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I'm making a poster (using baposter) where I wish to include some microscopy images. To make the fluorescent signals stand out better, I changed the background of that cell to "pitchblack", which I have defined as

\definecolor{pitchblack}{cmyk}{0,0,0,1}

I've also tried this with

\definecolor{pitchblack}{rgb}{0,0,0}

However, with my microscopy images in the foreground, I find out that "black" isn't really black: enter image description here

The baposterclass brings xcolor and tikz along for the ride, so I'm assuming my color issue is with those packages. So my question is, how I can make the "black" of the background frame black enough to match the background of these images?

Full code of the frame:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\headerbox{Preliminary Images}{name=prelim,column=2,row=0,boxColorOne=pitchblack}{
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\begin{center}
\renewcommand{\arraystretch}{0}
\begin{tabular}{@{}c@{}c@{}c@{}}
\raisebox{-2ex}{
\parbox[t]{0.25\textwidth}{\scriptsize\color{white} Representative volume renderings of HeLa cells imaged using TALEs
against the $\beta$-globin locus. Signals are approximately 0.3$\mu$m
in diameter.}} &
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_02_volume_view.jpg}} & 
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_04_volume_view.jpg}} \\ 
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_05_volume_view.jpg}} & 
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_07_volume_view.jpg}} & 
\imagetop{\includegraphics[width=0.3\textwidth]{dataimages/5_10_volume_view.jpg}}
\end{tabular}
\end{center}
}

As a final note, the problem appears to be independent of the pdf viewer (i.e. it looks this way in evince,acroread,and gimp).

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1  
In case the information is useful for you: The black in the microscopy image is R:0 G:0 B:0 D:255 100% and the pitchblack is R:35 G:31 B:32 D:255 100%. –  Stephen Oct 5 '12 at 18:06
    
That's interesting, but unsurprising. I guess my question really boils down to removing the minimal color that xcolor seems to insist on having. –  Mark Kelly Oct 5 '12 at 18:09
1  
Can you provide a full compilable minimal working example (MWE)? I get geometry errors after wrapping your frame code between \documentclass{baposter}\begin{document} and \end{document}. –  Qrrbrbirlbel Oct 5 '12 at 18:23
    
Have you tried to use the black color already defined in xcolor? I mean something like (in tikz) \fill[black] (0,0) circle 1;... this should give you real black. If not, then there will definitely be some more examination and a MWE needed. –  Benedikt Bauer Oct 5 '12 at 18:25
3  
baposter loads xcolor by default with the option cmyk. All color models used now don't get it right. If you load the package option rgb and use \definecolor{pitchblack}{gray}{0} (or the equivalent RGB command, or the already defined black) it is pitchblack. –  Qrrbrbirlbel Oct 5 '12 at 18:34
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1 Answer

up vote 23 down vote accepted

Short answer: “K does not really stand for black”

\definecolor{pitchblack}{cmyk}{1 1 1 1}

Long answer

This is a CMYK matter: baposter loads xcolor by default with the package option cmyk anymore.
Even passing rgb to xcolor doesn't help here.

Nonetheless, one should issue a test print, colors are not always what they appear to be.

A monitor uses the additive color model RGB, where your pixels consist of three sub-pixel. Additive means, that to have color one has to add light.
No output: black; full output: white.

A printer uses a subtractive color model like CMYK. Subtractive means removing light (when you add, for example, more cyan to your white paper, it will reflect less red).
No output: white (full reflection); full output: black.

Why is black not black?

Good question!
The output depends on so many factors like paper, printer, toner, …

Again, check your printed output. Even in different PDF viewer the colors that are not CMYK(0, 0, 0, 1) or CMYK(1, 1, 1, 1) are represented by different RGB values.

Examples

In the following MWE I provide three new “pitch”blacks to compare:

                         Name            CMYK             RGB
                    ———————————————————————————————————————————————
                      pitchblack0    0   0   0   1    .35 .31 .32
                      pitchblack1   .6  .4  .4   1    .13 .12 .12
                      pitchblack2    1   1   1   1    .0  .0  .0
                      pitchblack3   .75 .68 .67 .9    .16 .14 .13
                      black          0   0   0   1    .35 .31 .32

Code

\documentclass[tabular]{standalone}
\usepackage[cmyk]{xcolor}
\definecolor{pitch0}{cmyk}{0 0 0 1}
\definecolor{pitch1}{cmyk}{.6 .4 .4 1}
\definecolor{pitch2}{cmyk}{1 1 1 1}
\definecolor{pitch3}{cmyk}{.75 .68 .67 .9}
\newcommand*\blackbox[1]{\color{#1}\rule[-1ex]{3cm}{2em}}
\begin{document}\ttfamily
\begin{tabular}{lrrc}
    Color  &           CMYK &         RGB &      Output       \\
    pitch0 &        0 0 0 1 & .35 .31 .32 & \blackbox{pitch0} \\
    pitch1 &     .6 .4 .4 1 & .13 .12 .12 & \blackbox{pitch1} \\
    pitch2 &        1 1 1 1 &  .0  .0  .0 & \blackbox{pitch2} \\
    pitch3 & .75 .68 .67 .9 & .16 .14 .13 & \blackbox{pitch3} \\
    black  &        0 0 0 1 & .35 .31 .32 & \blackbox{black}  \\
\end{tabular}
\end{document}

Output

Fifty Shades of Grey

share|improve this answer
    
Thank you! I was about to give up and fix it by hand in gimp. –  Mark Kelly Oct 5 '12 at 19:03
    
@MarkKelly I've updated my answer with, hopefully, helpful details. I might add that common-sense black seems to be pitch1 (.6 .4 .4 .1). In Adobe's Reader this is even rendered as RGB (0 0 0). If you can, issue a test print with my table and on of your space pictures. In the professional field, a print with pitch2 (0 0 0 0) may rejected, because the “full load” of all colors is used to plot printer's marks (such as crop and registration marks). –  Qrrbrbirlbel Oct 6 '12 at 1:43
2  
I fear that this would not be black but a very thick layer of ink when printed. :) –  percusse Oct 6 '12 at 1:44
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