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spec.html
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<!DOCTYPE html>
<html lang="en">
<head>
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<meta name="author" content="Jon Sneyers, other contributors: add yourself here">
<title>FLIF16 Specification</title>
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<body class="article toc2 toc-right">
<div id="header">
<h1>FLIF16 Specification</h1>
<div class="details">
<span id="author" class="author">Jon Sneyers</span><br>
<span id="email" class="email"><a href="mailto:jon@cloudinary.com">jon@cloudinary.com</a></span><br>
<span id="author2" class="author">other contributors: add yourself here</span><br>
<span id="revdate">Work in progress</span>
</div>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_overview_of_the_format">Overview of the format</a></li>
<li><a href="#_notational_conventions">Notational conventions</a></li>
<li><a href="#_part_1_main_header">Part 1: main header</a></li>
<li><a href="#_part_2_metadata">Part 2: metadata</a></li>
<li><a href="#_symbol_encoding">Symbol encoding</a>
<ul class="sectlevel2">
<li><a href="#_range_encoder">Range encoder</a></li>
<li><a href="#_uniform_symbol_coding">Uniform Symbol Coding</a></li>
<li><a href="#_near_zero_symbol_coding">Near-zero symbol coding</a></li>
</ul>
</li>
<li><a href="#_part_3_second_header">Part 3: second header</a>
<ul class="sectlevel2">
<li><a href="#_transformation_0_channelcompact">Transformation 0: ChannelCompact</a></li>
<li><a href="#_transformation_1_ycocg">Transformation 1: YCoCg</a></li>
<li><a href="#_transformation_3_permuteplanes">Transformation 3: PermutePlanes</a></li>
<li><a href="#_transformation_4_bounds">Transformation 4: Bounds</a></li>
<li><a href="#_transformation_5_palettealpha">Transformation 5: PaletteAlpha</a></li>
<li><a href="#_transformation_6_palette">Transformation 6: Palette</a></li>
<li><a href="#_transformation_7_colorbuckets">Transformation 7: ColorBuckets</a></li>
<li><a href="#_transformation_10_duplicateframe">Transformation 10: DuplicateFrame</a></li>
<li><a href="#_transformation_11_frameshape">Transformation 11: FrameShape</a></li>
<li><a href="#_transformation_12_framelookback">Transformation 12: FrameLookback</a></li>
</ul>
</li>
<li><a href="#_maniac_entropy_coding">MANIAC entropy coding</a></li>
<li><a href="#_part_4_pixel_data">Part 4: pixel data</a>
<ul class="sectlevel2">
<li><a href="#_non_interlaced_method">Non-Interlaced method</a></li>
<li><a href="#_interlaced_method">Interlaced method</a></li>
<li><a href="#_pixel_encoding">Pixel encoding</a></li>
<li><a href="#_pixel_predictors">Pixel predictors</a></li>
<li><a href="#_maniac_properties">MANIAC properties</a></li>
<li><a href="#_maniac_tree_encoding">MANIAC tree encoding</a></li>
<li><a href="#_checksum">Checksum</a></li>
</ul>
</li>
</ul>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph">
<p>The Free Lossless Image Format (FLIF) is a lossless image compression format.
It supports grayscale, RGB and RGBA, with a color depth from 1 to 16 bits per channel.
Both still images and animations are supported.
FLIF can handle sparse-color images (e.g. 256 color palettes) effectively.
It has an interlaced and a non-interlaced mode; unlike PNG, interlacing usually does not come with worse compression.
