<abstract>This specification provides a set of algorithms to consistently generate colors given a string. The string can be a nickname, a JID or any other piece of information. All entities adhering to this specification generate the same color for the same string, which provides a consistent user experience across platforms.</abstract>
<remark><p>Accepted as Experimental by Council.</p></remark>
</revision>
<revision>
<version>0.0.1</version>
<date>2017-09-14</date>
<initials>jwi</initials>
<remark><p>First draft.</p></remark>
</revision>
</header>
<section1topic='Introduction'anchor='intro'>
<p>Colors provide a valuable visual cue to recognize text. Recognition of colors works much faster than recognition of text. Together with the length and overall shape of a piece of text (such as a nickname), a color provides a decent amount of entropy to distinguish a reasonable amount of entities.</p>
<p>Clients have been using randomly or deterministically chosen colors for users in multi-user situations for a long time already. However, since there has been no standard for how this is implemented, the experience differs across platforms. The goal of this XEP is to provide a uniform, platform-independent, stateless and easy-to-implement way to map arbitrary bytestrings to colors, as well as give recommendations how this is applied to color names of participants in conversations, roster entries and other pieces of text.</p>
<p>To allow cross-client use, it is important that the color scheme can be adapted to different environments. This specification provides means to adapt colors to different background colors as well as &cvds;.</p>
<p>In no way is the system presented in this specification a replacement for names. It only serves as an additional visual aid.</p>
<p>The color generation mechanism should provide the following features:</p>
<ul>
<li>Consistent generation of color across all platforms depending solely on the identifier used as input for the algorithm.</li>
<li>The system should be reasonably fast; it must be possible to, for example, apply it to all roster entries even of very large rosters in reasonable amount of time.</li>
<li>It must be able to provide decent contrast on any background.</li>
<li>The implementation should be stateless and not be complex.</li>
<li>A fallback path for users with common types of &cvds; must be provided.</li>
<li>A fallback path for systems which can only use colors from a restricted palette must be provided.</li>
</ul>
</section1>
<section1topic='Use Cases'anchor='usecases'>
<section2topic='Generating a color'anchor='usecase-textcolor'>
<p>To generate a color from a string of text, the follownig algorithms are applied in order:</p>
<ol>
<li><linkurl='#algorithm-angle'>Generate an angle in the CbCr plane from the text</link>.</li>
<li>If enabled, <linkurl='#algorithm-cvd'>apply configured corrections for &cvds;</link>.</li>
<li><linkurl='#algorithm-cbcr'>Convert the angle to a CbCr pair</link>.</li>
<li>If the output device only supports a small palette of colors, <linkurl='#algorithm-mappalette'>map the CbCr value to the closest palette color</link>.</li>
<li>If the output device supports RGB output, <linkurl='#algorithm-rgb'>convert the CbCr pair to an RGB triple</link>.</li>
</ol>
</section2>
<section2topic='Adding colors to participants of a conversation'anchor='usecase-nickcolor'>
<p>Implementations may colorize the participants of a conversation with an individual color to make them easier to distinguish.</p>
<p>In such cases, the color SHOULD be generated as described in the <linkurl='#usecase-textcolor'>Generating a color</link> section. The input used SHOULD be, in descending order of preference, (a) the nickname from the conversation, (b) the bare JID.</p>
<p>Implementations may want to show a picture in connection with a contact even if the contact does not have an avatar defined (e.g. via &xep0084;).</p>
<p>In such cases, auto-generating an avatar SHOULD happen as follows:</p>
