Make white transparent in a tikz picture

Question

I have some complex code that creates a tikz picture, called image A. The background areas of this picture are white, as are some foreground areas, which are painted with a white fill. I now want to lay this picture on top of image B, such that all white parts of image A are rendered transparent.

It would be ideal if image A could be created so that the white areas were simply not painted, but this is not possible, because many of the white areas are curve paths---so it's easy to draw them directly, but hard to draw their complement.

Can this be done? At the moment I'm having to render images A and B separately, take screenshots, and do this layering in GIMP.

Answer 1

I'm not aware of a way to do this while keeping the images in a vector format. However, as you are already using GIMP, and thus converting to bitmap, I assume you don't mind that too much. If so, the process can be automated using some command line tools. Here is a bash script that does this:

#!/bin/bash
# input files expected in pdf, output in png
bg_pdf=$1
fg_pdf=$2
output_png=$3

# generate png file names
bg_png=${bg_pdf%.pdf}.png
fg_png=${fg_pdf%.pdf}.png

# first, convert both pdf files to png. The best tool for this
# is pdftoppm from popplerutils. We switch off antialasing
# of fonts (-aa no) and other elements (--aaVector no) in
# order to avoid ghosting at the edges. The -r option sets the
# resolution (pixels in the output image per inch of input image).
# Use a high enough resolution, you can always reduce image size 
# (and maybe apply antialising) later. 

pdftoppm -r 300 -png -aa no -aaVector no $bg_pdf > $bg_png
pdftoppm -r 300 -png -aa no -aaVector no $fg_pdf > $fg_png

# now, we use imagemagick's convert to convert white to transparency in the
# foreground image

convert $fg_png -transparent white $fg_png

# finally, we use imagemagick's composite to stack them on top of each other.
# the -gravity center option centers the images to each other; it is not
# needed if they are of equal size.

composite -gravity center $fg_png $bg_png $output_png

# you could also use convert to reduce size and apply antialias
# convert $output_png -antialias -resize 50% halfsize.png

Save this as script.sh and make executable (chmod +x script.sh). Call this like

./script.sh background.pdf foreground.pdf composite.png

A breathtakingly beautiful example (input files uploaded as PNG, but I tested at home on the PDF originals):

Background

Foreground

Composite