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<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"><title>8.3. DGA</title><link rel="stylesheet" href="default.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.75.2"><link rel="home" href="index.html" title="MPlayer - The Movie Player"><link rel="up" href="video.html" title="Chapter 8. Video output devices"><link rel="prev" href="xv.html" title="8.2. Xv"><link rel="next" href="sdl.html" title="8.4. SDL"><link rel="preface" href="howtoread.html" title="How to read this documentation"><link rel="chapter" href="intro.html" title="Chapter 1. Introduction"><link rel="chapter" href="install.html" title="Chapter 2. Installation"><link rel="chapter" href="usage.html" title="Chapter 3. Usage"><link rel="chapter" href="advaudio.html" title="Chapter 4. Advanced audio usage"><link rel="chapter" href="cd-dvd.html" title="Chapter 5. CD/DVD usage"><link rel="chapter" href="tv.html" title="Chapter 6. TV"><link rel="chapter" href="radio.html" title="Chapter 7. Radio"><link rel="chapter" href="video.html" title="Chapter 8. Video output devices"><link rel="chapter" href="ports.html" title="Chapter 9. Ports"><link rel="chapter" href="mencoder.html" title="Chapter 10. Basic usage of MEncoder"><link rel="chapter" href="encoding-guide.html" title="Chapter 11. Encoding with MEncoder"><link rel="chapter" href="faq.html" title="Chapter 12. Frequently Asked Questions"><link rel="appendix" href="bugreports.html" title="Appendix A. How to report bugs"><link rel="appendix" href="skin.html" title="Appendix B. MPlayer skin format"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">8.3. DGA</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="xv.html">Prev</a> </td><th width="60%" align="center">Chapter 8. Video output devices</th><td width="20%" align="right"> <a accesskey="n" href="sdl.html">Next</a></td></tr></table><hr></div><div class="sect1" title="8.3. DGA"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="dga"></a>8.3. DGA</h2></div></div></div>PREAMBLE.

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).

WHAT IS DGA.

<acronym class="acronym">DGA</acronym> 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 <code class="systemitem">root</code> or by setting the SUID bit on the

MPlayer executable (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 X server, 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 X server's video

driver that controls this chip. So it does not work on every system...

INSTALLING DGA SUPPORT FOR MPLAYER.

First make sure X loads the DGA extension, see in

<tt class="filename">/var/log/XFree86.0.log</tt>:

<pre class="programlisting">(II) Loading extension XFree86-DGA</pre>

See, XFree86 4.0.x or greater is

highly recommended!

MPlayer's DGA driver is autodetected by

<tt class="filename">./configure</tt>, or you can force it

with <tt class="option">--enable-dga</tt>.

If the driver couldn't switch to a smaller resolution, experiment with

options <tt class="option">-vm</tt> (only with X 3.3.x), <tt class="option">-fs</tt>,

<tt class="option">-bpp</tt>, <tt class="option">-zoom</tt> to find a video mode that

the movie fits in. There is no converter right now :(

Become <code class="systemitem">root</code>. DGA needs root

access to be able to write directly video memory. If you want to run it as

user, then install MPlayer SUID root:

chown root <code>/usr/local/bin/mplayer</code>

chmod 750 <code>/usr/local/bin/mplayer</code>

chmod +s <code>/usr/local/bin/mplayer</code>

</pre>

Now it works as a simple user, too.

<div class="caution" title="Security risk" style="margin-left: 0.5in; margin-right: 0.5in;"><h3 class="title">Security risk</h3>

This is a big security risk!

Never do this on a server or on a computer

that can be accessed by other people because they can gain root privileges

through SUID root MPlayer.

</div>

Now use <tt class="option">-vo dga</tt> option, and there you go! (hope so:) You

should also try if the <tt class="option">-vo sdl:driver=dga</tt> option works for you!

It's much faster!

<a name="dga-modelines"></a>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 honoring aspect ratio) of the video data, but the X

server only allows switching to resolutions predefined in

<tt class="filename">/etc/X11/XF86Config</tt>

(<tt class="filename">/etc/X11/XF86Config-4</tt> for XFree 4.X.X respectively).

Those are defined by so-called modelines and depend on

the capabilities of your video hardware. The X server 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: <tt class="filename">/var/log/XFree86.0.log</tt>.

These entries are known to work fine with a Riva128 chip, using the nv.o X

server driver module.

Section "Modes"

Identifier "Modes[0]"

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

</pre>DGA & MPLAYER.

