Twitter image encoding challenge

Question

If a picture's worth 1000 words, how much of a picture can you fit in 140 characters?

Note: That's it folks! Bounty deadline is here, and after some tough deliberation, I have decided that Boojum's entry just barely edged out Sam Hocevar's. I will post more detailed notes once I've had a chance to write them up. Of course, everyone should feel free to continue to submit solutions and improve solutions for people to vote on. Thank you to everyone who submitted and entry; I enjoyed all of them. This has been a lot of fun for me to run, and I hope it's been fun for both the entrants and the spectators.

I came across this interesting post about trying to compress images into a Twitter comment, and lots of people in that thread (and a thread on Reddit) had suggestions about different ways you could do it. So, I figure it would make a good coding challenge; let people put their money where their mouth is, and show how their ideas about encoding can lead to more detail in the limited space that you have available.

I challenge you to come up with a general purpose system for encoding images into 140 character Twitter messages, and decoding them into an image again. You can use Unicode characters, so you get more than 8 bits per character. Even allowing for Unicode characters, however, you will need to compress images into a very small amount of space; this will certainly be a lossy compression, and so there will have to be subjective judgements about how good each result looks.

Here is the result that the original author, Quasimondo, got from his encoding (image is licensed under a Creative Commons Attribution-Noncommercial license):

Can you do better?

Rules

1. Your program must have two modes: encoding and decoding.
2. When encoding:
   1. Your program must take as input a graphic in any reasonable raster graphic format of your choice. We'll say that any raster format supported by ImageMagick counts as reasonable.
   2. Your program must output a message which can be represented in 140 or fewer Unicode code points; 140 code points in the range U+0000–U+10FFFF, excluding non-characters (U+FFFE, U+FFFF, U+nFFFE, U+nFFFF where n is 1–10 hexadecimal, and the range U+FDD0–U+FDEF) and surrogate code points (U+D800–U+DFFF). It may be output in any reasonable encoding of your choice; any encoding supported by GNU iconv will be considered reasonable, and your platform native encoding or locale encoding would likely be a good choice. See Unicode notes below for more details.
3. When decoding:
   1. Your program should take as input the output of your encoding mode.
   2. Your program must output an image in any reasonable format of your choice, as defined above, though for output vector formats are OK as well.
   3. The image output should be an approximation of the input image; the closer you can get to the input image, the better.
   4. The decoding process may have no access to any other output of the encoding process other than the output specified above; that is, you can't upload the image somewhere and output the URL for the decoding process to download, or anything silly like that.

4. For the sake of consistency in user interface, your program must behave as follows:

   1. Your program must be a script that can be set to executable on a platform with the appropriate interpreter, or a program that can be compiled into an executable.
   2. Your program must take as its first argument either encode or decode to set the mode.

   3. Your program must take input in one or more of the following ways (if you implement the one that takes file names, you may also read and write from stdin and stdout if file names are missing):

      1. Take input from standard in and produce output on standard out.

         my-program encode <input.png >output.txt
         my-program decode <output.txt >output.png
         

      2. Take input from a file named in the second argument, and produce output in the file named in the third.

         my-program encode input.png output.txt
         my-program decode output.txt output.png
         

5. For your solution, please post:
   1. Your code, in full, and/or a link to it hosted elsewhere (if it's very long, or requires many files to compile, or something).
   2. An explanation of how it works, if it's not immediately obvious from the code or if the code is long and people will be interested in a summary.
   3. An example image, with the original image, the text it compresses down to, and the decoded image.
   4. If you are building on an idea that someone else had, please attribute them. It's OK to try to do a refinement of someone else's idea, but you must attribute them.