<li>Obtain a name for the contact, in descending order of preference, (a) by using the actual bare JID of the contact (<em>not</em> the bare JID of the conference in case of a XEP-0045 MUC), (b) by using the nickname from the conversation.</li>
<p>Input: An identifier, encoded as octets of UTF-8 (&rfc3269;).</p>
<p>Output: Angle in the CbCr plane.</p>
<p>Note: The goal of this algorithm is to convert arbitrary text into a scalar value which can then be used to calculate a color. As it happens, the CbCr plane of the YCbCr space determines the color (while Y merely defines the lightness); thus, an angle in the CbCr plane serves as a good scalar value to select a color.</p>
<ol>
<li>Run the input through CRC32 as defined by zlib (TODO: add citation).</li>
<li>Take the lower 16 bits and XOR them with the upper 16 bits.</li>
<li>Divide the value by 65535 (use float division) and multiply it by 2π (two Pi).</li>
</ol>
</section2>
<section2topic='Corrections for &cvds;'anchor='algorithm-cvd'>
<p>Input: Angle in the CbCr plane.</p>
<p>Output: Angle in the CbCr plane.</p>
<p>Note: This algorithm will re-map the angle to map it away from ranges which can not be distinguished by people with the respective &cvds;.</p>
<p>Form a vector from the angle and project it to edges of a quad in 2D space with edge length 1 around (0, 0). The resulting coordinates are Cb and Cr:</p>
<code><![CDATA[float cr = sin(angle);
float cb = cos(angle);
float factor;
if (abs(cr) > abs(cb)) {
factor = 0.5 / abs(cr);
} else {
factor = 0.5 / abs(cb);
}
cb = cb * factor;
cr = cr * factor;
]]></code>
</section2>
<section2topic='CbCr to RGB'anchor='algorithm-rgb'>
<section2topic='Conversion of an RGB color palette to a CbCr color palette'anchor='algorithm-genpalette'>
<p>Input: A set of RGB colors (each component from 0 to 1).</p>
<p>Output: A mapping from CbCr pairs (each component from -0.5 to 0.5) to RGB colors.</p>
<p>Note: when the algorithm finishes, the mapping maps CbCr values (rounded to two decimal places) to the R, G, B triples which come closest to the desired color and lightness.</p>
<ol>
<li>Create an empty mapping M which maps from pairs of CbCr values to quadruples of Y, R, G and B.</li>
<li>For each color R, G, B from the input palette:
<ol>
<li>Calculate Y, Cb and Cr from R, G, B as described in <linkurl='#algorithm-rgb2cbcr'>RGB to YCbCr</link>.</li>
<li>Round Cb and Cr to two decimal places as Cb' and Cr'.</li>
<li>If the (Cb', Cr') pair is not in the mapping M yet, or if the Y value of the existing entry is farther away from 0.732 than the new Y value, put the Y, R, G, and B values as value for the (Cb', Cr') pair into the mapping.</li>
</ol>
</li>
<li>Strip the Y values from the values of mapping M.</li>
<li>Return M as the result of the algorithm.</li>
</ol>
</section2>
<section2topic='Mapping of a CbCr color to closest palette color'anchor='algorithm-mappalette'>
<p>Input: A set of colors (the palette) as tuples of Cbp and Crp and a color to map to the closest palette color as Cb and Cr value.</p>
<p>Output: A palette color as Cbr and Crr values.</p>
<ol>
<li>For each color as pair Cbp, Crp in the palette, calculate the distance metric: <code>D = sqrt((Cbp-Cb)*(Cbp-Cb) + (Crp-Cr)*(Crp-Cr))</code>.</li>
<li>Pick the palette color Cbp, Crp with the smallest distance metric D as result color Cbr, Crr.</li>
</ol>
<p>Note: the distance metric is simply the euclidian distance in the CbCr plane.</p>
<p>An implementation may choose a different value for Y depending on whether the sink for the R, G and B values expects Gamma Encoded or Gamma Decoded values. The recommended default of 0.732 is 0.5 to the power of 0.45, that is, a Gamma Encoded 0.5.</p>
<p>Modifications to Y SHOULD NOT be used to correct for bright/dark backgrounds. Implementations SHOULD instead use the algorithm described in <linkurl='#algorithm-bg'>Adapting the Color for specific Background Colors</link> for that.</p>
<p>As outlined above, implementations SHOULD offer the &rgblind; and &bblind; corrections as defined in the <linkurl='#algorithm-cvd'>Corrections for &cvds;</link> section. Users SHOULD be allowed to choose between:</p>
<li>disabling all corrections (skip the Corrections for &cvds; step entirely),</li>
