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



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 mean 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 just allowed by the kernel
 if you have superuser privileges. You can get these either by logging in 
 as root or by setting the suid bit on the mplayer executable.

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

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

 DGA2 incorporated 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 those 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 -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.

 ... to be continued during the next days ...

# An advantage of this method is that you are in full control of the 
# framebuffer and can put your hand on the code that is used for copying
# to the framebuffer. However there are two disadvantages:
 
# First, you control the screen - no more windowmanager functionally - just
# the one application that uses DGA is visible.

# Second you 
 
 
 
# Do it like this (you got to be root for this):

 # chown root /usr/local/bin/mplayer
 # chmod 750 /usr/local/bin/mplayer 
 # chmod +s /usr/local/bin/mplayer 
 
# Now it works for simple users, too.
# !!!! BUT STAY TUNED !!!!
# This is a BIG security risk! Never do this on a server or on a computer
# can be accessed by more people than only you because they can gain root
# privilegies through suid root mplayer.
# !!!! SO YOU HAVE BEEN WARNED ... !!!!
#

A. Sample modelines
~~~~~~~~~~~~~~~~~~~
Section "Modes"
  Identifier    "Modes[0]"
  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      "352x240"  15.750 352 368 416 432   240 244 246 262 Doublescan
  # 800x600 @ 60 Hz, 37.8 kHz hsync
  Modeline	"800x600"  40     800 840 968 1056  600 601 605 628 
  Modeline	"352x288"  25.10  352 368 416 432   288 296 290 310
EndSection


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

 Acki (acki@acki-netz.de, www.acki-netz.de)                  17.5.2001