DGA is used in two places with MPlayer: The SDL

driver can be made to make use of it (<tt class="option">-vo sdl:driver=dga</tt>) and

within the DGA driver (<tt class="option">-vo dga</tt>). The above said is true

for both; in the following sections I'll explain how the DGA driver for

MPlayer works.

100

FEATURES.

101

The DGA driver is invoked by specifying <tt class="option">-vo dga</tt> at the

102

command line. The default behavior is to switch to a resolution matching

103

the original resolution of the video as close as possible. It deliberately

104

ignores the <tt class="option">-vm</tt> and <tt class="option">-fs</tt> options

105

(enabling of video mode switching and fullscreen) - it always tries to

106

cover as much area of your screen as possible by switching the video mode,

107

thus refraining from using additional cycles of your CPU to scale the

108

image. If you don't like the mode it chooses you may force it to choose

109

the mode matching closest the resolution you specify by <tt class="option">-x</tt>

110

and <tt class="option">-y</tt>. By providing the <tt class="option">-v</tt> option, the

111

DGA driver will print, among a lot of other things, a list of all

112

resolutions supported by your current <tt class="filename">XF86Config</tt> file.

113

Having DGA2 you may also force it to use a certain depth by using the

114

<tt class="option">-bpp</tt> option. Valid depths are 15, 16, 24 and 32. It

115

depends on your hardware whether these depths are natively supported or if

116

a (possibly slow) conversion has to be done.

117

118

If you should be lucky enough to have enough offscreen memory left to

119

put a whole image there, the DGA driver will use double buffering, which

120

results in much smoother movie playback. It will tell you whether

121

double buffering is enabled or not.

122

123

Double buffering means that the next frame of your video is being drawn in

124

some offscreen memory while the current frame is being displayed. When the

125

next frame is ready, the graphics chip is just told the location in memory

126

of the new frame and simply fetches the data to be displayed from there.

127

In the meantime the other buffer in memory will be filled again with new

128

video data.

129

130

Double buffering may be switched on by using the option

131

<tt class="option">-double</tt> and may be disabled with

132

<tt class="option">-nodouble</tt>. Current default option is to disable

133

double buffering. When using the DGA driver, onscreen display (OSD) only

134

works with double buffering enabled. However, enabling double buffering may

135

result in a big speed penalty (on my K6-II+ 525 it used an additional 20%

136

of CPU time!) depending on the implementation of DGA for your hardware.

137

SPEED ISSUES.

138

Generally spoken, DGA framebuffer access should be at least as fast as

139

using the X11 driver with the additional benefit of getting a fullscreen

140

image. The percentage speed values printed by

141

MPlayer have to be interpreted with some care,

142

as for example, with the X11 driver they do not include the time used by

143

the X server needed for the actual drawing. Hook a terminal to a serial

144

line of your box and start top to see what is really

145

going on in your box.

146

147

Generally spoken, the speedup done by using DGA against 'normal' use of X11

148

highly depends on your graphics card and how well the X server module for it

149

is optimized.

150

151

If you have a slow system, better use 15 or 16 bit depth since they require

152

only half the memory bandwidth of a 32 bit display.

153

154

Using a depth of 24 bit is a good idea even if your card natively just supports

155

32 bit depth since it transfers 25% less data compared to the 32/32 mode.

156

157

I've seen some AVI files be played back on a Pentium MMX 266. AMD K6-2

158

CPUs might work at 400 MHZ and above.

159

KNOWN BUGS.

160

Well, according to some developers of XFree, DGA is quite a beast. They

161

tell you better not to use it. Its implementation is not always flawless

162

with every chipset driver for XFree out there.

163

164

With XFree 4.0.3 and <tt class="filename">nv.o</tt> there is a bug resulting

165

in strange colors.

166

</li><li class="listitem">

167

ATI driver requires to switch mode back more than once after finishing

168

using of DGA.

169

</li><li class="listitem">

170

Some drivers simply fail to switch back to normal resolution (use

171

Ctrl+Alt+Keypad +

172

and

173

Ctrl+Alt+Keypad -

174

to switch back manually).

175

</li><li class="listitem">

176

Some drivers simply display strange colors.

177

</li><li class="listitem">

178

Some drivers lie about the amount of memory they map into the process's

179

address space, thus vo_dga won't use double buffering (SIS?).

180

</li><li class="listitem">

181

Some drivers seem to fail to report even a single valid mode. In this

182

case the DGA driver will crash telling you about a nonsense mode of

183

100000x100000 or something like that.

184

</li><li class="listitem">

185

OSD only works with double buffering enabled (else it flickers).

186

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