(No version information available, might only be in SVN)
Imagick::transparentPaintImage — Paints pixels transparent
Paints pixels matching the target color transparent. This method is available if Imagick has been compiled against ImageMagick version 6.3.8 or newer.
target
     The target color to paint
alpha
     The level of transparency: 1.0 is fully opaque and 0.0 is fully transparent.
fuzz
     The amount of fuzz. For example, set fuzz to 10 and the color red at intensities of 100 and 102 respectively are now interpreted as the same color.
invert
     
       If TRUE paints any pixel that does not match the target color.
      
   Returns TRUE on success.