Guidelines

These are basically rules that may be broken, suggestions, or scoring criteria:

1. Aesthetics are important. I'll be judging, and suggest that other people judge, based on:
   1. How good the output image looks, and how much it looks like the original.
   2. How nice the text looks. Completely random gobbledigook is OK if you have a really clever compression scheme, but I also want to see answers that turn images into mutli-lingual poems, or something clever like that. Note that the author of the original solution decided to use only Chinese characters, since it looked nicer that way.
   3. Interesting code and clever algorithms are always good. I like short, to the point, and clear code, but really clever complicated algorithms are OK too as long as they produce good results.
2. Speed is also important, though not as important as how good a job compressing the image you do. I'd rather have a program that can convert an image in a tenth of a second than something that will be running genetic algorithms for days on end.
3. I will prefer shorter solutions to longer ones, as long as they are reasonably comparable in quality; conciseness is a virtue.
4. Your program should be implemented in a language that has a freely-available implementation on Mac OS X, Linux, or Windows. I'd like to be able to run the programs, but if you have a great solution that only runs under MATLAB or something, that's fine.
5. Your program should be as general as possible; it should work for as many different images as possible, though some may produce better results than others. In particular:
   1. Having a few images built into the program that it matches and writes a reference to, and then produces the matching image upon decoding, is fairly lame and will only cover a few images.
   2. A program that can take images of simple, flat, geometric shapes and decompose them into some vector primitive is pretty nifty, but if it fails on images beyond a certain complexity it is probably insufficiently general.
   3. A program that can only take images of a particular fixed aspect ratio but does a good job with them would also be OK, but not ideal.
   4. You may find that a black and white image can get more information into a smaller space than a color image. On the other hand, that may limit the types of image it's applicable to; faces come out fine in black and white, but abstract designs may not fare so well.
   5. It is perfectly fine if the output image is smaller than the input, while being roughly the same proportion. It's OK if you have to scale the image up to compare it to the original; what's important is how it looks.
6. Your program should produce output that could actually go through Twitter and come out unscathed. This is only a guideline rather than a rule, since I couldn't find any documentation on the precise set of characters supported, but you should probably avoid control characters, funky invisible combining characters, private use characters, and the like.

Scoring rubric

As a general guide to how I will be ranking solutions when choosing my accepted solution, lets say that I'll probably be evaluating solutions on a 25 point scale (this is very rough, and I won't be scoring anything directly, just using this as a basic guideline):

• 15 points for how well the encoding scheme reproduces a wide range of input images. This is a subjective, aesthetic judgement
  • 0 means that it doesn't work at all, it gives the same image back every time, or something
  • 5 means that it can encode a few images, though the decoded version looks ugly and it may not work at all on more complicated images
  • 10 means that it works on a wide range of images, and produces pleasant looking images which may occasionally be distinguishable
  • 15 means that it produces perfect replicas of some images, and even for larger and more complex images, gives something that is recognizable. Or, perhaps it does not make images that are quite recognizable, but produces beautiful images that are clearly derived from the original.
• 3 points for clever use of the Unicode character set
  • 0 points for simply using the entire set of allowed characters
  • 1 point for using a limited set of characters that are safe for transfer over Twitter or in a wider variety of situations
  • 2 points for using a thematic subset of characters, such as only Han ideographs or only right-to-left characters
  • 3 points for doing something really neat, like generating readable text or using characters that look like the image in question
• 3 points for clever algorithmic approaches and code style
  • 0 points for something that is 1000 lines of code only to scale the image down, treat it as 1 bit per pixel, and base64 encode that
  • 1 point for something that uses a standard encoding technique and is well written and brief
  • 2 points for something that introduces a relatively novel encoding technique, or that is surprisingly short and clean
  • 3 points for a one liner that actually produces good results, or something that breaks new ground in graphics encoding (if this seems like a low number of points for breaking new ground, remember that a result this good will likely have a high score for aesthetics as well)
• 2 points for speed. All else being equal, faster is better, but the above criteria are all more important than speed
• 1 point for running on free (open source) software, because I prefer free software (note that C# will still be eligible for this point as long as it runs on Mono, likewise MATLAB code would be eligible if it runs on GNU Octave)
• 1 point for actually following all of the rules. These rules have gotten a bit big and complicated, so I'll probably accept otherwise good answers that get one small detail wrong, but I will give an extra point to any solution that does actually follow all of the rules

Reference images

Some folks have asked for some reference images. Here are a few reference images that you can try; smaller versions are embedded here, they all link to larger versions of the image if you need those:

Prize

I am offering a 500 rep bounty (plus the 50 that StackOverflow kicks in) for the solution that I like the best, based on the above criteria. Of course, I encourage everyone else to vote on their favorite solutions here as well.

