module ChunkyPNG::Canvas::PNGEncoding

def determine_png_encoding(constraints = {})
  
  if constraints == :fast_rgb
    encoding = { :color_mode => ChunkyPNG::COLOR_TRUECOLOR }
  elsif constraints == :fast_rgba
    encoding = { :color_mode => ChunkyPNG::COLOR_TRUECOLOR_ALPHA }
  else
    encoding = constraints
  end
  
  # Do not create a pallete when the encoding is given and does not require a palette.
  if encoding[:color_mode]
    if encoding[:color_mode] == ChunkyPNG::COLOR_INDEXED
      self.encoding_palette = self.palette 
    end
  else
    self.encoding_palette = self.palette
    encoding[:color_mode] ||= encoding_palette.best_colormode
  end
  
  encoding[:interlace] = case encoding[:interlace]
    when nil, false, ChunkyPNG::INTERLACING_NONE then ChunkyPNG::INTERLACING_NONE
    when true, ChunkyPNG::INTERLACING_ADAM7      then ChunkyPNG::INTERLACING_ADAM7
    else encoding[:interlace]
  end
  return encoding
end