Replay file format

From RacerWiki

A replay is a sampling of a race. During a race, Racer periodically stores the current state of each car, plus other relevant data, in a memory buffer. The sampling frequency is usually 10 times per second. When you save the replay, the buffer is written to a file.

Replay files are written in a "chunked" format: after the file header, the file is just a sequence of chunks of bytes, each chunk preceded by it size. Each chunk is a sequence of commands (indicating the type of data that follows) plus a number of bytes, whose meaning depends on the command.

The file format version is stored in the header. Version 1.1 was used until Racer 0.5.2. beta 8.7. From beta 8.8, format version 2.1 is used.

Contents 1 File Header 2 Chunks 3 Commands 4 Remarks

File Header

Field	Data type and size	Remarks
Marker	string, 4	"RPLY"
Minor version	shortint	Second part of the file format version
Major version	shortint	First part of the file format version
Track name	string, 64	Name of the track's subdirectory
Number of cars	integer
Car name	string, 64	Name of the car's subdirectory. Repeated for each car.
Marker	string, 4	"DATA"

Chunks

Field	Data type and size	Remarks
Chunk size	shortint	Number of bytes in this chunk
Time	integer	Time from start of replay, in milliseconds

Commands

Each command is a shortint. The bytes that follow are:

Number	Meaning	Followed by
0	Set current car	Car ID (shortint)
1	Set position (obsolete)
2	Car state for the current car	Car position (3 * float) Car orientation (4 * float) Steer angle (float) Vertical wheel position, for each wheel (float) Wheel rotation angle, for each wheel (float) Wheel rotation speed, for each wheel (float) Rpm (float) Speed (3 * float) Brake light and skid factor (shortint) Gear (shortint)
3	End of replay (obsolete or unused)
4	Smoke particle spawn	Position (3 * float) Speed (3 * float) Lifetime (float)
5	Start of skid strip	Strip numer (shortint)
6	Point of skidmark	Strip numer (shortint) Position (3 * float) Transparency (float)
7	End of skid strip	Strip numer (shortint)
8	Skid volume (obsolete or unused)
9	Car enters game	Car name (string, 64) Car ID (integer)
10	Crossing start/finish (obsolete or unused)
11	Crossing a timeline (obsolete or unused)

Remarks

• The numbers in the file are stored in "little-endian" order, so Racer for the Mac has to reverse the byte order when reading/writing replay files. Strings are zero-terminated, single-byte character strings. Integers are 4 bytes wide, shortint are 2 bytes, and floats are 4 bytes.
• In format version 1.1, the car speed is only 1 float (scalar instead of 3-dimensional vector), and brake light, skid factor and gear are not present.