-- Kuwahara filter
--
-- 
-- Performs the Kuwahara Filter. This filter is an edge-preserving filter.
--
--
-- ( a  a  ab   b  b)
-- ( a  a  ab   b  b)
-- (ac ac abcd bd bd)
-- ( c  c  cd   d  d)
-- ( c  c  cd   d  d)
--
-- In each of the four regions (a, b, c, d), the mean brightness and the variance are calculated. The
-- output value of the center pixel (abcd) in the window is the mean value of that region that has the
-- smallest variance.
--
-- description copied from http://www.incx.nec.co.jp/imap-vision/library/wouter/kuwahara.html
--
-- implemented by Oeyvind Kolas <oeyvindk@hig.no> 2004
-- modified for Pixopedia 24 by Sinisa Petric, 2008  


-- the sampling window is:  width=size*2+1  height=size*2+1

-- local function to get the mean value, and
-- variance from the rectangular area specified

image_OUT = 0
image_INP = 1

function rgb_mean_and_variance (x0,y0,x1,y1)
 local variance
 local mean
 local r_mean
 local g_mean
 local b_mean 
 
 local min           = 1.0
 local max           = 0.0
 local accumulated_r = 0
 local accumulated_g = 0
 local accumulated_b = 0
 local count         = 0
   
 local x, y

 for y=y0,y1 do
   for x=x0,x1 do
     local v     = fp24_getImageLuminance01(image_INP, x,y)
     local r,g,b = fp24_getImageRGB(image_INP, x,y)
     accumulated_r   = accumulated_r + r
     accumulated_g   = accumulated_g + g
     accumulated_b   = accumulated_b + b
     count         = count + 1
      
     if v<min then min = v end
     if v>max then max = v end

    end
  end

  variance = max-min
  mean_r   = accumulated_r /count
  mean_g   = accumulated_g /count
  mean_b   = accumulated_b /count
    
  return mean_r, mean_g, mean_b, variance
end


-- return the kuwahara computed value
function rgb_kuwahara(x, y, size)
  local best_r, best_g, best_b
  local best_variance = 1.0

  local r,g,b, variance

  r,g,b, variance = rgb_mean_and_variance (x-size, y-size, x, y)

  if variance < best_variance then
    best_r, best_g, best_b = r, g, b
    
    best_variance = variance
  end

  r,g,b, variance = rgb_mean_and_variance (x, y-size, x+size,y)

  if variance < best_variance then
    best_r, best_g, best_b = r, g, b
    best_variance = variance
  end

  r,g,b, variance = rgb_mean_and_variance (x, y, x+size, y+size)

  if variance < best_variance then
    best_r, best_g, best_b = r, g, b
    best_variance = variance
  end

  r,g,b, variance = rgb_mean_and_variance (x-size, y, x,y+size)

  if variance < best_variance then
    best_r, best_g, best_b = r, g, b
    best_variance = variance
  end

  return best_r, best_g, best_b
end
function kuwahara(radius)
  for y=0, vp24_height-1 do
    for x=0, vp24_width-1 do
      r,g,b = rgb_kuwahara (x,y, radius)
      fp24_setImageRGB(image_OUT, x,y, r,g,b)
    end
    fp24_showProgress (y, vp24_height)
  end
end

kuwahara(2)