Here are the complete DEM specs! I have written the DEMo-Editor Descent Manager DEMOEDIT/DOS using this knowledge, with which you should be able, for example, to make a DEMo->AVI renderer... If you need any vartype definitions or constants, used in these specs, you can find them here!

The specs are for Descent 2 Full Version, Descent 1 Full Version and Descent 1 Shareware. Differences in the game format between the games are marked as red C-style "#IFDEF" and "#IFNDEF" commands. So here are the definements:

D1SHARE = Descent 1 Shareware Demo format
D1FULL  = Descent 1 Commercial Demo format
D2FULL  = Descent 2 Commercial Demo format

I don't have specs for Descent 2 Interactive Demo, but according to the version numbering, it seems that D2 Interactive demos have the same structure as D2 Commercial demos (the mission_name is "D2DEMO" then)... But this has to be tested, of course!

In the demo format, Destination Saturn is handled as Descent 1 Commercial, Destination Quartzon as Descent 2 Commercial.

eMail author: Heiko Herrmann
Descent Manager DEMOEDIT/DOS - An editor to cut *.DEM files, written by Heiko Herrmann

The demo file contains nothing else than a number of events! Descent has 32-51 (dependant of which version you use) different events. In the demo file is only saved what YOU - the player - actually see from YOUR point of view! That means, if you are PLAYER_1 of a 3 player multiplayer game, and PLAYER_2 kills PLAYER_3 in a different part of the mine, and you cannot see their fight, then the ONLY thing, that is actually saved in the demo file, is a) an event ND_EVENT_MULTI_DEATH, with the increased number of deaths of PLAYER_3, b) an event ND_EVENT_MULTI_KILL, with the increased number of kills of PLAYER_2 and c) an event ND_EVENT_HUD_MESSAGE, that lets the HUD display the message "PLAYER_2 killed PLAYER_3" at the top of the screen! Nothing else is saved here... so you have no chance to reconstruct, which weapon did actually kill PLAYER_3, nor can you check out, how many shield PLAYER_2 had left at the end of fight!

That means: You only know YOUR shield, energy, afterburner, weapons and so on... Well, you might wonder, how does Descent know while the playback of demos, what a killed player drops? The answer is, the demo playback routine doesn't know it! It doesn't even know which powerups are where, even if there are still laying in the mine; not picked up by anyone...

So here is the idea of the demo playback routine: Each frame that was recorded is saved extra. Now each frame starts with the position of PLAYER_1 in the mine, the event ND_EVENT_VIEWER_OBJECT! Now Descent knows, which room has to be displayed. Now every single object, that is in this room at this time is listed seperately using the ND_EVENT_RENDER_OBJECT! So if a frame shows 2 enemies, 4 powerups laying down plus 8 laser shots, then (2+4+8=)14 events of type ND_EVENT_RENDER_OBJECT follow the ND_EVENT_VIEWER_OBJECT! Of course the more fights are happening in one frame, the greater is the amount of bytes used for this frame!

After the ND_EVENT_RENDER_OBJECTs there follow events, that are noting actions happened in this frame: killing, changings of shield/energy/ammo/weapon/afterburner, status changes (e.g. collecting/loosing a flag, invulnerability on/off, cloaking on/off, headlight on/off etc.), scoring, getting messages on the HUD (not only from multiplayer games, also messages like "You already have 20 concussion missiles", "Afterburner!", "Ship destroyed" and "Guide-Bot: Finding blue key") and so on... Even if the screen flashes due to being hit, loading the fushion, being flashed or getting a powerup is an own event.

The demo begin with the header, which itself is an own event, ND_EVENT_START_DEMO! It also sets the JustStartedPlayback to 1. After that, the ND_EVENT_NEW_LEVEL event is called, which loads also all walls; after that JustStartedPlayback is set back to 0, and the first ND_EVENT_START_FRAME is fired!

In Descent 1 there is no JustStartedPlayback, so it just goes through: ND_EVENT_START_DEMO, ND_EVENT_NEWLEVEL, ND_EVENT_START_FRAME in that order.

Descent saves the action happening in each frame as OBJECTs. Each of the 2D and 3D objects, that are visible in the current frame including the shots, the items, the robots, the other players and the position of the viewer's eyes (yourself!) are saved in one object. The structure of the object type is NOT a fixed size one, it varies and some flags actually are defining the exact object structure of one specific object. Here is the OBJECT structure, that Descent uses for the DEMo files (The constants can be viewn here!):

The data "Robot_info[object_id].model_num" and "Polygon_models[object_rtype_pobj_info_model_num].n_models; i++)" must be get from the HAM file, or rather -if available- the HXM file for that specific level! View the HAM specs and the HXM specs for reading the info...

You do need to know the data to know how many ANGVEC data the object follows...

• Descent 1 and doors
  When you look into event 28 (ND_EVENT_NEW_LEVEL) you will see that no door states are loaded/saved at the beginning of the DEM file. This is why e.g. when you opened the hostage door before starting the demo recording, it will be closed when looking at it in the demo. You will just fly through a closed door. In Descent 2 this was fixed by adding the additional information and the variable "JustStartedPlayback".

• Descent 2 and hoard
  Hoard was implemented in V1.2 Vertigo Enhanced, and so was a very late implementation. Unfortunately Parallax forgot to implement hoard handling code into the DEMo file. This is why no orbs are counted in the demo; there would be the need of a new event (lets say ND_EVENT_ORB), which would make the demo format incompatible to earlier version of D2, so we would need a version "16". This wouldn't be that unwise, because earlier versions didn't support hoard nor did they have the textures/sounds needed for the hoard mode.
  But things are even worse: The textures are not displayed when playing hoard demos and the sound is totally disturbed. This is a hard bug and would be really a reason for a V1.21 or V1.3! I talked to Parallax before but they said no, they won't release a fix :((...

• Descent 1 Shareware and multiplayer games
  The support for playing multiplayer demos in D1 Shareware was in a very early state, as you can see in the missing events 32-42 and the missing data in the event 0 (ND_EVENT_EOF): D1 Shareware saves nothing about the players, not even their nicknames or scores.

• Descent 1 Shareware and ammo restrictions
  Even worse: D1 Shareware does not save the ammo nor the number of remaining missiles. Looks quite funny: You shoot and your cockpit displays "0 homings"; before and after...

The demo files start with a version number:

• Descent 1 Shareware: version=5, game_type=1, events=0-31
• Descent 1 Full Version: version=13, game_type=2, events=0-42
• Descent 2 Interactive Demo: version=15, game_type=3, events=?
• Descent 2 Full Version: version=15, game_type=3, events=0-50

The main demo now is a couple of events! Each event is initialized by a single byte, which defines the type of event following! Each number from 0-50 defines one event! This continues until ND_EVENT_EOF is fired! Here are the events together with their parameters (for declarations of the variable types click here):