Meatpack

A pure Rust implementation of Scott Mudge's MeatPack algorithm. The crate works in both std and no_std environments. A CLI is provided and bindings for other languages are in the pipeline. The Packer and Unpacker structs are configurable allowing you to set it up according to your embedded system resource constraints.

Support

Please consider supporting the crate by:

• Downloading and using the crate.
• Raising issues and improvements on the GitHub repo.
• Recommending the crate to others.
• ⭐ the crate on GitHub.
• Sponsoring the maintainer.

The Algorithm

Gcode uses a restricted alphabet and are mostly made up of numbers, decimal point, a few letters ('G', 'M', 'E', etc.), and other utilitiy characters (newline, space, etc.). Scott's histographic analysis of a dozen or so gcode files found that ~93% of all gcode use the same 15 characters. Gcode is stored as a sequence of u8 ascii characters that can fit a 256-character alphabet.

MeatPack takes advantage of this histographic property and uses a lookup table that takes the 15 most popular characters and matches them to a 4-bit representation (capable of representing 16 characters). This 4-bit representations are packed into 8-bit/1-byte characters. This effectively doubles the data density of a gcode file.

The 16th character (0b1111) of the 4-bit representation is reserved for telling the unpacker that it should expect a full-width character in the following byte.

The packer reorders some characters if the full width character is surrounded by packable characters. This is to avoid 4 bits being wasted telling the packer that only one full width character is coming up.

If the lower 4-bits contains 0b1111 then we unpack the full width character followed by the upper 4-bit character. If the upper 4-bit contains 0b1111 then we unpack the lower 4-bit character followed by the full width character. This minor reordering allows slightly more data to the be packed at the cost of a little complexity.

Example

Take the following "G1" command.

G1 X113.214 Y91.45 E1.3154

which consists of 27 bytes (paranthesis indicate 1 byte),

(G) (1) ( ) (X) (1) (1) (3) (.) (2) (1) (4) ( ) (Y) (9) (1) (.) (4) (5) ( ) (E) (1) (.) (3) (1) (5) (4) (\n)

Applying the packing algorithm we can pack the command into 16 bytes as follows:

(1G) (X ) (11) (.33) (12) ( 4) (9#) (Y) (.1) (54) (# ) (E) (.1) (13) (45) (\n)

The characters are ordered in bit order (upper, lower) and would be unpacked lower then upper. With "Whitespace Removal" active, we can further reduce this to 13 bytes.

(1G) (1X) (31) (2.) (41) (9#) (Y) (.1) (54) (1E) (3.) (51) (\n4)

The packed command is now less than half the size of the original command.

Command Patterns

MeatPack provides a communication/control layer identified by 2 255 (OxFF) signal bytes followed by a command byte. 0xFF is virtually never found in gcode, so it is can be considered a reserved character. The following command bytes exist:

Examples

use std::process;

use meatpack::{MeatPackResult, Packer, Unpacker};

fn main() {
	let gcode = "M73 P0 R3
M73 Q0 S3 ; Hello
M201 X4000 Y4000 Z200 E2500
M203 X300 Y300 Z40 E100
M204 P4000 R1200 T4000
";
	let mut packer = Packer::<64>::default();
	let mut out: Vec<u8> = vec![];

	out.extend(packer.header());

	for byte in gcode.as_bytes() {
		let packed = packer.pack(byte);
		match packed {
			Ok(MeatPackResult::Line(line)) => {
				println!("{:?}", line);
				out.extend(line);
			}
			Ok(MeatPackResult::WaitingForNextByte) => {}
			Err(e) => println!("{:?}", e),
		}
	}

	println!("{:?}", out);

	let mut unpacker = Unpacker::<64>::default();
	for byte in out {
		let res = unpacker.unpack(&byte);
		match res {
			Ok(MeatPackResult::WaitingForNextByte) => {}
			Ok(MeatPackResult::Line(line)) => {
				// If in std.
				for byte in line {
					let c = char::from(*byte);
					print!("{}", c);
				}
			}
			Err(e) => {
				println!("{:?}", e);
				process::exit(0)
			}
		}
	}
}

References

• https://github.com/scottmudge/OctoPrint-MeatPack
• https://github.com/prusa3d/libbgcode/blob/main/src/LibBGCode/binarize/meatpack.cpp
• https://github.com/scottmudge/Prusa-Firmware-MeatPack/blob/MK3_sm_MeatPack/Firmware/meatpack.cpp