2.9 KiB
Message Format
Data type
The datatype consists of 4-bit (nibble) 2 parts: the most significant nibble is the syntax, while the least significant nibble is the sematics.
| 7 . 6 | 5 . 4 | 3. 2. 1. 0 |
|---|---|---|
| angular | linear | semantics |
When the Data type is 0x00, this indicates that a second byte will follow which defines further syntax options. Currently, this has not been defined yet, but can be used to define high-resolution angles using 2 bytes per dimension.
Syntax
The syntax defines the format of the data sent and makes it possible to parse or skip parts of the message. Every value can consist of two parts: the angular and linear part, each represented by 2 bits.
| Angular part | |
|---|---|
| 00 (0x0) | No angle |
| 01 (0x4) | 1-byte, 1D angle |
| 10 (0x8) | 2-byte, 2D angle |
| 11 (0xC) | 3-byte, 3D angle |
| Linear part | |
|---|---|
| 00 (0x0) | No magnitude |
| 01 (0x1) | 1 byte magnitude |
| 10 (0x2) | 2 byte magnitude |
| 11 (0x3) | 4 byte magnitude |
Note that these values do not actually say how the angular and linear values are encoded. A 4-byte magnitude may be a 32-bit unsigned int or a single precision float for example. This depends on the semantics.
When the angular and linear parts are combined, the size of the message can be determined. A few examples:
| Data type | Size | Description |
|---|---|---|
| 0001 (0x1) | 1 byte | A single byte of linear data, like an intensity value |
| 0100 (0x4) | 1 byte | An angle, like an angle between 0 and 360 degrees |
| 0101 (0x5) | 2 bytes | An angle with a distance, like a polar coordinate |
| 1101 (0xD) | 4 bytes | For example a quaternion, where the x,y,z = angular and w = linear part |
| 0002 (0x2) | 2 bytes | A 16-bit value, like a single short int |
| 1011 (0xB) | 6 bytes | 2D angles and a 32-bit distance, for example a spherical coordinate |
Semantics
The semantics defines how the data should be interpreted. The following are defined:
| Semantics | |
|---|---|
| 0b0001 (0x1) | Position |
| 0b0010 (0x2) | Linear velocity |
| 0b0011 (0x3) | Linear acceleration |
| 0b0101 (0x5) | Orientation or rotation |
| ob0110 (0x6) | Angular velocity |
| 0b0111 (0x7) | Angular acceleration |
| 0b1000 (0x8) | Direction |
Currently implemented data types
| Data type | Message |
|---|---|
| 0x62 | Polar velocity with 2-byte distance |
Typical data types
| Data type | Interpretation | | 0x48 | A direction in 2D space, represented by an angle. | | 0x88 | A direction in 3D space, represented by 2 angles. | | 0x83 | A position in 3D space, using Spherical coordinates | | 0xD4 | An orientation represented by a quaternion | | 0xE5 | Angular verlocity, represented in Angle-axis format | | 0xF5 | Angular verlocity, represented in Angle-axis format using a float for the magnitude |
Note that a 2D or 3D vector is not directly supported. In alignment to the coordinate sytem of Roboid controle, these need to be converted to Polar or Spherical coordinates before they can be sent.