Note on deadline

This contest will run until the bounty runs out, about 6 PM on Saturday, May 30. I can't say the precise time it will end; it may be anywhere from 5 to 7 PM. I will guarantee that I'll look at all entries submitted by 2 PM, and I will do my best to look at all entries submitted by 4 PM; if solutions are submitted after that, I may not have a chance to give them a fair look before I have to make my decision. Also, the earlier you submit, the more chance you will have for voting to be able to help me pick the best solution, so try and submit earlier rather than right at the deadline.

Unicode notes

There has also been some confusion on exactly what Unicode characters are allowed. The range of possible Unicode code points is U+0000 to U+10FFFF. There are some code points which are never valid to use as Unicode characters in any open interchange of data; these are the noncharacters and the surrogate code points. Noncharacters are defined in the Unidode Standard 5.1.0 section 16.7 as the values U+FFFE, U+FFFF, U+nFFFE, U+nFFFF where n is 1–10 hexadecimal, and the range U+FDD0–U+FDEF. These values are intended to be used for application-specific internal usage, and conforming applications may strip these characters out of text processed by them. Surrogate code points, defined in the Unicode Standard 5.1.0 section 3.8 as U+D800–U+DFFF, are used for encoding characters beyond the Basic Multilingual Plane in UTF-16; thus, it is impossible to represent these code points directly in the UTF-16 encoding, and it is invalid to encode them in any other encoding. Thus, for the purpose of this contest, I will allow any program which encodes images into a sequence of no more than 140 Unicode code points from the range U+0000–U+10FFFF, excluding all noncharacters and surrogate pairs as defined above.

I will prefer solutions that use only assigned characters, and even better ones that use clever subsets of assigned characters or do something interesting with the character set they use. For a list of assigned characters, see the Unicode Character Database; note that some characters are listed directly, while some are listed only as the start and end of a range. Also note that surrogate code points are listed in the database, but forbidden as mentioned above. If you would like to take advantage of certain properties of characters for making the text you output more interesting, there are a variety of databases of character information available, such as a list of named code blocks and various character properties.

Since Twitter does not specify the exact character set they support, I will be lenient about solutions which do not actually work with Twitter because certain characters count extra or certain characters are stripped. It is preferred but not required that all encoded outputs should be able to be transferred unharmed via Twitter or another microblogging service such as identi.ca. I have seen some documentation stating that Twitter entity-encodes <, >, and &, and thus counts those as 4, 4, and 5 characters respectively, but I have not tested that out myself, and their JavaScript character counter doesn't seem to count them that way.

Tips & Links

• The definition of valid Unicode characters in the rules is a bit complicated. Choosing a single block of characters, such as CJK Unified Ideographs (U+4E00–U+9FCF) may be easier.
• You may use existing image libraries, like ImageMagick or Python Imaging Library, for your image manipulation.
• If you need some help understanding the Unicode character set and its various encodings, see this quick guide or this detailed FAQ on UTF-8 in Linux and Unix.
• The earlier you get your solution in, the more time I (and other people voting) will have to look at it. You can edit your solution if you improve it; I'll base my bounty on the most recent version when I take my last look through the solutions.
• If you want an easy image format to parse and write (and don't want to just use an existing format), I'd suggest using the PPM format. It's a text based format that's very easy to work with, and you can use ImageMagick to convert to and from it.

Edit log

May 30: Bounty deadline up, picked winner
May 28: More precise details on deadline
May 28: Cleanup of links, mention PPM
May 27: Reorganized and cleaned up rules and guidelines; added tips and links
May 26: Mentioned deadline issues and removed the soft cap of 100 lines
May 22: Added reference images, clarifications about Unicode, and a rough scoring rubric
May 21: Updated to allow 140 arbitrary Unicode characters. While the Twitter web form counts by UTF-16 code units, the API allows 140 arbitrary Unicode characters, so we'll use that definition to allow more flexibility.
May 21: Added deadline information
May 21: Posted original challenge

Answer 1

The general overview of my solution would be:

