PHP将24位颜色转换为4位颜色

Question

Background, I am converting images to ascii art. This works perfectly and even works with 24-bit color, converting the colors to the right rgb values. However, I now want to render the ascii art in 4-bit color palette rather than 24-bit.

How do I convert 24-bit colors to 4-bit with PHP?

More specifically, I have the standard IRC color pallet which I need to convert any given Hexidecimal or RGB value to. It is preferred that the colors match as best as possible when converted to the 4-bit color.

Other ideas I have had on this are to convert the image itself to a 4-bit palette (using GD, which is what I use to read in the colors right now) before trying to grab colors off of it. And another idea might be to define a color range for each of the following color and just check that the given 24-bit color is in the range, however I wouldn't know how to get the ranges for all colors into that palette.

Answer 1

In the end, despite the wonderful suggestions surrounding imagemagick I found a good solution using straight php. I was able to calculate the closest color through the use of delta E 2000 with a modified version of php-color-difference library found on github, here is my fork: https://github.com/nalipaz/php-color-difference

The pertinent example is:

<?php
include('lib/color_difference.class.php');

$palette = array(
  '00' => array(255, 255, 255),
  '01' => array(0, 0, 0),
  '02' => array(0, 0, 139),
  '03' => array(0, 128, 0),
  '04' => array(255, 0, 0),
  '05' => array(139, 0, 0),
  '06' => array(128, 0, 128),
  '07' => array(255, 165, 0),
  '08' => array(255, 255, 0),
  '09' => array(50, 205, 50),
  '10' => array(0, 128, 128),
  '11' => array(173, 216, 230),
  '12' => array(0, 0, 255),
  '13' => array(255, 105, 180),
  '14' => array(128, 128, 128),
  '15' => array(211, 211, 211),
);

$color_rgb = array(255, 255, 128);
$color_delta_e = new color_difference($color_rgb);
$match_index = $color_delta_e->getClosestMatch($palette);
$color = $palette[$match_index];

I am pretty happy with this solution and smaller amount of overhead. Thanks for the suggestions guys.