In terms of compression, FLIF usually outperforms other lossless compression formats like PNG, lossless WebP, BPG, JPEG 2000, JPEG XR, JPEG-LS, APNG, MNG, or GIF.</p>
</div>
<div class="paragraph">
<p>This document specifies the FLIF16 bitstream, as implemented by version 0.2 of the reference encoder (<a href="https://github.com/FLIF-hub/FLIF" class="bare">https://github.com/FLIF-hub/FLIF</a>), which was released in September 2016.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_overview_of_the_format">Overview of the format</h2>
<div class="sectionbody">
<div class="paragraph">
<p>A FLIF file consists of 4 parts, with increasingly complicated encoding methods:</p>
</div>
<div class="olist arabic">
<ol class="arabic">
<li>
<p>The main header, containing basic image metadata: width, height, color depth, number of frames. This information is encoded in a straightforward way, to make it easy to quickly identify a file.</p>
</li>
<li>
<p>Optional metadata chunks. These chunks contain non-pixel data: Exif/XMP metadata, an ICC color profile, …​ This information is encoded using DEFLATE compression.</p>
</li>
<li>
<p>Second header, containing further information about how the actual pixel data is encoded. This header also describes a chain of transformations that were applied before encoding, which have to be reversed after decoding (e.g. the YCoCg color transform). This information is encoded using CABAC entropy coding.</p>
</li>
<li>
<p>The actual pixel data. This information is encoded using MANIAC entropy coding.</p>
</li>
</ol>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_notational_conventions">Notational conventions</h2>
<div class="sectionbody">
<div class="paragraph">
<p>We use '/' to denote integer division (rounding towards zero) and '>>' to denote arithmetic right shift. This corresponds to the usual operator semantics in e.g. the C programming language.</p>
</div>
<div class="admonitionblock important">
<table>
<tr>
<td class="icon">
<i class="fa icon-important" title="Important"></i>
</td>
<td class="content">
>>1 denotes arithmetic right shift by one, which corresponds to dividing by two <strong>rounding down</strong>.
For positive integers this is the same as /2, but for negative integers it is not, e.g. -1 / 2 = 0 while -1 >> 1 = -1.
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_part_1_main_header">Part 1: main header</h2>
<div class="sectionbody">
<table class="tableblock frame-all grid-all spread">
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
<th class="tableblock halign-left valign-top">Value</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">4 bytes</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Magic</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>"FLIF"</code></p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">4 bits</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Interlacing, animation</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">3 = ni still; 4 = i still; 5 = ni anim; 6 = i anim</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">4 bits</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Number of channels (<strong>nb_channels</strong>)</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">1 = Grayscale; 3 = RGB; 4 = RGBA</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">1 byte</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Bytes per channel (<strong>Bpc</strong>)</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>'0'</code>,<code>'1'</code>,<code>'2'</code> (<code>'0'</code>=custom)</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">varint</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Width</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><strong>width</strong>-1</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">varint</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Height</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><strong>height</strong>-1</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">varint</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Number of frames (<strong>nb_frames</strong>)</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><strong>nb_frames</strong>-2 (only if animation)</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>The fifth byte is an ASCII character that can be interpreted as follows:
for non-interlaced still images: <code>'1'</code>=Grayscale, <code>'3'</code>=RGB, <code>'4'</code>=RGBA,
for interlaced images add +0x10 (so <code>'A'</code>=Grayscale, <code>'C'</code>=RGB, <code>'D'</code>=RGBA),
for animations add +0x20 (so <code>'Q'</code>, <code>'S'</code>, <code>'T'</code> for non-interlaced, <code>'a'</code>,<code>'c'</code>,<code>'d'</code> for interlaced).</p>
</div>
<div class="paragraph">
<p>Variable-size integer encoding (varint):</p>
</div>
<div class="ulist">
<ul>
<li>
<p>An unsigned integer (Big-Endian, MSB first) stored on a variable number of bytes.</p>
</li>
<li>
<p>All the bytes except the last one have a '1' as their first bit.</p>
</li>
<li>
<p>The unsigned integer is represented as the concatenation of the remaining 7 bit codewords.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p><strong>nb_frames</strong> is 1 for a still image, > 1 for an animation.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_part_2_metadata">Part 2: metadata</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Chunks are defined similar to PNG chunks, except the chunk size is encoded with a variable number of bytes, and there is no chunk CRC.
