+ Fix wording in angle generation section which did still use CRC32. + Rework palette mapping after with implementation experience. +
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.
+Colors provide a valuable visual cue to recognize shapes. 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, without having to actually read the text.
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.
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;.
In no way is the system presented in this specification a replacement for names. It only serves as an additional visual aid.
@@ -75,9 +84,8 @@Output: Angle in the CbCr plane.
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.
Input: A set of RGB colors (each component from 0 to 1).
-Output: A mapping from CbCr pairs (each component from -0.5 to 0.5) to RGB colors.
-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.
+Output: A mapping from angles (from 0 to 2π) to RGB colors.
+Note: when the algorithm finishes, the mapping maps angles (rounded to two decimal places) to the R, G, B triples which come closest to the desired color and lightness.
Convert Cb and Cr to an angle:
+ 0:
+ cr /= magn
+ cb /= magn
+angle = atan2(cr, cb) % (2*pi)
+]]>
+ Here, % is the floating point modulo operator. Since atan2 may return negative values, it is used to put the values into the range from 0 to 2π. ** is the exponentiation operator (cb**2 is thus cb squared).
+Implementations are free to choose a representation for palette colors different from R, G, B triplets. The exact representation does not matter, as long as it can be converted to an angle in the CbCr plane accordingly.
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.
-Output: A palette color as Cbr and Crr values.
+Input: (a) A mapping which maps angles to R, G, B triplets and (b) a color to map to the closest palette color as angle alpha.
+Output: A palette color as R, G, B triplet.
+Note: See Conversion of an RGB color palette to a CbCr color palette on how to convert an R, G, B triplet or a CbCr pair to an angle.
D = sqrt((Cbp-Cb)*(Cbp-Cb) + (Crp-Cr)*(Crp-Cr)).D = min((alpha - beta) % (2*pi), (beta - alpha) % (2*pi)).Note: the distance metric is simply the euclidian distance in the CbCr plane.
+Implementations are free to choose a representation for palette colors different from R, G, B triplets. The exact representation does not matter, as long as it can be converted to an angle in the CbCr plane accordingly.