[cairo] Best strategy for complex screen drawing

[Jim Gettys]

Actually, on X, there is the double buffer extension.  Composite is in
fact for different purposes.
			- Jim


On Mon, 2005-11-21 at 12:43 -0800, Bill Spitzak wrote:
> 
> Timothée Lecomte wrote:
> 
> > 1) Would it be more efficient to use a gdk/cairo surface for the screen
> > (via gdk_cairo_create) and use cairo_set_source_surface to copy the
> > image surface to the screen ? (instead of the image buffer conversion) I
> > suppose the answer is no.
> > 
> > 2) Would it be more efficient to use a specific backend ? For example,
> > should I draw to an offscreen Xlib pixmap, and use it as a source
> > surface ? Will I benefit from possible render acceleration ?
> > 
> > 3) Are there other solutions that I might have forgotten ?
> 
> What you want is double buffering, or backing store, or offscreen 
> rendering, or any of a dozen other names. This should be provided by the 
> system or the toolkit as some kind of modification to an existing window 
> object.
> 
> You do not want to do your own "fake" double buffering by allocating 
> some other object and doing explicit copies. The reason is that the 
> "real" double buffering knows that the image and the window are 
> explicitly linked, and can be setup to copy very fast, and it can take 
> advantage of the fact that the backing store will not be used for any 
> purpose other than to update the visible window. It can resize and 
> delete it in sync with the real window, and free it if your window has 
> not been displayed for a long time.
> 
> It is obvious to me that all future systems will always be in permanent 
> double-buffered mode (again cryptically named, in X they call this the 
> "composite extension" and in the Windows world they talk a lot about 
> transparency, thought that is a trivial side effect). If this is true, 
> unless a system is *very* clever and able to figure out that the extra 
> buffer you are allocating is being used as a double buffer, then any 
> attempts you make to create the double buffer will be a waste of time on 
> them, and memory as *two* images will be created, and your code will 
> copy from one invisible one to another, and then the system will copy 
> from there to the screen window.
> 
> So the answer is: if you think you have to create two objects, that is 
> wrong. Figure out a way so that there is one object per screen area. 
> Examine your system and/or toolkit to find out how to turn this on. If 
> it does not seem possible, you must switch toolkits.
> 
> You may also find that the toolkit provides the interface but internally 
> it is using a pixmap just like you were going to. This is ok. It 
> indicates the toolkit authors are thinking about the future and plan to 
> rewrite this area, so you should plan for it by using their interface.
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