Bug 57244

Created attachment 70211 [details]
PDF with text in solid black

CMYK solid black (i.e. C=0, M=0, Y=0, K=100%), or some other kinds of solid blacks (separation, index/DeviceN with K-only CMYK black, etc) convert to dark-gray-ish RGB instead of pure RGB black.

While not a bug, as that original black in the original color space (determined by the source —default or declared— ICC profile, I guess) may indeed not map to pure black, such conversions are usually inappropriate even if arguably accurate, since many (most?) times one would want to display those blacks as pure RGB black, no matter how that original black actually looked on paper.

Pretty much all documents with text will produce this unintended dark-gray color when viewed on screen, since most of times that text will be solid black.

Many software capable of converting PDFs from one color space to another usually have an option to "preserve blacks" (i.e. add an exception to how color management should deal with solid blacks and grayscale objects). Acrobat X has it in its dialog for Tools > Print production > Color conversion (although I cannot even get Acrobat to do what I expect by this option with the embedded file); GhostScript's source code has seemingly portions for that at http://svn.ghostscript.com/ghostscript/trunk/gs/lcms2/src/cmscnvrt.c (which I cannot tell how relevant it actually is: I stumbled into it when googling "PDF preserve black" and cannot read C). There are countless articles on the web on why "preserving blacks" is an important feature when converting color spaces.

Here I attach a PDF created with Adobe Illustrator CS5.5 with solid black CMYK text, which produces this dark-gray RGB when converted with `pdftocairo` and `pdftoppm`.