其实我目前还没有实现。

references:

http://qt-project.org/faq/answer/how_can_i_convert_a_colored_qpixmap_into_a_grayscaled_qpixmap

http://www.qtcentre.org/threads/46596-How-to-convert-a-32-bit-RGB-image-to-8-bit-gray-scale-image

http://www.qtcentre.org/threads/19296-is-there-a-simple-and-quick-way-to-convert-color-images-to-gray-image

http://www.qtcentre.org/threads/15186-QImage-Greyscale-8-bits-pixel

http://stackoverflow.com/questions/15672743/convert-16-bit-grayscale-to-qimage