Also, the first chunk (which is always "FLIF") has no explicit chunk size and is not DEFLATE-compressed.</p>
</div>
<div class="paragraph">
<p>Chunk names are either 4 letters (4 bytes), indicating an optional chunk, or 1 byte with a value below 32, indicating a non-optional chunk.</p>
</div>
<div class="paragraph">
<p>The convention of using upper and lower case letters is kept, but the meaning of the bits is slightly different.</p>
</div>
<div class="ulist">
<ul>
<li>
<p>First letter: uppercase=critical, lowercase=non-critical → non-critical chunks can be stripped while keeping a valid file (it might be rendered differently though)</p>
</li>
<li>
<p>Second letter: uppercase=public, lowercase=private</p>
</li>
<li>
<p>Third letter: uppercase=needed to correctly display the image (e.g. a color profile), lowercase=can be stripped safely without changing anything visually</p>
</li>
<li>
<p>Fourth letter: uppercase=safe to copy blindly (when editing the actual image data), lowercase=may depend on image data (e.g. contain a thumbnail)</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>This optional part is a concatenation of chunks that each look like this:</p>
</div>
<table class="tableblock frame-all grid-all spread">
<colgroup>
<col style="width: 50%;">
<col style="width: 50%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Type</th>
<th class="tableblock halign-left valign-top">Description</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">4 bytes</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Chunk name</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">varint</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Chunk length (<em>size</em>)</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock"><em>size</em> bytes</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">DEFLATE-compressed chunk content</p></td>
</tr>
</tbody>
</table>
<div class="paragraph">
<p>The optional chunks are followed by one non-optional chunk with a single-byte chunk name.
Non-optional chunks do not have an explicit chunk size and their content is not DEFLATE-compressed.
Currently the only non-optional chunk has name 0x00 (a NUL byte), which indicates a FLIF16 bitstream.
Future (non-backwards compatible) revisions of the FLIF format will use a different name for this non-optional chunk.</p>
</div>
<div class="paragraph">
<p>If a decoder encounters an unknown optional chunk, it should proceed decoding (optionally it can produce a warning),
as long as the chunk is non-critical.
If a decoder encounters an unknown non-optional chunk, it should fail and produce an error message.</p>
</div>
<div class="paragraph">
<p>The following optional chunks are defined:</p>
</div>
<table class="tableblock frame-all grid-all spread">
<colgroup>
<col style="width: 33.3333%;">
<col style="width: 33.3333%;">
<col style="width: 33.3334%;">
</colgroup>
<thead>
<tr>
<th class="tableblock halign-left valign-top">Chunk name</th>
<th class="tableblock halign-left valign-top">Description</th>
<th class="tableblock halign-left valign-top">Content (after DEFLATE-decompression)</th>
</tr>
</thead>
<tbody>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">iCCP</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">ICC color profile</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">raw ICC color profile data</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">eXif</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">Exif metadata</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock"><code>"Exif\0\0"</code> header followed immediately by a TIFF header and the EXIF data</p></td>
</tr>
<tr>
<td class="tableblock halign-left valign-top"><p class="tableblock">eXmp</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">XMP metadata</p></td>
<td class="tableblock halign-left valign-top"><p class="tableblock">XMP contained within a read-only xpacket with no padding</p></td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="sect1">
<h2 id="_symbol_encoding">Symbol encoding</h2>
<div class="sectionbody">
<div class="paragraph">
<p>From here on, all values are encoded using arithmetic coding.</p>
</div>
<div class="paragraph">
<p>uni_int(<em>min</em>,<em>max</em>) denotes an integer in the interval <em>min</em>..<em>max</em> (inclusive), encoded using Uniform symbol coding (see below) and a 24-bit RAC. This encoding is not context-adaptive, and every number in the interval gets a fixed chance (close to a uniform distribution, hence the name).
