Detecting all pages which contain color

Question

In an larger LaTeX document there are often only some pages with color content (mainly figures) and the remaining ones are only black and white. Because printing costs for color pages are much higher than for black and white it would be good to be able to extract all pages with color and print them separately. The first step for this is to be able to detect if a page contains color or not. This could be in a form of an text list of page number suitable to be read by a PDF page extraction script (using e.g. pdftk).

A simple solution sufficient for many people would be to detect all pages which contain a figure and assume that only these have color. However, a general solution would be nice. Only color elements which are printed should be taken into account, while e.g. the color frames around link by hyperref should not. It is OK that the solution would disable these for the detection.

Answer 1

Newer versions of Ghostscript (version 9.05 and later) include a "device" called inkcov. It calculates the ink coverage of each page (not for each image) in Cyan (C), Magenta (M), Yellow (Y) and Black (K) values, where 0.00000 means 0%, and 1.00000 means 100%.

Example commandline:

gs -o - -sDEVICE=inkcov /path/to/your.pdf

Example output:

Page 1
0.00000  0.00000  0.00000  0.02230 CMYK OK
Page 2
0.02360  0.02360  0.02360  0.02360 CMYK OK
Page 3
0.02525  0.02525  0.02525  0.00000 CMYK OK
Page 4
0.00000  0.00000  0.00000  0.01982 CMYK OK

You can see here that the page 4 is using no color, while pages 1+2+3 do. This case is particularly 'nasty' for people who want to save on color ink: because all the C, M, Y (and K) values are exactly the same for each of the pages 1-3, they possibly could appear to the human eye not as color pages, but as ("rich") grayscale anyway (if each single pixel is mixed with these color values).

Ghostscript can also convert color into grayscale. Example commandline:

gs \
 -o grayscale.pdf \
 -sDEVICE=pdfwrite \
 -sColorConversionStrategy=Gray \
 -sProcessColorModel=/DeviceGray \
  /path/to/your.pdf

Checking for the ink coverage distribution again (note how the addition of -q to the parameters slightly changes the output format):

gs -q  -o - -sDEVICE=inkcov grayscale.pdf
 0.00000  0.00000  0.00000  0.02230 CMYK OK
 0.00000  0.00000  0.00000  0.02360 CMYK OK
 0.00000  0.00000  0.00000  0.02525 CMYK OK
 0.00000  0.00000  0.00000  0.01982 CMYK OK

Answer 2

For the general case it seem to be indeed better to use an external tool to test for all pages which contain colors. This is the topic of the mentioned SO question How do I know if PDF pages are color or black-and-white?. I now wrote an answer to it which includes small script for this.

However, it is much easier to get a list of all pages containing figures. Here I use the zref-abspage package to get an absolute page counter. The normal \write command can be used which will expand its content when the surrounding content is really placed on a page. Therefore the page counters will have the correct value. Then the end-macro of figure can simply be patched to hold this code.

\documentclass{book}
\usepackage{mwe}

\usepackage{zref-abspage}% absolute page counter
\newwrite\figpages
\openout\figpages=\jobname.fpg
\makeatletter
\g@addto@macro\endfigure{%
    % Write absolute page number and page label to file
    % Do not use \immediate!
    \write\figpages{\number\value{abspage}: \thepage}%
}
\makeatother

\newcount\mycount% for example loop
\begin{document}
\frontmatter
\Blindtext

\begin{figure}
    \centering
    \includegraphics[width=.8\textwidth,height=5cm]{example-image}
    \caption{Some caption}
\end{figure}

\mainmatter
\Blindtext

\loop% keep MWE small by using a loop

\begin{figure}
    \centering
    \includegraphics[width=.8\textwidth,height=5cm]{example-image}
    \caption{Some caption}
\end{figure}

{\Blindtext}

\advance\mycount by 1
    \ifnum\mycount<20\relax
\repeat

\backmatter
\appendix
\Blindtext

\begin{figure}
    \centering
    \includegraphics[width=.8\textwidth,height=5cm]{example-image}
    \caption{Some caption}
\end{figure}

\end{document}

This generates a .fpg file (for figure pages) which looks like:

2: ii
4: 2
5: 3
7: 5
8: 6
10: 8
11: 9
13: 11
14: 12
16: 14
18: 16
19: 17
21: 19
22: 20
24: 22
25: 23
27: 25
28: 26
30: 28
31: 29
33: 31
38: 36

The format can be changed if required.

Answer 3

There's a rather useful python script at http://homepages.inf.ed.ac.uk/imurray2/code/hacks/pdfcolorsplit which uses pdftk to split into colour and b&w files, though it doesn't deal with the boxes around hyperrefs. If you have access to the LaTeX source, why not turn off the colour in hyperref anyway - I do it like this:

\usepackage[colorlinks=true,
            linkcolor=black,
            citecolor=black,
            filecolor=black,
            urlcolor=black]{hyperref}

IIRC if you just set [colorlinks=false] they're not clickable.