MDL file format (FSX)

This document describes the structure of FsX MDL files. This document is mainly useful for people who want to read or decode these MDL files in a tool.

Compared to the Fs2004 MDL format there are quite a few changes. One of the biggest is that aircraft and scenery MDL files now have the same format. At the moment the knowledge in this document mainly comes from evaluating simple scenery objects, so it is certainly not yet complete. If you have anything to add, please do so.

MDL sections

The MDL file uses the RIFF format, this means that the file consists of different sections. Each section starts with a four character label, followed by the size of the content in bytes (this size is thus without the length of the header itself). The tabel below shows how the different section can be nested.

Name	Description	Parent section
RIFF	General RIFF section (covers entire file)	None
MDLH	MDL header	RIFF
MDLG	Object GUID	RIFF
MDLN	Object friendly name	RIFF
PARA	Parameters Block	RIFF
CRAS	Object crashbox	RIFF
BBOX	Object boundingbox	RIFF
RADI	Object radius	RIFF
MDLD	Exterior Node Root	RIFF
TEXT	Texture list	MDLD
MATE	Material list	MDLD
INDE	Triangle index list	MDLD
VERB	Vertex buffer list	MDLD
VERT	Vertex list	VERB
TANS	Tangent Space list (?)	VERB
TRAN	Static transformation matrices	MDLD
AMAP	Animation Map	MDLD
SCEN	Scenegraph Node Root	MDLD
SGAL	Scenegraph Animation Linkage	MDLD
SGVL	Scenegraph Visibility Linkage	MDLD
SGJC	Scenegraph Joint Constraints	MDLD
SGBR	Scenegraph Bone Reference	MDLD
LODT	LOD table	MDLD
LODE	LOD Entry	LODT
PART	Part list	LODE
ANIB	Animation Block	MDLD

The main RIFF section itself has one additional parameter, this is a four character string that indicates the type of RIFF MDL file. For an FsX MDL file the value should always be MDLX.

The following subsection show more details about the content of these different sections.

MDLH

This the FS MDL object header. It seems to contain of an integer that always has the value of 123456 and a float of value 10. The purpose of these parameter is unknown.

MDLG

This section stores the object GUID. The GUID is stored in the following parts:

unsigned long data1
unsigned short data2
unsigned short data3
unsigned char data4[8]

In hexadecimal form the GUID is then written as:

{data1-data2-data3-data4[0:1]-data4[2:7]}

MDLN

This section contains a string with the friendly name of the object.

PARA

This is the ParamsBlock section, containing parameter block data.

CRAS

This section stores the crashtree of the object. It has first two unknown integer values, then there are 3 float values giving the front lower left corner of the object, then 3 float values giving the length, height, and width, then another unknown integer, and then the branches of the tree, which are all integer values (instead of bytes which had been used in FS2004). If the value of a cell is -1 the cell is solid, if it is -2 the cell is free, every other value is the offset of the octet of the next lower node in the branch.

BBOX

This section stores the bounding box of the object in the following format:

float xmin
float ymin
float zmin
float xmax
float ymax
float zmax

RADI

This section stores the radius of the object as a float.

MDLD

This section contains the actual data defining the object.

TEXT

This section contains the list of all textures of the object. Each texture is given as a 64 character string. Unlike the Fs2004 format no additional parameters are stored in the texture list, only the texture name.

MATE

This section contains a list of all materials of the object. Each material is defined by a 120 byte long record. The definition of this record is given below:

short  material_flags
short  (?)
float  (?)
int    diffuse_texture_index
float  (?)
float  (?)
float  (?)
int    emissive_texture_index
float  (?)
float  (?)
float  diffuse_color_R
float  diffuse_color_G
float  diffuse_color_B
float  diffuse_color_A
float  specular_color_R
float  specular_color_G
float  specular_color_B
float  specular_color_A
float  specular_power
float  (?)
float  (?)
float  (?)
float  (?)
float  specular_bloom_floor
float  ambient_light_scale
float  (?)
int    source_blend
int    destination_blend
int    alpha_test_function
float  alpha_test_threshold
float  z_write_alpha

material_flags is a bitmask:

 0x01: material has a base color
 0x02: material has a diffuse texture (the diffuse color is then only used as a fallback)
 0x80: material has an emissive texture (for night)

The source_blend and destination_blend parameters seem to use the following enumeration:

 1 zero
 2 once
 3 srcColor
 4 invSrcColor
 5 srcAlpha
 6 invSrcAlpha
 7 destAlpha
 8 invDestAlpha
 9 destColor
10 invDestColor

The alpha_test_function parameter seems to use the following enumeration:

1 never
2 less
3 equal
4 lessEqual
5 greater
6 notEqual
7 greaterEqual
8 always

The z_write_alpha parameter has a value of 1 (float) when it is enabled (TRUE).

INDE

This section contains of a lot of triangles, each given by three short values. These values are the indices of the vertices to use. In the PART list an offset is specified for these indices, so the first vertex of a part seems to be zero always.

VERB

This section gives the vertex buffer of the object.

VERT

This section contains a list of all vertices of the object. Each vertex is given by a 32 byte record. The definition of this record is given below:

float position_x
float position_y
float position_z
float normal_x
float normal_y
float normal_z
float texture_mapping_x
float texture_mapping_y

TANS

This section is contains tangent space list data. The first paramter of this list seems to be a zero. Then for each vertex two vectors are given that define a plane that is tangent to the vertex (these vectors are defined as three floats). This data only seems to be set for textured polygons, untextured polygons have null vectors defined here.

TRAN

This section stores the static transformation matrices. Each matrix is defined by 16 floats. For a definition of these matrices see the DirectX SDK.

AMAP

This section contains animation map data.

SCEN

Unknown at the moment.

SGAL

This is the scenegraph animation linkage section.

SGVL

This is the scenegraph visibility linkage section.

SGJC

This scenegraph section contains IK joint constraint data.

SGBR

This is the scenegraph bone reference section.

LODT

This section defines the LOD table of the object. It is followed by a LODE section for each LOD level in the object.

LODE

This section defines a LOD level for the object. It contains an int that indicates the value of this LOD. The default value if only one LOD is used in the object is 100. This section is followed by a PART section for each part in this LOD level.

PART

This section defines an object part. The part definition contains the following parameters:

int  type (1 = Triangle List, 2 = Triangle Fan, 3 = Triangle Strip)
int  scenegraph_reference (?)
int  material_index
int  vertex_buffer_index
int  vertex_offset
int  vertex_count  
int  index_offset
int  index_count
int  mouse_rectangle_ref (?)

vertex_buffer_index tells which VERT structure inside the VERB section is to be used.

The index_offset and index_count are given in indices, so it should be divided by three to get a triangle offset and count.

ANIB

Animation block section. Data type is uint8.