1. I start with calculating the maximum amount of raw data that you can fit into 140 utf8 characters.
   • (I am assuming utf8, which is what the original website claimed twitter stored it's messages in. This differs from the problem statement above, which asks for utf16.)
   • Using this utf8 faq, I calculate that the maximum number of bits you can encode in a single utf8 character is 31 bits. In order to do this, I would use all characters that are in the U-04000000 – U-7FFFFFFF range. (1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx, there are 31 x's, therefore I could encode up to 31 bits).
   • 31 bits times 140 characters equals 4340 bits. Divide that by 8 to get 524.5, and round that down to 542 bytes.
   • (If we restrict ourselves to utf16, then we could only store 2 bytes per character, which would equal 280 bytes).
2. Compress the image down using standard jpg compression.
   • Resize the image to be approximately 50x50px, then attempt to compress it at various compression levels until you have an image that is as close to 542 bytes as possible without going over.
   • This is an example of the mona lisa compressed down to 536 bytes.
3. Encode the raw bits of the compressed image into utf-8 characters.
   • Replace each x in the following bytes: 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx, with the bits from the image.
   • This part would probably be the part where the majority of the code would need to be written, because there isn't anything that currently exists that does this.

I know that you were asking for code, but I don't really want to spend the time to actually code this up. I figured that an efficient design might at least inspire someone else to code this up.

I think the major benefit of my proposed solution is that it is reusing as much existing technology as possible. It may be fun to try to write a good compression algorithm, but there is guaranteed to be a better algorithm out there, most likely written by people who have a degree in higher math.

One other important note though is that if it is decided that utf16 is the preferred encoding, then this solution falls apart. jpegs don't really work when compressed down to 280 bytes. Although, maybe there is a better compression algorithm than jpg for this specific problem statement.

Answer 2

This genetic algorithm that Roger Alsing wrote has a good compression ratio, at the expense of long compression times. The resulting vector of vertices could be further compressed using a lossy or lossless algorithm.

http://rogeralsing.com/2008/12/07/genetic-programming-evolution-of-mona-lisa/

Would be an interesting program to implement, but I'll give it a miss.

Answer 3

I'd be interested in seeing how much hidden data could be encoded in what would be a human readable message. Even if it was enough for just a url it would be useful.

Answer 4

Hi very intersting challenge,

I have several stupid questions as I am trying to size the challenge:

• where is the original picture (the one above has two images, including your result).
• how many pixels does the original picture have ? how many bytes goes into the encoding of each of the three RGB colors in the original picture ?
• I have a specific solution in mind but, in your challenge is there some type of requirement that the encoder is lightweight (i.e. simple, not CPU intensive) ?

Cheers,

Igor.

Answer 5

Brian,

So if I am reading this challenge correctly, we are looking to see how large of an image we can get by using only 350 bytes of space. Thanks.

Igor.

Answer 6

In the original challenge the size limit is defined as what Twitter still allows you to send if you paste your text in their textbox and press "update". As some people correctly noticed this is different from what you could send as a SMS text message from your mobile.

What is not explictily mentioned (but what my personal rule was) is that you should be able to select the tweeted message in your browser, copy it to the clipboard and paste it into a text input field of your decoder so it can display it. Of course you are also free to save the message as a text file and read it back in or write a tool which accesses the Twitter API and filters out any message that looks like an image code (special markers anyone? wink wink). But the rule is that the message has to have gone through Twitter before you are allowed to decode it.

Good luck with the 350 bytes - I doubt that you will be able to make use of them.

Answer 7

Who is willing to post sample code to read and write image files - I'm sure there are people with ideas (like me) but getting the data into a usable structure is a barrier to entry (mainly due to the time frame).

Answer 8

Posting a Monochrome or Greyscale image should improve the size of the image that can be encoded into that space since you don't care about colour.

Possibly augmenting the challenge to upload three images which when recombined give you a full colour image while still maintaining a monochrome version in each separate image.

Add some compression to the above and It could start looking viable...

Nice!!! Now you guys have piqued my interest. No work will be done for the rest of the day...

