mpv/DOCS/DGA

Contents
~~~~~~~~

 1. Summary
 2. What is DGA
 3. Resolution switching
 4. DGA & MPlayer
 5. Features of the DGA driver
 6. Speed issues
 7. Known bugs
 8. Future work
 
 A. Some modelines
 B. Bug Reports


1. Summary
~~~~~~~~~~
 This document tries to explain in some words what DGA is in general and
 what the DGA video output driver for mplayer can do (and what it can't).

2. What is DGA?
~~~~~~~~~~~~~~~

 DGA is short for Direct Graphics Access and is a means for a program to
 bypass the X-Server and directly modifying the framebuffer memory. 
 Technically spoken this happens by mapping the framebuffer memory into
 the memory range of your process. This is allowed by the kernel only
 if you have superuser privileges. You can get these either by logging in 
 as root or by setting the suid bit on the mplayer excecutable (NOT
 recommended!).

 There are two versions of DGA: DGA1 is used by XFree 3.x.x and DGA2 was 
 introduced with XFree 4.0.1.

 DGA1 provides only direct framebuffer access as described above. For 
 switching the resolution of the video signal you have to rely on the 
 XVidMode extension.

 DGA2 incorporates the features of XVidMode extension and also allows
 switching the depth of the display. So you may, although basically 
 running a 32 bit depth XServer, switch to a depth of 15 bits and vice 
 versa. 

 However DGA has some drawbacks. It seems it is somewhat dependent on the
 graphics chip you use and on the implementation of the XServer's video 
 driver that controls this chip. So it does not work on every system ...


3. Resolution switching
~~~~~~~~~~~~~~~~~~~~~~~
 The DGA driver allows for switching the resolution of the output signal.
 This avoids the need for doing (slow) software scaling and at the same
 time provides a fullscreen image. Ideally it would switch to the exact
 resolution (except for honouring aspect ratio) of the video data, but the
 XServer only allows switching to resolutions predefined in 
 /etc/X11/XF86Config (/etc/X11/XF86Config-4 for XFree 4.0.X respectively).
 Those are defined by so-called modelines and depend on the capabilites
 of your video hardware. The XServer scans this config file on startup and
 disables the modelines not suitable for your hardware. You can find 
 out which modes survive with the X11 log file. It can be found at:
 /var/log/XFree86.0.log
 See appendix A for some sample modeline definitions.

4. DGA & MPlayer
~~~~~~~~~~~~~~~~

 DGA is used in two places with MPlayer: The SDL driver can be made to make
 use of it (-vo sdl:dga) and within the DGA driver (-vo dga).
 The above said is true for both; in the following sections I'll explain
 how the DGA driver for MPlayer works.


5. Features of the DGA driver
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The DGA driver is invoked by specifying -vo dga at the command line.
 The default behaviour is to switch to a resolution matching the original 
 resolution of the video as close as possible. It deliberately ignores the 
 -vm and -fs switches (enabling of video mode switching and fullscreen) - 
 it always tries to cover as much area of your screen as possible by switching
 the video mode, thus refraining to use a single additional cycle of your CPU 
 to scale the image.
 If you don't like the mode it chooses you may force it to choose the mode
 matching closest the resolution you specify by -x and -y. 
 By providing the -v option, the DGA driver will print, among a lot of other 
 things, a list of all resolutions supported by your current XF86-Config 
 file.
 Having DGA2 you may also force it to use a certain depth by using the -bpp 
 option. Valid depths are 15, 16, 24 and 32. It depends on your hardware 
 whether these depths are natively supported or if a (possibly slow) 
 conversion has to be done.
 
 If you should be lucky enough to have enough offscreen memory left to 
 put a whole image there, the DGA driver will use doublebuffering, which 
 results in much smoother movie replaying. It will tell you whether double-
 buffering is enabled or not.

 Doublebuffering means that the next frame of your video is being drawn in
 some offscreen memory while the current frame is being displayed. When the
 next frame is ready, the graphics chip is just told the location in memory 
 of the new frame and simply fetches the data to be displayed from there.
 In the meantime the other buffer in memory will be filled again with new 
 video data.

 Doublebuffering may be switched on by using the option -double and may be 
 disabled with -nodouble. Current default option is to disable 
 doublebuffering. When using the DGA driver, onscreen display (OSD) only 
 works with doublebuffering enabled. However, enabling doublebuffering may
 result in a big speed penalty (on my K6-II+ 525 it used an additional 20% of
 CPU time!) depending on the implementation of DGA for your hardware. 
 


6. Speed issues
~~~~~~~~~~~~~~~
 Generally spoken, DGA framebuffer access should be at least as fast as using
 the X11 driver with the additional benefit of getting a fullscreen image.
 The percentage speed values printed by mplayer have to be interpreted with 
 some care, as for example, with the X11 driver they do not include the time
 used by the X-Server needed for the actual drawing. Hook a terminal to a 
 serial line of your box and start top to see what is really going on in your 
 box ...

