(PECL gmagick >= Unknown)
Gmagick::writeimage — Writes an image to the specified filename
Writes an image to the specified filename. If the filename parameter is null,
the image is written to the filename set by Gmagick::readimage() or Gmagick::setimagefilename().
filenameThe image filename.
The Gmagick object.
错误时抛出 GmagickException。