libcapt

Implementation of the Canon CAPT v1 protocol and SCoA compression based on reverse engineering of the original driver.

Important

This project is currently in an EXPERIMENTAL state.

NOTE: This is not a full-fledged driver that you can install into your system. It is an implementation of the compression algorithm and protocol, abstracted from any operating system or transmission channel.

Target devices

Currently, the target devices are printers with CNTblModel = 0 (see Canon's driver PPD).

Model	Cartridge	PPM (A4)	Max Resolution	Year (approx.)
LBP800	EP-22	8	600 dpi	1999-2001
LBP810	EP-22	8	600 dpi	2001-2002
LBP1120	EP-22	10	600 dpi	2003-2004
LBP1210	EP-25	14	600 dpi	~2002
LBP3200	EP-26/EP-27	18	600 dpi	2004-2006

Compilation

cmake -S. -B build
cmake --build build

Dependencies

Compile-time

• gcc >= 11 or clang >= 16 or MSVC (tested on 19.44.35217.0)
• cmake >= 3.21

Testing

• gtest (+gmock)

For compression tests:

• gdb
• ghostscript
• poppler-utils (pdfseparate, pdfinfo, etc.)
• captfilter deps (libpopt0:i386 on Debian)

Testing

Core tests

cmake -S. -B build -DLIBCAPT_BUILD_TESTS=ON
cmake --build build
ctest --test-dir build

Compression tests

cmake -S. -B build -DLIBCAPT_BUILD_TESTS=ON -DLIBCAPT_COMPRESSION_TESTS=ON
cmake --build build
ctest --test-dir build

Compression test files are located at tests/Compression/data. If you are adding your own files, don't forget to add them into the CMakeLists.txt file.

Usage

There are several options if you want to print on your Canon LBP printer (except for the original driver):

• darkvision77/captppd — CUPS driver based on libcapt (CAPTv1 only)
• UoWPrint — print server that supports all Canon LBP models (and other printers)
• mounaiban/captdriver — CUPS driver for the newer Canon LBP models (early alpha stage)

Also, the examples folder contains an example program for printing PBM files.
Keep in mind that this program does not implement all the features and is used only for printing testing.

cmake -S. -B build -DLIBCAPT_BUILD_EXAMPLES=ON
cmake --build build
./build/examples/printpbm /dev/usb/lp0 ./examples/data/example-1.pbm

To rasterize PDF files into PBM, you can use the pdf2pbm.sh script.
Also, you can find some test PBM files in the examples/data folder.

Decoder verification

The correct operation of the compressor can be verified using the decoder.
But how can we ensure that the decoder itself is functioning correctly?

General principle:

1. The original PBM is converted to filtered.bin using Canon's captfilter, extracting its «command sequence log».
2. The decoder receives filtered.bin, decodes the raster, and writes the read commands to its command log.
3. If the command logs of the captfilter and decoder are equal, then the commands were correctly recognized.
4. The original raster is compared with the decoded one from filtered.bin.

Of course, the original captfilter does not write any logs. So...

Fine-tuning Canon's captfilter

The captfilter is not being run directly. Instead, it is run under a special gdb python script that is responsible for:

1. Command log: the captfilter binary file contains some debug prints, but the DebugPrint function is disabled in the release build, so for each compression command there is a breakpoint that makes the corresponding entry in the log.
2. Resolve «Buffer shift bug»: there is a bug in captfilter related to the *ThenRaw* commands that affects the final raster.
   Let's say there is a buffer rawData[256]. It can be represented as the following commands:

   char* rawData;
   CopyThenRawLong(0, rawData, 255); // writes {1, 2, 3, ..., 255}
   rawData += 255;
   CopyThenRaw(0, rawData, 1); // writes {256}

   But for some reason, there is no buffer offset, and the output is something like this:

   char* rawData;
   CopyThenRawLong(0, rawData, 255); // writes {1, 2, 3, ..., 255}
   // Oops...
   CopyThenRaw(0, rawData, 1); // writes {1} again...

   This bug can be found by compressing files with random data (see rand{A..F}.pbm).
3. Zero margins: to prevent captfilter from resizing the raster, you need to set the margins to zero. But since the printer does not support zero margins, captfilter does not support them either. Therefore, we have to use the magic of gdb scripts.

License

libcapt is licensed under a 2-clause BSD license.