 Generally spoken, the speedup done by using DGA against 'normal' use of X11 
 highly depends on your graphics card and how well the X-Server module for it 
 is optimized.

 If you have a slow system, better use 15 or 16bit depth since they require 
 only half the memory bandwidth of a 32 bit display. 
 
 Using a depth of 24bit is even a good idea if your card natively just supports 
 32 bit depth since it transfers 25% less data compared to the 32/32 mode. 
 
 I've seen some avi files already be replayed on a Pentium MMX 266. AMD K6-2
 CPUs might work at 400 MHZ and above.

7. Known Bugs
~~~~~~~~~~~~~

 Well, according to some developpers of XFree, DGA is quite a beast. They 
 tell you better not to use it. Its implementation is not always flawless
 with every chipset driver for XFree out there.

 o with XFree 4.0.3 and nv.o there is a bug resulting in strange colors
 o ATI driver requires to switch mode back more than once after finishing 
   using of DGA
 o some drivers simply fail to switch back to normal resolution (use 
   Ctrl-Alt-Keypad +, - to switch back manually)
 o some drivers simply display strange colors
 o some drivers lie about the amount of memory they map into the process's
   address space, thus vo_dga won't use doublebuffering (SIS?)
 o some drivers seem to fail to report even a single valid mode. In this
   case the DGA driver will crash telling you about a nonsense mode of 
   100000x100000 or the like ...
 o OSD only works with doublebuffering enabled

8. Future work
~~~~~~~~~~~~~~

 o use of the new X11 render interface for OSD
 o where is my TODO list ???? :-(((


A. Sample modelines
~~~~~~~~~~~~~~~~~~~
Section "Modes"
  Identifier    "Modes[0]"
                # 800x600 @ 60 Hz, 37.8 kHz hsync
  Modeline	"800x600"  40     800 840 968 1056  600 601 605 628 
  Modeline      "712x600"  35.0   712 740 850 900   400 410 412 425
  Modeline      "640x480"  25.175 640 664 760 800   480 491 493 525 
  Modeline 	"400x300"  20     400 416 480 528   300 301 303 314 Doublescan
  Modeline	"352x288"  25.10  352 368 416 432   288 296 290 310
  Modeline      "352x240"  15.750 352 368 416 432   240 244 246 262 Doublescan
  Modeline	"320x240"  12.588 320 336 384 400   240 245 246 262 Doublescan
EndSection


 These entries work fine with my Riva128 chip, using nv.o XServer driver
 module.

B. Bug Reports
~~~~~~~~~~~~~~

 If you experience troubles with the DGA driver please feel free to file 
 a bug report to me (e-mail address below). Please start mplayer with the 
 -v option and include all lines in the bug report that start with vo_dga:
 
 Please do also include the version of X11 you are using, the graphics card 
 and your CPU type. The X11 driver module (defined in XF86-Config) might 
 also help. Thanks!

 
 Acki (acki@acki-netz.de, www.acki-netz.de)                  18.06.2001