MDL (GoldSrc)
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MDL (sometimes referred to as Half-Life MDL or HLMDL) is a model format used by 
GoldSrc to store dynamically lit models which can be animated using skeletal animation, along with their associated textures. It is a heavily modified version of the Quake MDL format.
Overview
Limits
HLMDL has the following limits per input SMD:
- 1 mesh
- 2048 vertices (and vertex normals) per mesh
- 4080 triangles per mesh
- 1 animation sequence
- 512 keyframes per sequence
- 128 bones
Additionally, the following limits apply to the whole MDL:
- 4 bone attachments
- 8 bone controllers
- 32 meshes across all submodels
- 256 animation sequences.
Textures must:
- Be 8-bit uncompressed BMP files.
- Have a resolution which is a multiple of 8 (multiples of 16 are preferred).
- Have a resolution which does not exceed 512x512 (soft limit; raised to 1024x1024 in
Sven Co-op and other engines like 
FTE). - Have a file name which is not longer than 64 characters (including the .bmp extension).
Important:Older versions of StudioMDL, including the version found in the Half-Life SDK on Steam, do not support tiling UVs, clamping them to the edge of the texture. This is not a limitation of the HLMDL file format; the Sven Co-op StudioMDL can create MDLs compatible with vanilla GoldSrc which have tiled UVs.
Bug:The latest version of the Sven Co-op StudioMDL (from December 2020), as found in the Sven Co-op SDK, has some bugs with bounding boxes; use the version linked above instead (from November 2018). [todo tested in ?]
Tip:To create a model or submodel with more than 4080 tris, split the model into multiple SMD meshes, with a separate $body or $bodygroup entry for each mesh in the QC file.
Warning:Avoid going over 9000 tris!While the HLMDL format can handle over 100K tris, the old OpenGL renderer struggles with such "high-poly" models, even on high-end GPUs.[1]
Note:Third party games that upgrade the renderer (ex: 
, 
or 
) can handle higher polygon MDLs better. This does not include Sven Co-op, which uses a mostly vanilla renderer.This is obviously not a problem for games on engines much newer than
GoldSrc that use the HLMDL format, such as 
Codename: Loop (
) or 
FreeCS (). 
Confirm:Is this soft limit 9000 total, or 9000 being displayed simultaneously? Confirm:Can custom renderers without engine access, such as Trinity Engine, improve performance for higher-poly MDLs?
Code:While MDL does not natively support animated textures, a model's skin can be quickly switched between in game code to animate between them. For example, monster_houndeye uses skin switching to blink. Confirm:Max textures, max skins, and other limits?Flags
Model flags
Particle effects cannot be combined, but all others can.[Clarify]
| Bit | Flag name | Particle effect? | Description |
|---|---|---|---|
| 1 | EF_ROCKET | Yes |  Quake I rocket trail particles and dynamic light
|
| 2 | EF_GRENADE | Yes | Quake I grenade trail smoke particles
|
| 4 | EF_GIB | Yes | Quake I gib blood trail particles
|
| 8 | EF_ROTATE | No | Rotate this model around its origin, like the weapon pickups in Quake I; requires supported game code.
|
| 16 | EF_TRACER | Yes | Quake I skrag spit particles
|
| 32 | EF_ZOMGIB | Yes | Quake I zombie blood; like EF_GIB, but darker
|
| 64 | EF_TRACER2 | Yes | Quake I death knight slashing particles
|
| 128 | EF_TRACER3 | Yes | Like EF_GRENADE, but only has one trail |
| 256 | EF_NOSHADELIGHT | No | Reduces intensity of shadows on the model, without making the model flat Confirm:Half Lambert? |
| 512 | EF_HITBOXCOLLISIONS | No | Use hitbox for collision instead of bounding box |
| 1024 | EF_FORCESKYLIGHT | No | Always use skybox lighting, regardless of model location |
Texture flags
| Bit | Flag name | Description |
|---|---|---|
| 1 | STUDIO_NF_FLATSHADE | Shade the model a single tint, without any shadowing, resulting in the model looking "flat" like a cartoon character. |
| 2 | STUDIO_NF_CHROME | Use matcap environment mapping to render the texture as a sphere map.
Bug:Limited to 64x64 in vanilla GoldSrc; this is fixed in Sven Co-op. Chrome resolutions higher than 256x256 tend to have diminishing returns. [todo tested in ?]Note:Automatically set for any textures that contain the string chrome somewhere in their name.Important:Setting the chrome flag via StudioMDL will also add the flatshade flag, as well as move all UV coordinates to the top-left corner (chrome doesn't use UV mapping). |
| 4 | STUDIO_NF_FULLBRIGHT | Display texture at maximum brightness, ignoring lighting.
Bug:Doesn't work in vanilla GoldSrc; fixed in Sven Co-op and custom renderers. [todo tested in ?] |
| 8 | STUDIO_NF_MIPMAPS | Generates mipmaps for the texture, greatly reducing texture aliasing at a distance. No effect in Sven Co-op, which always generates mipmaps for models, regardless of this flag.Bug:This flag was actually originally intended to disable mipmaps for a texture, but this behavior was accidentally inverted. Confirm:Allegedy fixed in 25th Anniversary Update.Tip:Recommended for most textures except alphatested ones. Due to the way textures are stored, enabling mipmaps of alphatested texture will result in the texture transparency becoming more exaggerated at a distance, with fine details like thin foliage and wire fences disappearing, and shadowing occurring around the edges occurring due to transparent texels not being dilated. |
| 16 | STUDIO_NF_ALPHA | Unused? |
| 32 | STUDIO_NF_ADDITIVE | Use additive translucency. Still affected by entity lighting; use the Additive rendermode on an entity to ignore lighting.
Bug:Sometimes breaks normals of non-additive faces on the same mesh.
 Workaround:Split the additive portions onto a different mesh, by listing it as a separate SMD with a separate $body line (usually after the main mesh).
Tip:This affects the order in which faces are loaded into the z-buffer, which can be used for manual alpha sorting. In practice, this means defining an additive chrome overlay (to simulate specular highlights) as a separate mesh after the main mesh will prevent  z-fighting, even if the mesh is coplanar. |
| 64 | STUDIO_NF_MASKED | Use binary alphatest transparency, wherein the last palette entry of the texture is transparent. |
In other engines
Half-Life MDLs are supported by FTE and the Unity game Codename: Loop.
External links
References
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