black channel in CMYK

[ArtoI]

Hello,

I try to convert rgb.png files to cmyk.tiff. (convert -colorspace CMYK rbg.png cmyk.tiff). The problem is that black channel is zero and all black pixels are mixture of cyan, magenta and yellow. How can I make CMYK files, where black pixels are only in the K-channel and all other colors are zero at these points.

Regards,
Arto

[anthony]

What version of IM are you using?

can you give a link to an example image (small example).

[ArtoI]

What version of IM are you using?

Examples are made by the latest version 6.3.5-Q8 (Windows).

can you give a link to an example image (small example).

Small example of the original .png can be found from:

http://www.keskiespoo.net/~arinkala/im/rgb.png

at the same directory cmyk.tif is made by command

convert -colorspace CMYK rgb.png cmyk.tif

after that color channels are separated by both images.

convert rgb.png -colorspace CMYK -separate rgb_sep_%d.tif
convert cmyk.tif -colorspace CMYK -separate cmyk_sep_%d.tif

The files separated from rgb.png seems to be fine, but from cmyk.tif K-channel is zero.

[anthony]

Okay so reading in a CMYK version of TIFF then applying -colorspace CMYK
stuffs up the image.

I would report exactly that in the bugs section.

[ArtoI]

The error might appear already in the conversion from .png. So, that the colorspace in the .tif file is CMY instead of CMYK.