Answer 9

The idea of storing a bunch of reference images is interesting. Would it be so wrong to store say 25Mb of sample images, and have the encoder try and compose an image using bits of those? With such a minuscule pipe, the machinery at either end is by necessity going to be much greater than the volume of data passing through, so what's the difference between 25Mb of code, and 1Mb of code and 24Mb of image data?

(note the original guidelines ruled out restricting the input to images already in the library - I'm not suggesting that).

Answer 10

Is it acceptable to use something like PIL (python imaging library) for all the imaging functions, and then a custom function or two to actually encode the image into unicode?

EDIT: just read the above post, already been answered

Answer 11

My first thought was along the lines of fractal compression, but the maths behind it are quite complex and often patented (grrr...) and I didn't have the time to reasearch the subject and try an implementation in the time limit for this challenge. From what I've seen over the years, the results are quite good, the decompressed image can be upscaled without becoming blocky. Here's Wikipedia's article an fractal compression for anyone interested - there's quite a few links at the bottom of the page for further reading.

Anyway, here's my rule breaking entry. It's a zipped exe command line tool written in C# using .Net 2.0 - tested on WinXP. Here's the command to 'decode' one of the sample images above:

TwitterCodec "4,-5A*>8=D/1/DB=F:.834=3B-A54B<95<3453;2G@210"

Type the command with no arguments for command line format.

Skizz

EDIT: OK, maybe that was breaking the rules a lot. Still, it allows you to 'encode' a file into a small string and 'decode' it anywhere else. It just doesn't do any compression.

EDIT 2: The code is just an FTP client that copies the image and image name to a server and encodes the URL as output for retrieval at a later date.

Answer 12

Alright, here's mine: nanocrunch.cpp and the CMakeLists.txt file to build it using CMake. It relies on the Magick++ ImageMagick API for most of its image handling. It also requires the GMP library for bignum arithmetic for its string encoding.

I based my solution off of fractal image compression, with a few unique twists. The basic idea is to take the image, scale down a copy to 50% and look for pieces in various orientations that look similar to non-overlapping blocks in the original image. It takes a very brute force approach to this search, but that just makes it easier to introduce my modifications.

The first modification is that instead of just looking at ninety degree rotations and flips, my program also considers 45 degree orientations. It's one more bit per block, but it helps the image quality immensely.

The other thing is that storing a contrast/brightness adjustment for each of color component of each block is way too expensive. Instead, I store a heavily quantized color (the palette has only 4 * 4 * 4 = 64 colors) that simply gets blended in in some proportion. Mathematically, this is equivalent to a variable brightness and constant contrast adjustment for each color. Unfortunately, it also means there's no negative contrast to flip the colors.

Once it's computed the position, orientation and color for each block, it encodes this into a UTF-8 string. First, it generates a very large bignum to represent the data in the block table and the image size. The approach to this is similar to Sam Hocevar's solution -- kind of a large number with a radix that varies by position.

Then it converts that into a base of whatever the size of the character set available is. By default, it makes full use of the assigned unicode character set, minus the less than, greater than, ampersand, control, combining, and surrogate and private characters. It's not pretty but it works. You can also comment out the default table and select printable 7-bit ASCII (again excluding <, >, and & characters) or CJK Unified Ideographs instead. The table of which character codes are available is stored a run-length encoded with alternating runs of invalid and valid characters.

Anyway, here are some images and times (as measured on my old 3.0GHz P4), and compressed to 140 characters in the full assigned unicode set described above. Overall, I'm fairly pleased with how they all turned out. If I had more time to work on this, I'd probably try to reduce the blockiness of the decompressed images. Still, I think the results are pretty good for the extreme compression ratio. The decompressed images are bit impressionistic, but I find it relatively easy to see how bits correspond to the original.

