https://github.com/dbohdan/ptjd
A pure Tcl JPEG decoder
https://github.com/dbohdan/ptjd
jim-tcl jpeg jpeg-decoder tcl
Last synced: 3 months ago
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A pure Tcl JPEG decoder
- Host: GitHub
- URL: https://github.com/dbohdan/ptjd
- Owner: dbohdan
- License: mit
- Created: 2017-04-28T08:39:09.000Z (about 9 years ago)
- Default Branch: master
- Last Pushed: 2024-05-15T11:25:15.000Z (about 2 years ago)
- Last Synced: 2025-11-27T21:18:10.829Z (6 months ago)
- Topics: jim-tcl, jpeg, jpeg-decoder, tcl
- Language: Tcl
- Homepage: https://wiki.tcl-lang.org/page/Pure+Tcl+JPEG+decoder
- Size: 458 KB
- Stars: 2
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
- License: LICENSE
- Authors: AUTHORS
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README
# Pure Tcl JPEG decoder
A single-file pure Tcl baseline JPEG decoder library.
Works in Tcl 8.5, Tcl 8.6, Tcl 9, and Jim Tcl 0.75 and later.
## Q & A
### What is supported?
- JPEG/JFIF and JPEG/Exif
- Huffman coding
- YCbCy (standard three-channel color) and grayscale
- 8-bit color channels
- Chroma subsampling (horizontal and vertical 4:2:2 subsampling and 4:2:0 subsampling has been tested)
- Restart markers
### What isn't?
- Progressive and lossless encoding
- Arithmetic coding
- CMYK color
- 12-bit color channels
### Why?
To learn how to write a JPEG decoder.
### What is it good for?
The decoder is too slow to replace JPEG decoders for Tcl written in C for anything but the smallest images.
However, it can be used as a benchmark to compare different Tcl implementations and different versions of the same implementation.
(See [below](#what-is-the-performance-like).)
The code may help you understand JPEG compression.
It is small (around 850 lines),
written in a [functional](https://en.wikipedia.org/wiki/Functional_programming) style
(the decoder consists of pure functions [insofar as they exist](https://wiki.tcl-lang.org/page/trace) in Tcl),
and stores data in easy-to-inspect immutable data structures.
(If you want to understand how a JPEG decoder works,
based on my experience I recommend reading the [Wikipedia article on JPEG](https://en.wikipedia.org/wiki/JPEG)
and following the [JPEG Huffman coding tutorial](https://web.archive.org/web/20190118205958/https://www.impulseadventure.com/photo/jpeg-huffman-coding.html) by Calvin Hass before anything else.
If you don't know Tcl and want to read the code, [Learn Tcl in Y Minutes](https://learnxinyminutes.com/docs/tcl/) should teach you enough to get started.
Tcl is pretty much a collection of independent commands;
once you know the syntax, you can [look them up](https://www.tcl.tk/man/tcl8.6/TclCmd/contents.htm) as you go.)
### What is the performance like?
The script `benchmark.tcl` can evaluate the performance of the decoder by timing how long it takes to decode several test images included in the repository.
Here are the results gathered on an [AMD Phenom II](https://www.cpubenchmark.net/cpu.php?cpu=AMD+Phenom+II+X4+955&id=368) CPU.
```none
Running in Tcl 8.5.19 (64-bit) on x86_64 Linux
5 iterations per image
==========================
grayscale.jpg 10 ms
landscape.jpg 19548 ms
landscape-q.jpg 11159 ms
63.6 MB
```
```none
Running in Tcl 8.6.5 (64-bit) on x86_64 Linux
5 iterations per image
==========================
grayscale.jpg 10 ms
landscape.jpg 18758 ms
landscape-q.jpg 10587 ms
75.3 MB
```
```none
Running in Jim 0.75 (64-bit) on x86_64 linux
5 iterations per image
==========================
grayscale.jpg 19 ms
landscape.jpg 38508 ms
landscape-q.jpg 22832 ms
88.8 MB
```
## License
MIT.
The `test-data/landscape*` photo by [m wrona](https://unsplash.com/@mwrona), licensed under [CC0](https://creativecommons.org/publicdomain/zero/1.0/).