uni_int(<em>b</em>) denotes an integer of <em>b</em> bits, i.e. it’s a synonym for uni_int(0,2^<em>b</em>-1).</p>
</div>
<div class="paragraph">
<p>nz_int_<strong>CONTEXT</strong>(<em>min</em>,<em>max</em>) denotes an integer in the interval <em>min</em>..<em>max</em> (inclusive), encoded using Near-zero symbol coding (see below) and a 24-bit RAC, where the chance table is given by <strong>CONTEXT</strong>. This encoding is context-adaptive, i.e. the chance table gets updated when encoding or decoding a number. Numbers are encoded using a zero-sign-exponent-mantissa representation, which favors distributions in which near-zero values have a higher chance (hence the name).</p>
</div>
<div class="paragraph">
<p>gnz_int_<strong>CONTEXT</strong>(<em>min</em>,<em>max</em>) denotes a simple generalization of Near-zero symbol coding that can be used in case the interval does not necessarily include zero. It is defined as follows:</p>
</div>
<div class="paragraph">
<p>gnz_int_<strong>CONTEXT</strong>(<em>min</em>,<em>max</em>) =</p>
</div>
<div class="hdlist">
<table>
<tr>
<td class="hdlist1">
nz_int_<strong>CONTEXT</strong>(0, <em>max</em> - <em>min</em>) + <em>min</em>
</td>
<td class="hdlist2">
<p>if <em>min</em> > 0</p>
</td>
</tr>
<tr>
<td class="hdlist1">
nz_int_<strong>CONTEXT</strong>(<em>min</em> - <em>max</em>, 0) + <em>max</em>
</td>
<td class="hdlist2">
<p>if <em>max</em> < 0</p>
</td>
</tr>
<tr>
<td class="hdlist1">
nz_int_<strong>CONTEXT</strong>(<em>min</em>,<em>max</em>)
</td>
<td class="hdlist2">
<p>otherwise</p>
</td>
</tr>
</table>
</div>
<div class="sect2">
<h3 id="_range_encoder">Range encoder</h3>
<div class="paragraph">
<p>TODO: describe the 24-bit RAC and the 12-bit chances.</p>
</div>
<div class="sect3">
<h4 id="_chance_table_update">Chance table update</h4>
<div class="paragraph">
<p>TODO: describe how <strong>cutoff</strong> and <strong>alpha</strong> influence this.</p>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_uniform_symbol_coding">Uniform Symbol Coding</h3>
<div class="paragraph">
<p>An integer encoded with Uniform symbol coding uses a series of 50% chance bits (chance is 2048/4096, non-adaptive)
to recursively narrow down the input interval until it is a singleton.
The following code describes how to decode such an integer:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="CodeRay highlight"><code data-lang="c"> <span class="comment">// assumption: min <= max</span>
<span class="predefined-type">int</span> read_uniform_int(<span class="predefined-type">int</span> min, <span class="predefined-type">int</span> max) {
<span class="keyword">if</span> (min == max) <span class="keyword">return</span> min;
<span class="comment">// split in [min..mid] [mid+1..max]</span>
<span class="predefined-type">int</span> med = min + (max-min)/<span class="integer">2</span>;
<span class="keyword">if</span> (rac.read_bit()) {
<span class="keyword">return</span> read_uniform_int(mid+<span class="integer">1</span>, max);
} <span class="keyword">else</span> {
<span class="keyword">return</span> read_uniform_int(min, mid);
}
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>There is no compression advantage of using a RAC to encode these 50% chance bits (just regular bits would be just as efficient),
but it is convenient to use the RAC anyway to avoid having to mix the RAC bitstream with regular bits.</p>
</div>
<div class="paragraph">
<p>For intervals of the form 0..2^<em>k</em> this representation boils down to writing down the number as <em>k</em> binary bits;
for other intervals it can be slightly more concise than binary notation.</p>
</div>
<div class="paragraph">
<p>This representation is used mostly for simple signaling, where context adaptation would be of little use.</p>
</div>
</div>
<div class="sect2">
<h3 id="_near_zero_symbol_coding">Near-zero symbol coding</h3>
<div class="paragraph">
<p>This is the representation used for the bulk of the data, including the pixel values. It is very efficient for values that
are close to zero (e.g. well-predicted pixels).</p>
</div>
<div class="paragraph">
<p>An integer encoded with Near-zero symbol coding uses a zero-sign-exponent-mantissa representation, where the exponent is represented
in unary notation.
Every bit position of this representation has its own RAC context (so their chances adapt separately); the exponent bits use different chances
not just depending on their position but also depending on the sign.
