory from which the video card 
starts to send data to the monitor. In order to make sure that the flip 
doesn't happen in the middle of the screen retrace, we have to wait for the 
hotspot to be at the bottom of the screen.

In windowed mode, we don't use two pages of video memory because we need to 
preserve the Windows desktop around the window. So we _emulate_ 
page-flipping by using a backbuffer which is block-copied onto the screen 
when the flip() method is invoked. To sum up, page-flipping is actually 
double-buffering in windowed mode and we don't need to bother with the 
vertical retrace when double-buffering because the video card does it for 
us.

-- 
Eric Botcazou