DEM (file format)

Note that it is not possible to edit what happens in a Source demo, it basically stores the packets right off the wire, and we don't know the packet format.

For the purpose of this, I will be referring to each collection of events as a frame. This may or may not add up to the number of frames given by the header.

Demo Header

Type	Field	Value
String	Header	7+NULL characters, should be "HL2DEMO"
Int	Demo Protocol	Demo protocol version, should be '3'
Int	Network Protocol	Network protocol version number
String	Server name	260 characters long
String	Client name	260 characters long
String	Map name	260 characters long
String	Game directory	260 characters long
Float	Playback time	The length of the demo, in seconds
Int	Ticks	The number of ticks in the demo
Int	Frames	The number of frames in the demo
Int	Sign on length	?

Frame

Each frame begins with 0 or more of these commands: These are described in hl2sdk\utils\demoinfo\demoformat.h

Type	Value
dem_signon	1
dem_packet	2
dem_synctick	3
dem_consolecmd	4
dem_usercmd	5
dem_datatables	6
dem_stop	7
dem_lastcommand	dem_stop

Depending on the command that was received, a different action needs to be taken.

dem_stop, dem_lastcommand

This is a signal that the demo is over, and there is no more data to be parsed.

dem_consolecmd, dem_datatables, dem_usercmd

Read a standard data packet, deal with it as necessary.

dem_synctick, dem_signon, dem_packet

Ignore.

Standard Data "Packet"

Any time there is more than one byte to be read in, a standard format is used.

This begins with an integer that has the number of bytes in this packet.

Example code to handle this:

int ReadData(char **buffer)
{
	int len;
	fread (&len,sizeof(int),1,fDemo);
	if (len > 0)
	{
		*buffer = new char[len];
		fread(*buffer,len,1,fDemo);
	}
	return len;
}