<li>applying one of the &cvd; correction profiles and</li>
<li>disabling colorization altogether.</li>
</ul>
<p>The last option is important for users with monochromatic view.</p>
<p>Some sources on the internet indicate that people with &cvds; may profit from having larger areas of color to be able to recognize them. This should be taken into consideration when selecting font weights and line widths for colored parts.</p>
<p>This specification extracts a bit more information from an entity and shows it alongside the existing information to the user. As the algorithm is likely to produce different colors for look-alikes (see &xep0165; for examples) in JIDs, it may add additional protection against attacks based on those.</p>
<p>Due to the limited set of distinguishable colors and only extracting 16 bits of the hash function output, possible &cvds; and/or use of palettes, entities MUST NOT rely on colors being unique in any context.</p>
<p>This section provides an overview of design considerations made while writing this specification. It shows alternatives which have been considered, and eventually rejected.</p>
<section2topic='Other variants of the YCbCr color space'anchor='design-other-ycbcr'>
<p>The other common YCbCr variants, BT.709 and BT.2020, do not achieve a brightness across the color space as uniform as &BT.601; does. Adapting the Y value for uniform luminosity across the range for CbCr would have complicated the algorithm with little or no gain.</p>
<section2topic='Hue-Saturation-Value/Lightness color space'anchor='design-hsv'>
<p>The HSV and HSL color spaces fail to provide uniform luminosity with fixed value/lightness and saturation parameters. Adapting those parameters for uniform luminosity across the hue range would have complicated the algorithm with litte to no gain.</p>
</section2>
<section2topic='Palette-based and context-aware coloring'anchor='design-context'>
<p>Given a fixed-size and finite palette of colors, it would be possible to ensure that, until the number of entities to color exceeds the number of colors, no color collisions happen.</p>
<p>There are issues with this approach when the set of entities is dynamic. In such cases, it is possible that an entity changes its associated color (for example by re-joining a colored group chat), which defeats the original purpose.</p>
<p>In addition, more state needs to be taken into account, increasing the complexity of choosing a color.</p>
</section2>
<section2topic='Choice of mixing function in angle generation'anchor='design-mixing'>
<p>This specification needs to collapse an arbitrarily long string into just a few bits (the angle in the CbCr plane). To do so, SHA-1 (&rfc3174;) is used.</p>
<p>CRC32 and Adler32 have been considered as faster alternatives. Downsides of these functions:</p>
<ul>
<li>Bad mixing without additional entropy.</li>
<li>Adler32 is rarely available in standard libraries.</li>
<li>CRC32 is ambiguous: there are multiple polynomials in widespread use (e.g. the Ethernet and the zlib polynomials). Often it is not clear which polynomial is used by a library.</li>
</ul>
<p>SHA-1 is widely available. From a security point of view, the exact choice of hash function does not matter here, since it is truncated to 16 bits. At this length, any cryptographic hash function is weak.</p>
<p>This section holds test vectors for the different configurations. The test vectors are provided as Comma Separated Values. Strings are enclosed by single quotes ('). The first line contains a header. Each row contains, in that order, the original text, the text encoded as UTF-8 as hexadecimal octets, the angle in radians, and the Cb, Cr, Red, Green, and Blue values.</p>
<section2topic='Test Vectors for mapping to 216 color palette'anchor='testvectors-palette'>
<p>The used palette can be generated by sampling the RGB cube evenly with six samples on each axis (resulting in 216 colors). The resulting palette is commonly known as the palette of so-called "Web Safe" colors.</p>
<p>The format of the test vectors is the same as in the full range case above.</p>
