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List:       xine-devel
Subject:    Re: [xine-devel] OpenGL output
From:       Billy Biggs <vektor () dumbterm ! net>
Date:       2006-02-06 23:53:50
Message-ID: 20060206235350.GC11527 () dumbterm ! net
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Marcel Janssen (korgull@home.nl):

> On Monday 06 February 2006 23:29, Billy Biggs wrote:
> > Furthermore, XVIDEO will always be higher quality than OpenGL.
> 
> Just curious, but why is Xvideo better in quality ?
> I wonder if that's true for most modern cards. I can imagine that development 
> of the opengl part has much more attention nowadays than the Xvideo overlay.

  I shouldn't have said it so strongly, as of course this depends on
both the hardware and the driver, and of course I've only used a small
subset of both.

  However, most video cards I've seen have a separate piece of hardware
called a video overlay.  This is applied after the framebuffer and is
designed specifically to deal with video.  Overlays have some nice
properties:

  1. When you're at 16-bit colour, the overlay is still 24-bit, since it
gets applied on the way out to the monitor rather than writing bytes
into the framebuffer.

  2. Overlays usually page flip on retrace.

  3. For some NVIDIA cards, the overlay has a higher-order filter for
interpolation.

  4. From my own personal experience, I can say with confidence that the
overlay gives better colour and scaling on at least the Matrox G400,
Inetl i815 and i830, and NVIDIA Geforce 2 MX.

  I am currently using an NVIDIA Quadro FX 1400.  It doesn't seem to
have a hardware overlay, but it does have an "NV17 Video Texture", which
clearly uses the 3D engine but has nice options like "XV_ITURBT_709" to
flip between the 601 and 709 chromaticities for HDTV vs traditional
video content, as well as an "XV_SYNC_TO_VBLANK" option which
conveniently defaults to 1.  Clearly, the NVIDIA driver can optimize
this path better for video quality than an OpenGL path, which is more
general.

  I can't see how development effort has anything to do with it, as the
quality will be mostly related to the hardware capabilities, and how to
best take advantage of them.   Note that the "NV17 Video Texture", since
it is internally based on the 3D engine and not a true overlay, does not
show the problems of the bleeding colour key, only-on-one-head, or
moving out of sync with the window.  I am confident that other drivers
and cards with similar capabilities can do the same thing through their
XVIDEO implementations.

  -Billy



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