With this post there are 2 files attached:

posterize.c
CMakeLists.txt

When these files are placed in Freir0r .../src/filters/posterize
-folder they will compile into plugin for Frei0r.

The file .../src/CMakeLists.txt will need to be updated to include the
new folder for successful build.

I request that someone with commit access to Frei0r repository
includes these in the Frei0r distribution.


Janne Liljeblad
set (SOURCES posterize.c)
set (TARGET posterize)

if (MSVC)
set_source_files_properties (posterize.c PROPERTIES LANGUAGE CXX)
set (SOURCES ${SOURCES} ${FREI0R_DEF})
endif (MSVC)

add_library (${TARGET} MODULE ${SOURCES})
set_target_properties (${TARGET} PROPERTIES PREFIX "")

install (TARGETS ${TARGET} LIBRARY DESTINATION ${LIBDIR})
/*
* This file is a port of com.jhlabs.image.PosterizeFilter.java
* Copyright 2006 Jerry Huxtable
*
* posterize.c
* Copyright 2012 Janne Liljeblad
*
* This file is a Frei0r plugin.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
*/

#include <stdlib.h>
#include <assert.h>

#include "frei0r.h"
#include "frei0r_math.h"

typedef struct posterize_instance
{
unsigned int width;
unsigned int height;
double levels;
} posterize_instance_t;

int f0r_init()
{
return 1;
}

void f0r_deinit()
{ /* no initialization required */ }

void f0r_get_plugin_info(f0r_plugin_info_t* posterize_info)
{
posterize_info->name = "posterize";
posterize_info->author = "Janne Liljeblad";
posterize_info->plugin_type = F0R_PLUGIN_TYPE_FILTER;
posterize_info->color_model = F0R_COLOR_MODEL_RGBA8888;
posterize_info->frei0r_version = FREI0R_MAJOR_VERSION;
posterize_info->major_version = 0;
posterize_info->minor_version = 1;
posterize_info->num_params = 1;
posterize_info->explanation = "Posterizes image by reducing the number of available colors";
}

void f0r_get_param_info(f0r_param_info_t* info, int param_index)
{
  switch(param_index)
  {
  case 0:
    info->name = "levels";
    info->type = F0R_PARAM_DOUBLE;
    info->explanation = "Number of values available per channel";
    break;
  }
}

f0r_instance_t f0r_construct(unsigned int width, unsigned int height)
{
    posterize_instance_t* inst = (posterize_instance_t*)calloc(1, sizeof(*inst));
    inst->width = width; 
  inst->height = height;
    inst->levels = 5.0;
    return (f0r_instance_t)inst;
}

void f0r_destruct(f0r_instance_t instance)
{
free(instance);
}

void f0r_set_param_value(f0r_instance_t instance, 
                         f0r_param_t param, int param_index)
{
  assert(instance);
  posterize_instance_t* inst = (posterize_instance_t*)instance;

  switch(param_index)
  {
  case 0:
    inst->levels = *((double*)param);
    break;
  }
}

void f0r_get_param_value(f0r_instance_t instance,
                         f0r_param_t param, int param_index)
{
  assert(instance);
  posterize_instance_t* inst = (posterize_instance_t*)instance;

  switch(param_index)
  {
  case 0:
    *((double*)param) = inst->levels;
    break;
  }
}

void f0r_update(f0r_instance_t instance, double time,
                const uint32_t* inframe, uint32_t* outframe)
{
  assert(instance);
  posterize_instance_t* inst = (posterize_instance_t*)instance;
  unsigned int len = inst->width * inst->height;

  // create levels table
  int numLevels = (int)inst->levels;
  numLevels = CLAMP(numLevels, 2, 50);// "working" range of values for this param
  unsigned char levels[256];
  int i;
  for (i = 0; i < 256; i++)
  {
          levels[i] = 255 * (numLevels*i / 256) / (numLevels-1);
  }

  unsigned char* dst = (unsigned char*)outframe;
  const unsigned char* src = (unsigned char*)inframe;
  unsigned char r,g,b = 0;
  while (len--)
  {
    r = *src++;
    g = *src++;
    b = *src++;
    *src++;

    r = levels[r];
    g = levels[g];
    b = levels[b];

    *dst++ = r;
    *dst++ = g;
    *dst++ = b;
    *dst++; 
  }
}