Stack Overflow Logo (8.6s to encode, 7.9s to decode, 485 bytes):

Lena (32.8s to encode, 13.0s to decode, 477 bytes):

Mona Lisa (43.2s to encode, 14.5s to decode, 490 bytes):

Edit: CJK Unified Characters

Sam asked in the comments about using this with CJK. Here's a version of the Mona Lisa compressed to 139 characters from the CJK Unified character set:

咏璘驞凄脒鵚据蛥鸂拗朐朖辿韩瀦魷歪痫栘璯緍脲蕜抱揎頻蓼債鑡嗞靊寞柮嚛嚵籥聚隤慛絖銓馿渫櫰矍昀鰛掾撄粂敽牙稉擎蔍螎葙峬覧絀蹔抆惫冧笻哜搀澐芯譶辍澮垝黟偞媄童竽梀韠镰猳閺狌而羶喙伆杇婣唆鐤諽鷍鴞駫搶毤埙誖萜愿旖鞰萗勹鈱哳垬濅鬒秀瞛洆认気狋異闥籴珵仾氙熜謋繴茴晋髭杍嚖熥勳縿餅珝爸擸萿

The tuning parameters at the top of the program that I used for this were: 19, 19, 4, 4, 3, 10, 11, 1000, 1000. I also commented out the first definition of number_assigned and codes, and uncommented out the last definitions of them to select the CJK Unified character set.

Answer 13

Is it possible to use text compression on the resulting unicode string?

Answer 14

Stupid idea, but sha1(my_image) would result in a "perfect" representation of any image (ignoring collisions). The obvious problem is the decoding process requires inordinate amounts of brute-forcing..

1-bit monochrome would be a bit easier.. Each pixel becomes a 1 or 0, so you would have 1000 bits of data for a 100*100 pixel image. Since the SHA1 hash is 41 characters, we can fit three into one message, only have to brute force 2 sets of 3333 bits and one set of 3334 (although even that is probably still inordinate)

It's not exactly practical. Even with the fixed-length 1-bit 100*100px image there is.., assuming I'm not miscalculating, 49995000 combinations, or 16661667 when split into three.

def fact(maxu):
        ttl=1
        for i in range(1,maxu+1):
                ttl=ttl*i
        return ttl

def combi(setsize, length):
    return fact(length) / (fact(setsize)*fact(length-setsize))

print (combi(2, 3333)*2) + combi(2, 3334)
# 16661667L
print combi(2, 10000)
# 49995000L

Answer 15

Here this compression is good.

www.intuac.com/userport/john/apt/

http://img86.imageshack.us/img86/4169/imagey.jpg

I used the following batch file:

capt mona-lisa-large.pnm out.cc 20
dapt out.cc image.pnm
Pause

The resulting filesize is 559 bytes.

Answer 16

Regarding the encoding/decoding part of this challenge. base16b.org is my attempt to specify a standard method for safely and efficiently encoding binary data in the higher Unicode planes.

Some features :

• Uses only Unicode's Private User Areas
• Encodes up to 17 bits per character; nearly three times more efficient than Base64
• A reference Javascript implementation of encode/decode is provided
• Some sample encodings are included, including Twitter and Wordpress

Sorry, this answer comes way too late for the original competition. I started the project independently of this post, which I discovered half-way into it.

Answer 17

Okay, I'm late to the game, but nevertheless I made my project.

It's a toy genetic algorithm that uses translucent colorful circles to recreate the initial image.

Features:

• pure Lua. Runs anywhere where a Lua interpreter runs.
• uses netpbm P3 format
• comes with a comprehensive suite of unit tests
• preserves original image size

Mis-feautres:

• slow
• at this space constraints it preserves only the basic color scheme of the initial image and a general outline of few features thereof.

Here's an example twit that represents Lena: 犭楊谷杌蒝螦界匘玏扝匮俄归晃客猘摈硰划刀萕码摃斢嘁蜁嚎耂澹簜僨砠偑婊內團揕忈義倨襠凁梡岂掂戇耔攋斘眐奡萛狂昸箆亲嬎廙栃兡塅受橯恰应戞优猫僘瑩吱賾卣朸杈腠綍蝘猕屐稱悡詬來噩压罍尕熚帤厥虤嫐虲兙罨縨炘排叁抠堃從弅慌螎熰標宑簫柢橙拃丨蜊缩昔儻舭勵癳冂囤璟彔榕兠摈侑蒖孂埮槃姠璐哠眛嫡琠枀訜苄暬厇廩焛瀻严啘刱垫仔

The code is in a Mercurial repository at bitbucket.org. Check out http://bitbucket.org/tkadlubo/circles.lua