PDF without color operators
If you are lucky and the PDF image does not contain any color operators and you are using pdfTeX/LuaTeX, then a simple
PDF with color operators
The color operators can be indentified and removed from the PDF file. Usually the page contents streams are compressed.
pdftk can uncompress them:
pdftk myimage.pdf output temp.pdf uncompress
Then you need to find the page object. Look for
4 0 obj <<
/Contents 5 0 R
/Resources 3 0 R
/MediaBox [0 0 595.276 841.89]
/Parent 7 0 R
Then the number after
/Contents is the object number for the page contents stream.
In this case the number is
5 and we search for
5 0 obj:
5 0 obj <<
0 g 0 G
1 0 0 1 14.944 500.863 cm
10 0 0 10 0 0 cm
Now we have 254 bytes between
endstream to scan for color operators.
The operators follows the operands (postfix notation). The main color operators
<n> is a number between 0 and 1):
<n> g for gray color model
<n> <n> <n> RG,
<n> <n> <n> rg for RGB color model
<n> <n> <n> <n> K,
<n> <n> <n> <n> k for CMYK color model
- others are
scn with parameters each.
In the case of the monochrome black image, I would expect
0 g and
0 G near the beginning of the stream. The above example contains the two operations:
0 g 0 G.
If the PDF file is edited great care is needed the size of the object is not changed.
First the length of the stream is given in the
/Length entry. And the file offsets of the objects are written in the
xref table of the PDF file. Therefore the color operations are deleted by overwriting. Also the editor should not do its own editing by changing line end characters, for example.
If the color operations are on a line by its own, then it is enough to replace the
first character with the percent char, that is also a comment char in PostScript and PDF, in the example above:
% g 0 G
Or overwrite the entry by spaces (
, ordinary spaces with character code 32).
The the file is recompressed:
pdftk temp.pdf output myimage-colorless.pdf compress
And the trick with pdfTeX will work:
Low level editing of color operations via PostScript
If it is difficult to edit the PDF file directly (compressed, size requirements, editor requirement, adding colors, …), then the PDF can be converted to PostScript.
My first choice is
pdftops -eps myimage.pdf myimage.eps
It uses the same operator names as in the PDF file. In case of
pdftops the page starts after
pdfStartPage). PostScript is a programming language, in general it can be difficult to locate the color operators:
setcmykcolor. In case of
pdftops, new operators are defined with the same names as the PDF operators. Thus the same can be done as described in the previous section. However, the size of the file can easily change, also the line endings are less critical and new color operations can be inserted.
Then the file is reconverted to PDF, e.g.:
ps2pdf myimage.eps myimage-changedcolors.pdf
This way can also be used, if later a different driver than pdfTeX/LuaTeX is used that
always ensure the black is the default color. Then the color is changed in the PostScript