XEP-xxxx: Consistent Color Generation

1. Introduction
2. Requirements
3. Algorithms
    3.1. Angle generation
    3.2. Corrections for Color Vision Deficiencies
       3.2.1. Red/Green-blindness
       3.2.2. Blue-blindness
    3.3. CbCr generation
    3.4. CbCr to RGB
    3.5. Adapting the Color for specific Background Colors
4. Use Cases
    4.1. Adding colors to participants of a conversation
    4.2. Auto-Generating Avatars
5. Business Rules
6. Implementation Notes
    6.1. Gamma Correction
7. Accessibility Considerations
8. Security Considerations
9. Design Considerations
    9.1. Other variants of the YCbCr color space
    9.2. Hue-Saturation-Value/Lightness color space
10. IANA Considerations
11. XMPP Registrar Considerations

Appendices
    A: Document Information
    B: Author Information
    C: Legal Notices
    D: Relation to XMPP
    E: Discussion Venue
    F: Requirements Conformance
    G: Notes
    H: Revision History

3.1 Angle generation

Input: An identifier, encoded as octets of UTF-8 (RFC 3269 [1]).

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.

1. Run the input through CRC32 as defined by zlib (TODO: add citation).
2. Take the lower 16 bits and XOR them with the upper 16 bits.
3. Divide the value by 65535 (use float division) and multiply it by 2π (two Pi).

3.2 Corrections for Color Vision Deficiencies

Input: Angle in the CbCr plane.

Output: Angle in the CbCr plane.

Note: This algorithm will re-map the angle to map it away from ranges which can not be distinguished by people with the respective Color Vision Deficiencies.

3.3 CbCr generation

Input: Angle in the CbCr plane, from the previous algorithm.

Output: Values for Cb and Cr in the YCbCr BT.601 color space in the range from -0.5 to 0.5.

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:

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;

3.4 CbCr to RGB

Input: Values for Cb and Cr in the YCbCr BT.601 color space in the range from -0.5 to 0.5; Value for Y.

Output: Values for Red (R), Green (G) and Blue (B) in the RGB color space in the range from 0 to 1.

Note: The recommended value for Y is 0.732. See Gamma Correction for a discussion on the choice of Y.

1. Calculate r, g and b according to BT.601:

   float r = 2*(1 - KR)*cr + y;
   float b = 2*(1 - KB)*cb + y;
   float g = (y - KR*r - KB*b)/KG;
   

2. Clip the values of r, g and b to the range from 0 to 1.

3.5 Adapting the Color for specific Background Colors

Input: RGB values for the color to adapt (Ri, Gi, Bi) and for the background color to adapt to (Rb, Gb, Bb), in the range from 0 to 1 each.

Output: Values for Red (Rc), Green (Gc) and Blue (Bc) in the RGB color space in the range from 0 to 1.

1. Invert the background color by subtracting the individual channels from 1 each:

   rb = 1-rb;
   gb = 1-gb;
   bb = 1-bb;

2. Mix the inverted background with the color to adapt, using a mixing factor of 0.2:

   rc = 0.2*rb + 0.8*ri;
   gc = 0.2*gb + 0.8*gi;
   bc = 0.2*bb + 0.8*bi;

4.1 Adding colors to participants of a conversation

Implementations may colorize the participants of a conversation with an individual color to make them easier to distinguish.

In such cases, the color SHOULD be generated as described in the Algorithms section. The input used SHOULD be, in descending order of preference, (a) the name assigned in the roster, (b) the nickname from the conversation, (c) the bare JID.

4.2 Auto-Generating Avatars

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 User Avatar (XEP-0084) [2]).

In such cases, auto-generating an avatar SHOULD happen as follows:

1. Obtain a name for the contact, in descending order of preference, (a) from the roster, (b) by using the nickname from the conversation, (c) by using the bare JID.
2. Generate a color as described in the Algorithms section.
3. Fill a square with that color.
4. Render the first character of the name in white or black centered on the square.

6.1 Gamma Correction

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.

Modifications to Y SHOULD NOT be used to correct for bright/dark backgrounds. Implementations should instead use the algorithm described in Adapting the Color for specific Background Colors for that.

9.1 Other variants of the YCbCr color space

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.

9.2 Hue-Saturation-Value/Lightness color space

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.

Jonas Wielicki

Email: jonas@wielicki.name
JabberID: jonas@wielicki.name

Copyright

Permissions

Disclaimer of Warranty

Limitation of Liability

IPR Conformance

1. RFC 3269: UTF-8, a transformation format of ISO 10646 <http://tools.ietf.org/html/rfc3269>.

2. XEP-0084: User Avatar <https://xmpp.org/extensions/xep-0084.html>.

3. XEP-0165: Best Practices to Prevent JID Mimicking <https://xmpp.org/extensions/xep-0165.html>.

4. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

5. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.

Version 0.0.1 (2017-05-04)