Re: [Qemu-devel] [PATCH] opengl rendering in the sdl window

[Jamie Lokier <jamie@shareable.org>]

Anthony Liguori wrote:
> I'd like to see if it really makes a difference performance wise to 
> render with OpenGL.  I've had good results in the past using software 
> bilinear interpolation to do scaling.  If OpenGL doesn't provide a 
> measurable performance advantage, doing it in software may be the right 
> thing to do.

Software interpolation, especially bilinear or other filter, is
relatively slow if you're expanding a lot of pixels e.g. 640x384 to
1920x1200.  OpenGL will surely beat it.

On the other hand, OpenGL can to be relatively slow (in my experience)
if there is no colour conversion and scaling to do and X11-SHM would
be a straight 2d blit on the server side.

In principle OpenGL should as fast as the 2d blit or faster on
implementations where the image in client app's memory is a
memory-mapped texture, but that seems to be still a work in progress
on BSD and Linux.  DRI2 was dropped this summer because the TTM
texture memory manager was ousted in favour of the GEM texture memory
manager... and they haven't caught up with each other yet.  You need
some kind of good shared texture memory to get great speed out of
OpenGL displaying a texture which is being constantly updated by the
client app.

Imho, if there is an OpenGL backend (overlaid on SDL, Xlib, or
whatever), it should be a switch, for the next few years probably
defaulting to X11-SHM (or plain X11 when remote), and changing the
default to OpenGL in a few years when that's working really well
everywhere.

-- Jamie