Convert to grayscale

-- Description:
-- converts 32 bit image to grayscale
-- Parameters:
-- _image = 32 bit image
-- Note:
-- #grayscale can be replaced by any custom color palette, (sepia can be useful too)
 
on convertToGrayScale _image
  gray = image(_image.width, _image.height, 8,#grayscale)
  gray.copyPixels(_image, _image.rect, _image.rect , [#ink:#copy])  
  if _image.usealpha =0 then return gray -- not 32 bit image skipping copying
  i32 = image(_image.width, _image.height, 32)
  i32=_image.duplicate()
  i32.copyPixels(gray, i32.rect, i32.rect , [#ink:#copy,#mask:_image.extractALpha()])  
  i32.setalpha(_image.extractALpha())
  return i32  
end