[Poppler-bugs] [Bug 57244] New: Need option to "preserve blacks"

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=57244

          Priority: medium
            Bug ID: 57244
          Assignee: poppler-bugs at lists.freedesktop.org
           Summary: Need option to "preserve blacks"
          Severity: normal
    Classification: Unclassified
                OS: Mac OS X (All)
          Reporter: chocolate.camera at gmail.com
          Hardware: x86 (IA32)
            Status: NEW
           Version: unspecified
         Component: general
           Product: poppler

Created attachment 70211
  --> https://bugs.freedesktop.org/attachment.cgi?id=70211&action=edit
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`.

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