Transparent background in GR and PGFPlotsX

[ivan-boikov]

Description

• Use the default color palette (dark) with a fully transparent background.
  I imagine it's reasonable, since we can't know what colorscheme a user wants with such a background.
• Allow full transparency in GR's PDFs and PNGs.
• Allow full and partial transparency in PGFPlotsX's PDFs.

Related issues #2725 #2899, but won't close, since it's a partial fix.

Code to generate images

using Plots

function test_plot(; kwargs...)
    plot([0,1], [0,1]; xticks=[0,1], yticks=[0,1], legend=:top, kwargs...)
    plot!(Shape(0.3 .+ [0,0.3,0.3,0], 0.3 .+ [0,0,0.2,0.2]))
end
function gen_files(; backend="", kwargs...)
    test_plot(; kwargs...);
    savefig(backend * "-default.pdf")
    savefig(backend * "-default.png")
    test_plot(; background=:transparent, kwargs...);
    savefig(backend * "-transparent.pdf")
    savefig(backend * "-transparent.png")
    for alpha in 0:0.25:1.0
        test_plot(; background=RGBA{Float64}(0.0, 0.0, 0.5, alpha), kwargs...);
        name = backend * "-" * string(alpha)
        savefig(name * ".pdf")
        savefig(name * ".png")
    end
end
gr()
gen_files(; backend="gr", size=(4,3).*28)
pgfplotsx()
gen_files(; backend="pgfx", size=(4,3).*45)

and I check transparency with LaTeX

\documentclass{article}
\usepackage{geometry}
\geometry{a4paper, left=5mm, right=0mm, top=5mm, bottom=0mm} 

\usepackage{graphics}
\usepackage{xcolor}
\usepackage{float}
\usepackage[pages=some]{background}
\usepackage{lipsum}

\backgroundsetup{scale=1, color=black, opacity=0.8, 
    contents={%
        \includegraphics[width=\paperwidth,height=\paperheight]{example-image}
    }%
}

\begin{document}
\pagecolor{lightgray}
\BgThispage
\section{GR}
\begin{figure}[H]
    \includegraphics{gr-default.pdf}
    \includegraphics{gr-default.png}
    \includegraphics{gr-transparent.pdf}
    \includegraphics{gr-transparent.png}
    \caption{Default PDF, PNG, :transparent PDF, PNG}
\end{figure}
\begin{figure}[H]
    \includegraphics{gr-0.0.pdf}
    \includegraphics{gr-0.25.pdf}
    \includegraphics{gr-0.5.pdf}
    \includegraphics{gr-0.75.pdf}
    \includegraphics{gr-1.0.pdf}
    \caption{Blue w/ alpha 0.00, 0.25, 0.50, 0.75 and 1.00 (PDF)}
\end{figure}
\begin{figure}[H]
    \includegraphics{gr-0.0.png}
    \includegraphics{gr-0.25.png}
    \includegraphics{gr-0.5.png}
    \includegraphics{gr-0.75.png}
    \includegraphics{gr-1.0.png}
    \caption{Blue w/ alpha 0.00, 0.25, 0.50, 0.75 and 1.00 (PNG)}
\end{figure}

\section{PGFPlotsX}
\begin{figure}[H]
    \includegraphics{pgfx-default.pdf}
    \includegraphics{pgfx-default.png}
    \includegraphics{pgfx-transparent.pdf}
    \includegraphics{pgfx-transparent.png}
    \caption{Default PDF, PNG, :transparent PDF, PNG}
\end{figure}
\begin{figure}[H]
    \includegraphics{pgfx-0.0.pdf}
    \includegraphics{pgfx-0.25.pdf}
    \includegraphics{pgfx-0.5.pdf}
    \includegraphics{pgfx-0.75.pdf}
    \includegraphics{pgfx-1.0.pdf}
    \caption{Blue w/ alpha 0.00, 0.25, 0.50, 0.75 and 1.00 (PDF)}
\end{figure}
\begin{figure}[H]
    \includegraphics{pgfx-0.0.png}
    \includegraphics{pgfx-0.25.png}
    \includegraphics{pgfx-0.5.png}
    \includegraphics{pgfx-0.75.png}
    \includegraphics{pgfx-1.0.png}
    \caption{Blue w/ alpha 0.00, 0.25, 0.50, 0.75 and 1.00 (PNG)}
\end{figure}

\end{document}

Before fix (beware, it's hideous!):

After fix:

GR

I can't find the issue with a partial transparency in GR's PDFs. I tried using the C library directly and it works. Really strange.

#include "gr/include/gr.h"
int main(void) {  
    gr_axes(gr_tick(0, 1), gr_tick(0, 1), 0, 0, 1, 1, -0.01);

    // fill background akin to how Plots.jl does
    gr_setfillintstyle(1);  
    gr_setfillcolorind(2);
    gr_settransparency(0.1);
    gr_fillrect(0.0, 1.0, 0.0, 1.0);

    double x[] = {0, 0.2, 0.4, 0.6, 0.8, 1.0};
    double y[] = {0.3, 0.5, 0.4, 0.2, 0.6, 0.7};
    gr_settransparency(1.0);
    gr_polyline(6, x, y);

    gr_setfillcolorind(3);
    gr_settransparency(0.5);
    gr_fillrect(0.0, 0.5, 0.0, 0.5);

    gr_setfillcolorind(4);
    gr_settransparency(0.5);
    gr_fillrect(0.3, 0.8, 0.3, 0.8);

    return 0;
}

export GKS_FILEPATH="test.pdf"
export GKSwstype="pdf"
./main

PGFPlotsX

PGFPlotsX is using pdftocairo or pdftoppm for generating PNGs.
https://github.com/KristofferC/PGFPlotsX.jl/blob/366ccffcca413b5861c381564134855d823fb2c2/src/build.jl#L140

• pdftocairo draws transparent regions as white by default, but it can be overridden with -transp flag - it would require keyword passing to PGFPlotsX
• pdftoppm does not have such flag, but I've never used it, so can't tell what's default.

Attribution

• I am listed in .zenodo.json (see How to get acknowledged #3503)