Files and Directories

In this chapter of the manual we will cover the usage of files and directories by OpenMW CS. Files and directories are file system concepts of your operating system, so we will not be going into specifics about that, we will only focus on what is relevant to OpenMW CS.



OpenMW and OpenMW CS us multiple directories on the file system. First of all there is a user directory that holds configuration files and a number of different sub-directories. The location of the user directory is hard-coded into the CS and depends on your operating system.

Operating System User Dircetory
GNU/Linux <whatever>
OS X ~/Library/Application Support/openmw/
Windows <whatever>

In addition to to this single hard-coded directory both OpenMW and OpenMW CS need a place to seek for a actuals data files of the game: textures, 3D models, sounds and record files that store objects in game; dialogues an so one. These files are called content files. We support multiple such paths (we call them data paths) as specified in the configuration. Usually one data path points to the directory where the original Morrowind game is either installed or unpacked to. You are free to specify as many data paths as you would like, however, there is one special data path that, as described later, which is used to store newly created content files.

Content files

The original Morrowind engine by Bethesda Softworks uses two types of content files: esm (master) and esp (plugin). The distinction between those two is not clear, and often confusing. One would expect the esm (master) file to be used to specify one master, which is then modified by the esp plugins. And indeed: this is the basic idea. However, the official expansions were also made as ESM files, even though they could essentially be described as really large plugins, and therefore would rather use esp files. There were technical reasons behind this decision – somewhat valid in the case of the original engine, but clearly it is better to create a system that can be used in a more sensible way. OpenMW achieves this with our own content file types.

We support both ESM and ESP files, but in order to make use of new features in OpenMW one should consider using new file types designed with our engine in mind: game files and addon files, collectively called content files.

OpenMW content files

The concepts of Game and Addon files are somewhat similar to the old concept of ESM and ESP, but more strictly enforced. It is quite straight-formward: If you want to make new game using OpenMW as the engine (a so called total conversion) you should create a game file. If you want to create an addon for an existing game file create an addon file. Nothing else matters; the only distinction you should consider is if your project is about changing another game or creating a new one. Simple as that.

Another simple thing about content files are the extensions: we are using .omwaddon for addon files and .omwgame for game files.

Morrowind content files

Using our content files is recommended for projects that are intended to used with the OpenMW engine. However, some players might wish to still use the original Morrowind engine. In addition thousands of ESP/ESM files were created since 2002, some of them with really outstanding content. Because of this OpenMW CS simply has no other choice but to support ESP/ESM files. If you decid to choose ESP/ESM file instead of using our own content file types you are most likely aimng at compatibility with the original engine. This subject is covered in it own chapter of this manual.

The actual creation of new files is described in the next chapter. Here we are going to focus only on the details you need to know in order to create your first OpenMW CS file while fully understanding your needs. For now let’s jut remember that content files are created inside the user directory in the the data subdirectory (that is the one special data directory mentioned earlier).


Since an addon is supposed to change the game it follows that it also depends on the said game to work. We can conceptualise this with an examples: your modification is the changing prize of an iron sword, but what if there is no iron sword in game? That’s right: we get nonsense. What you want to do is tie your addon to the files you are changing. Those can be either game files (for example when making an expansion island for a game) or other addon files (making a house on said island). Obviously It is a good idea to be dependent only on files that are really changed in your addon, but sadly there is no other way to achieve this than knowing what you want to do. Again, please remember that this section of the manual does not cover creating the content files – it is only a theoretical introduction to the subject. For now just keep in mind that dependencies exist, and is up to you to decide whether your content file should depend on other content files.

Game files are not intend to have any dependencies for a very simple reasons: the player is using only one game file (excluding original and the dirty ESP/ESM system) at a time and therefore no game file can depend on other game file, and since a game file makes the base for addon files it can not depend on addon files.

Project files

Project files act as containers for data not used by the OpenMW game engine itself, but still useful for OpenMW CS. The shining example of this data category are without doubt record filters (described in a later chapter of the manual). As a mod author you probably do not need or want to distribute project files at all, they are meant to be used only by you and your team.

As you would imagine, project files make sense only in combination with actual content files. In fact, each time you start to work on new content file and a project file was not found, one will be created. The extensio of project files is .project. The whole name of the project file is the whole name of the content file with appended extension. For instance a swords.omwaddon file is associated with a swords.omwaddon.project file.

Project files are stored inside the user directory, in the projects subdirectory. This is the path location for both freshly created project files, and a place where OpenMW CS looks for already existing files.

Resource files

Unless we are talking about a fully text based game, like Zork or Rogue, one would expect that a video game is using some media files: 3D models with textures, images acting as icons, sounds and anything else. Since content files, no matter whether they are ESP, ESM or new OpenMW file type, do not contain any of those, it is clear that they have to be delivered with a different file. It is also clear that this, let’s call it “resources file“, has to be supported by the engine. Without code handling those files it is nothing more than a mathematical abstraction – something, that lacks meaning for human beings. Therefore this section must cover ways to add resources files to your content file, and point out what is supported. We are going to do just that. Later, you will learn how to make use of those files in your content.


OpenMW uses FFmpeg for audio playback, and so we support every audio type supported by that library. This makes a huge list. Below is only small portion of the supported file types.

mp3 (MPEG-1 Part 3 Layer 3)
A popular audio file format and de facto standard for storing audio. Used by the Morrowind game.
An open source, multimedia container file using a high quality Vorbis audio codec. Recommended.


Video As in the case of audio files, we are using FFmepg to decode video files. The list of supported files is long, we will cover only the most significant.

Videos used by the original Morrowind game.
A multimedia container which use more advanced codecs (MPEG-4 Parts 2, 3 and 10) with a better audio and video compression rate, but also requiring more CPU intensive decoding – this makes it probably less suited for storing sounds in computer games, but good for videos.
A new, shiny and open source video format with excellent compression. It needs quite a lot of processing power to be decoded, but since game logic is not running during cutscenes we can recommend it for use with OpenMW.
Alternative, open source container using Theora codec for video and Vorbis for audio.

Textures and images

The original Morrowind game uses DDS and TGA files for all kinds of two dimensional images and textures alike. In addition, the engine supported BMP files for some reason (BMP is a terrible format for a video game). We also support an extended set of image files – including JPEG and PNG. JPEG and PNG files can be useful in some cases, for instance a JPEG file is a valid option for skybox texture and PNG can useful for masks. However, please keep in mind that JPEG images can grow to large sizes quickly and are not the best option with a DirectX rendering backend. You probably still want to use DDS files for textures.