How do I wait for vertical retrace?

Carsten Haitzler (The Rasterman) raster at
Tue May 6 10:13:33 PDT 2008

On Tue, 06 May 2008 12:53:50 -0400 Chuck Robey <chuckr at> babbled:

> Hash: SHA1
> Carsten Haitzler (The Rasterman) wrote:
> > On Tue, 6 May 2008 18:43:52 +0200 Nicolas George
> > < at> babbled:
> > 
> >> L'octidi 18 floréal, an CCXVI, Johan Walles a écrit :
> >>> I don't care about vertical retrace in itself, waiting for vertical
> >>> retrace would only be a way for me to achieve flicker-free animation
> >>> in a 2D application.
> >> DOUBLE-BUFFER is no good for that: when used for continuous animation (such
> >> as video), it causes exactly as much tearing as a simple bitmap copy. At
> >> least it was true in November 2006.
> > 
> > it's been pretty much true as long as i have done anything with x (since
> > about 1995...). i have yet to see is not tear...
> > 
> Well, I guess the Xorg command to swap buffers, while it certainly does
> accomplish an immediate changing of any or all of the data in your screen
> buffers, it also does automatically do retrace sync.  You'd want to change
> your sits quite quickly as one requirements, but you'd want the data to be
> swapper NOT in the middle of writing a screen, no matter how fast it can
> happen.  You don't have to worry about syncing, OpenGL does that for you
> without your direct involvement.

opengl CAN vsync - IF you:

1. dont use a multihead xinerama/zaphod/twinview/whatever (1 root window across
all screens) display. (in my experience this instantly disables any vsync)
2. if you gl buffer is a fullscreen window you get a buffer swap if the driver
knows this and is smart - it just flips pointers for next retrace to pick up
3. if not u need to use the glx wait for vsync extension which means u will
likely sit in a polling loop waiting for vsync (not being able to do anything
else in your main loop while waiting).

it *IS* possible to do it with - if you take care. you could use the glx vsync
stuff in combination with 2d x and thus get close to vsync (as if u are fats
enough from vsync to actually sending your request to copy from pixmap to
window u'll do it before the trace hits the window).

so this is how to do it. as such we really could do with a standard 2d vsync
signal from x that is reliable and works the vast majority of the time and
doesn't involve anyone sitting and polling/spinning. :) i can dream... :)

------------- Codito, ergo sum - "I code, therefore I am" --------------
The Rasterman (Carsten Haitzler)    raster at

More information about the xorg mailing list