What is AEM?

AEM is an excellent model file format. 😉

It describes game-ready 3D models with everything they need - no more, no less:

• Vertex normals, tangents and bitangents for lighting and normal mapping
• Single-channel UV coordinates for texture mapping
• Separate index data to use with an index buffer
• Standard PBR material system with base color, normal, occlusion, roughness and metalness maps
• Skeletal animations

This repository contains:

• libaem: A minimal and dependency-free C library that can load and animate AEM models efficiently
• converter: A command-line tool that can convert GLB files into AEMs
• viewer: A viewer application that illustrates how to load and render AEM models with libaem and OpenGL 3.3 and can be used to inspect AEM models

Specification

Definitions

• All offsets and sizes in this specification are given in bytes.
• All bytes in AEM files are stored in little-endian order.
• All matrices in AEM files are stored in column-major order.

File Header

Offset	Size	Description	Data Type
0	3	Magic number	String
3	1	Version number	Unsigned integer
4	4	Number of vertices	Unsigned integer
8	4	Number of indices	Unsigned integer
12	4	Number of textures	Unsigned integer
16	4	Number of meshes	Unsigned integer
20	4	Number of materials	Unsigned integer
24	4	Number of joints	Unsigned integer
28	4	Number of animations	Unsigned integer
32	4	Number of tracks	Unsigned integer
36	4	Number of keyframes	Unsigned integer

The magic number is always "AEM" in ASCII (0x41 45 4D). This specification describes version 1 of the file format.

Vertex Section

Offset	Size	Description	Data Type
0	4	Position X	Float
4	4	Position Y	Float
8	4	Position Z	Float
12	4	Normal X	Float
16	4	Normal Y	Float
20	4	Normal Z	Float
24	4	Tangent X	Float
28	4	Tangent Y	Float
32	4	Tangent Z	Float
36	4	Bitangent X	Float
40	4	Bitangent Y	Float
44	4	Bitangent Z	Float
48	4	Texture U	Float
52	4	Texture V	Float
56	4	Joint index 1	Signed integer
60	4	Joint index 2	Signed integer
64	4	Joint index 3	Signed integer
68	4	Joint index 4	Signed integer
72	4	Joint weight 1	Float
76	4	Joint weight 2	Float
80	4	Joint weight 3	Float
84	4	Joint weight 4	Float
...	...	(repeat)	...

(The fields above are repeated for each vertex in the file.)

The joint indices 1-4 index into the joint section. The sum of all joint weights is 1.0 if the vertex is affected by at least 1 joint. Otherwise the sum of joint weights is 0.0. If a vertex is affected by less than 4 joints, the additional joint weights are 0.0, and the corresponding joint indices are -1.

Index Section

Offset	Size	Description	Data Type
0	4	Index	Unsigned integer
...	...	(repeat)	...

(The field above is repeated for each index in the file.)

The indices index into the vertex section.

Texture Section

Offset	Size	Description	Data Type
0	128	Filename	String
...	...	(repeat)	...

(The field above is repeated for each texture in the file.)

The filename for a texture is supplied with a null-terminated string of at most 128 characters (including the null terminator). Longer filenames are not supported.

Mesh Section

Offset	Size	Description	Data Type
0	4	First index	Unsigned integer
4	4	Number of indices	Unsigned integer
8	4	Material index	Unsigned integer
...	...	(repeat)	...

(The field above is repeated for each mesh in the file.)

Meshes consist of a range of indices in the index section. The material indices index into the material section.

Material Section

Offset	Size	Description	Data Type
0	4	Base color texture index	Signed integer
4	4	Normal texture index	Signed integer
8	4	ORM texture index	Signed integer
...	...	(repeat)	...

(The fields above are repeated for each material in the file.)

The texture indices index into the material section. An index of 255 indicates that the respective texture is not defined. In that case, your application should fall back to an appropriate default texture. (For example a solid 127/127/255 texture to use when a mesh in the model does not come with an actual normal map.) ORM stands for Occlusion, Roughness and Metalness: A texture image where red represents occlusion, green represents roughness and the blue channel represents metalness.

Joint Section

Offset	Size	Description	Data Type
0	128	Name	String
128	64	Inverse bind matrix	4x4 matrix
192	4	Parent joint index	Signed integer
...	...	(repeat)	...

(The fields above are repeated for each joint in the file.)

The joint name is supplied with a null-terminated string of at most 128 characters (including the null terminator). Longer names are not supported. The parent joint indices index into this same joint section, and is -1 for root joints without parent.

Animation Section

Offset	Size	Description	Data Type
0	128	Name	String
128	4	Duration	Float
...	...	(repeat)	...

(The fields above are repeated for each animation in the file.)

The animation name is supplied as a null-terminated string of at most 128 characters (including the null terminator). Longer names are not supported. The duration is defined in seconds.

Track Section

Offset	Size	Description	Data Type
0	4	First keyframe index	Unsigned integer
4	4	Number of translation keyframes	Unsigned integer
8	4	Number of rotation keyframes	Unsigned integer
12	4	Number of scale keyframes	Unsigned integer
...	...	(repeat)	...

(The fields above are repeated for each track in the file.)

Tracks index into the keyframe section. There is a track for each joint in each animation and the keyframes are always in the order: Translation keyframes, then rotation keyframes, then scale keyframes. The layout is as follows: Track for joint 1/2 in animation 1/2, track for joint 2/2 in animation 1/2, track for joint 1/2 in animation 2/2, track for joint 2/2 in animation 2/2 and so on.

Keyframe Section

Offset	Size	Description	Data Type
0	4	Time	Float
4	4	X	Float
8	4	Y	Float
12	4	Z	Float
16	4	W	Float
...	...	(repeat)	...

(The fields above repeated for each keyframe in the file.)

The time of the keyframe is defined in seconds. Keyframes are generic, they can represent position, rotation or scale keyframes, depending on how the track using the keyframe is indexing it. For position and scale keyframes, the last component W is 0. For rotation keyframes, the X, Y, Z, W values define a quaternion that expresses the rotation of the keyframe.