Originally, I thought this function would be similar to the Posterize function, or at least, any one of the Imagick class functions that produces an artistic effect (like the oilPaintImage function). This function can be used for those purposes, but it is mostly geared toward print production. The OrderedPosterize is simply a highly flexible dithering tool. The intention essentially is to produce high-resolution imagery by means of using constant dots across a medium that vary in size according to the detail of the imagery. Everyone has seen a dithered photograph inside of a newspaper, but the wiki page provides better examples: http://en.wikipedia.org/wiki/Dither .
The two parameters are unusual to someone who simply wants to get the use of this function without fussing too much about the Imagick class.
First parameter: The threshold_map is a set of "brushes" predefined by a very simple xml file. They are: threshold, checks, o2x2, o3x3, o4x4, o8x8, h4x4a, h6x6a, h8x8a, h4x4o, h6x6o, h8x8o, h16x16o, c5x5b, c5x5w, c6x6b, c6x6w, c7x7b, and c7x7w. These are the values you are expected to input as parameters for the Thershold_Map value. Much better descriptions of these brush shapes available at the Imagick site: http://www.imagemagick.org/Usage/quantize/tmaps_list.txt .
Second parameter: The channel is any of the constants as predefined by the Imagick class. In the code, the value looks like "imagick::CHANNEL_RED", but you have the "_value" options of: red, undefined, gray, cyan, green, magenta, blue, yellow, alpha, opacity, matte, black, index, all, and default. More info here: http://www.php.net/manual/en/imagick.constants.php#imagick.constants.channel .
Finally, don't forget you can use bitwise operators on the second parameter. That means you can use & to AND them, | to OR them, & to XOR them, and ~ to NEGATE them. A valid parameter for the second parameter would be: "((~imagick::CHANNEL_GREEN) ^ imagick::CHANNEL_YELLOW) | imagick::CHANNEL_MAGENTA)". You can get extremely creative in this particular parameter. And if you want to define your own brushes using simple XML, then that's also true of the first parameter, too.
Note: You can use this function artistically. How? Use the orderedPosterizeImage to give the image some texture (a photo of a vase, for instance), and then use your OilPoint, Sketch, or Standard Posterize to give the image a cool effect. Alone, though, seems pretty boring.
And now, a very simple demonstration :
<?php
// Author: holdoffhunger@gmail.com
// Filename
// ---------------------------------------------
$file_to_grab_with_location = "graphics_engine/image_workshop_directory/ordered_posterize_source.bmp"
$imagick_type = new Imagick();
// Open File
// ---------------------------------------------
$file_handle_for_viewing_image_file = fopen($file_to_grab_with_location, 'a+');
$imagick_type->readImageFile($file_handle_for_viewing_image_file);
// Perform Function
// ---------------------------------------------
$imagick_type->orderedPosterizeImage("o2x2", imagick::CHANNEL_GREEN);
// Save File
// ---------------------------------------------
$file_to_save_with_location = "graphics_engine/image_workshop_directory/ordered_posterize_result.bmp"
$file_handle_for_saving_image_file = fopen($file_to_save_with_location, 'a+');
$imagick_type->writeImageFile($file_handle_for_saving_image_file);
?>