Chances are represented as 12-bit numbers (1..4095), where 1 means that the bit is very likely to be zero (it has 1/4096 chance of being one),
while 4095 means that the bit is very likely to be one (the chance is 4095/4096).</p>
</div>
<div class="paragraph">
<p>The following code describes how to decode an integer in this representation:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="CodeRay highlight"><code data-lang="c"> <span class="comment">// assumption: min <= 0 <= max</span>
<span class="predefined-type">int</span> read_nearzero_int(<span class="predefined-type">int</span> min, <span class="predefined-type">int</span> max) {
<span class="keyword">if</span> (min == max) <span class="keyword">return</span> min;
<span class="keyword">if</span> (rac.read(ZERO)) <span class="keyword">return</span> <span class="integer">0</span>;
<span class="predefined-type">bool</span> sign; <span class="comment">// true = positive, false = negative</span>
<span class="keyword">if</span> (min < <span class="integer">0</span>) {
<span class="keyword">if</span> (max > <span class="integer">0</span>) {
<span class="comment">// only actually read the sign bit if both signs are possible</span>
sign = rac.read(SIGN);
} <span class="keyword">else</span> {
<span class="comment">// max <= 0 and the number is not zero, so it is negative</span>
sign = <span class="predefined-constant">false</span>;
}
} <span class="keyword">else</span> {
<span class="comment">// min >= 0 and the number is not zero, so it is positive</span>
sign = <span class="predefined-constant">true</span>;
}
<span class="comment">// highest possible absolute value</span>
<span class="directive">const</span> <span class="predefined-type">int</span> amax = (sign? max : -min);
<span class="comment">// highest possible exponent</span>
<span class="directive">const</span> <span class="predefined-type">int</span> emax = ilog2(amax);
<span class="comment">// exponent is represented in unary: a series of 1s followed by 0</span>
<span class="comment">// (the end 0 is dropped if e=emax)</span>
<span class="predefined-type">int</span> e;
<span class="keyword">for</span> (e = <span class="integer">0</span> ; e < emax ; e++) {
<span class="keyword">if</span> (rac.read(EXP(e,sign)) <span class="keyword">break</span>;
}
<span class="comment">// the first mantissa bit is always 1</span>
<span class="predefined-type">int</span> have = (<span class="integer">1</span> << e);
<span class="comment">// if all other mantissa bits are 1, then the total is have+left</span>
<span class="predefined-type">int</span> left = have-<span class="integer">1</span>;
<span class="comment">// read mantissa bits from most-significant to least-significant</span>
<span class="keyword">for</span> (<span class="predefined-type">int</span> pos = e; pos><span class="integer">0</span>;) {
left >>= <span class="integer">1</span>; pos--;
<span class="comment">// if the bit is 1, then the value will be at least minabs1</span>
<span class="predefined-type">int</span> minabs1 = have | (<span class="integer">1</span><<pos);
<span class="comment">// if the bit is 0, then the value will be at most maxabs0</span>
<span class="predefined-type">int</span> maxabs0 = have | left;
<span class="keyword">if</span> (minabs1 > amax) {
<span class="comment">// 1-bit is impossible (would bump value above maximum),</span>
<span class="comment">// so assume the bit is 0 without reading it</span>
} <span class="keyword">else</span> <span class="keyword">if</span> (maxabs0 >= <span class="integer">1</span>) {
<span class="comment">// 0-bit and 1-bit are both possible,</span>
<span class="comment">// so we read the bit and adjust what we have if it is a 1</span>
<span class="keyword">if</span> (rac.read(MANT(pos))) have = minabs1;
} <span class="keyword">else</span> {
<span class="comment">// 0-bit is impossible (would make the value zero),</span>
<span class="comment">// so assume the bit is 1 without reading it</span>
have = minabs1;
}
}
<span class="keyword">return</span> (sign ? have : -have);
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>The chances are initialized as follows:</p>
</div>
<table class="tableblock frame-all grid-all spread">
<colgroup>
<col style="width: 50%;">
<col style="width: 50%;">
</colgroup>
<thead>
<tr>