Bug 690278

This is probably not very difficult, but I recommend that, for consistency with
the 'bit***' devices and the ppmraw devices, the -dGrayValues= value be used.

While some devices described in 'doc/Devices.htm' use BitsPerSample and
BitsPerPixel, it would be better to stick to -dGrayValues= since this is
described in doc/Use.htm, which is a more general document, and is more likely
to be "discovered" by users.

For a bounty solution, I'd like to see the documentation in doc/Devices.htm
for the tiffsep device as well as improved documentation in the doc/Use.htm
section.

Note that in the meantime, programs such as ImageMagick 'convert' can be used
in a separate step to dither the gray images down to 1-bit per pixel.
For example,

convert bits-8.tif -ordered-dither o8x8 -depth 1 bits-1.tif

Currently is there any way to create CMYK Separate mono TIFF files?
I tried -dGrayValues BitsPerPixel and -BitsPerSample. all dosen't work,
I also tried to set Dithering Type as gsmono. No help either.

So currently for TIFF* device, no paramter can change the sample depth.
Is that correct?

I created the 'tiffsep1' device that creates 1-bit per pixel separations,
including CMYK and any named separations. I recommend rev 10394 or later
(that's the most recent change specific to tiffsep1).