Colorbar as included png instead of standard pgfplots output to avoid pdf viewer problems

Question

pgfplots offers really nice surface plots as vector graphics, but file size can easily blow up. it is probably quite common (at least for me) to include a bitmap, which was obtained beforehand by converting a pgfplots-pdf with convert (or sometimes preferably with pdftoppm, see comparison at the bottom of this post) to a png bitmap file.

The MWE code then looks like this, just a plot with an included example image and a colorbar:

\documentclass{standalone}
\usepackage{graphics,pgfplots}
\pgfplotsset{compat=newest}


\begin{document}
\begin{tikzpicture}
\begin{axis}[
             colorbar,
             xmin=0,xmax=1,
             ymin=0,ymax=1,
             colormap/viridis,
             colormap={mymap}{samples of colormap=(10 of viridis)},
             colormap access=piecewise constant,
            ]

\addplot graphics [xmin=0, xmax=1, ymin=0, ymax=1] {example-image-a};   
\end{axis}
\end{tikzpicture}
\end{document}

The only special thing is the piecewise constant colormap access. Unfortunately, this kind of colorbar causes problems for almost all pdf viewers I know of. Adobe and Okular work fine, but some examples are the Master PDF Editor (left picture), where the colorbar is just left empty, or evince (right picture), where the colorbar is not piecewise constant and shows a completely wrong color gradient.

The latter problem (i.e., wrong colors in colorbar) happens for most of the viewers I know, e.g. also the pdf-viewer integrated in Firefox. Since it sometimes works, I am quite sure that this is just a viewer problem. But nevertheless, it is probably better to solve it until viewers are fixed. I want everyone to enjoy my beautiful works of art created with pgfplots. :)

Is there a possibility to include a graphic into the colorbar axis, e.g., with \addplot graphics? It may be just the example-image-b.

For the real solution, one might use the colorbar created by pgfplots, see the next piece of code and its output below. (a pdfcrop-run seems to be necessary on the output pdf before converting it to a bitmap format)

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}


\begin{document}
\begin{tikzpicture}
\begin{axis}[
             colorbar,
             xmin=0,xmax=1,
             ymin=0,ymax=1,
             colormap/viridis,
             colormap={mymap}{samples of colormap=(10 of viridis)},
             colormap access=piecewise constant,
             colorbar style = {hide axis},
             hide axis
            ]
\end{axis}
\end{tikzpicture}
\end{document}

almost unrelated info

To highlight the implied problems of convert (i guess it is a problem of ghostscript used to render the pdf), I have converted the colorbar only to a png , on the left with pdftoppm -png -r 30 in.pdf out and on the right with convert -density 30 in.pdf out.png. convert has serious problems with color transitions (it stays the same if a larger dpi value is used). Especially steep transitions become very grainy.

original very small colorbars:

zoomed comparison (upscaled resolution only to make differences visible):

With only 30 dpi, the height/width ratio is not the same for both tools, but more importantly, the convert version has some wrong transitions. Furthermore, file size is 153 bytes for pdftoppm in contrast to 2742 bytes for convert.

Edit

When reading the manual, I noticed that the colorbar sampled option looks correct in all my viewers, even given that it should look the same as the piecewise-constant colorbar. Very strange...

Answer 1

Simple Answer

For me, the colorbar sampled looks the same as the piecewise-constant version. The only difference is that it is viewed correctly in the programs I tried so far. Code is as follows (pay attention that the number of samples must be 1 larger than the number of constant pieces before!)

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
             colorbar sampled,
             xmin=0,xmax=1,ymin=0,ymax=1,
             colormap/viridis,
            % colormap={mymap}{samples of colormap=(10 of viridis)},
            % colormap access=piecewise constant,
             colorbar style={samples=11}, %% 10 +1!
            ]

\addplot graphics [xmin=0, xmax=1, ymin=0, ymax=1] {example-image-a};   
\end{axis}
\end{tikzpicture}
\end{document}

And the output looks good:

Another version, more complicated

Reading the manual carefully enough, I have found a satisfying answer. Of course the colorbar is just an axis, and its settings can be modified. Section 4.9.13 is about colorbars.

This answer is just copied together from several pieces in the named section.

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}

%%% page 284 in 4.9.13 of manual -> definition of 'every colorbar' style
%%  not necessary!
\pgfplotsset{
colorbar style={at={(parent axis.right of north east)},anchor=north west,},
colorbar shift/.style={xshift=0.3cm},
}
%%% modifying the draw-code allows for custom things inside the colorbar    
\pgfplotsset{
colorbar/draw/.code={
\begin{axis} [every colorbar,]% first comes the "original" colorbar code
\node at (0.5,0.5) {\includegraphics[width=0.5cm,height=7cm]{example-image-b}};  
% this assumes standard settings of x = [0,1], y = [0,1], colorbar width =0.5cm and height=7cm

%\addplot graphics [xmin=0, xmax=1, ymin=0, ymax=1] {example-image-a};   
% addplot graphics always draws the real colorbar and not the given bitmap.
\end{axis}
},
}


\begin{document}
\begin{tikzpicture}
\begin{axis}[
             colorbar,
             xmin=0,xmax=1,ymin=0,ymax=1,
             colormap/viridis,
             colormap={mymap}{samples of colormap=(10 of viridis)},
             colormap access=piecewise constant,
            ]

\addplot graphics [xmin=0, xmax=1, ymin=0, ymax=1] {example-image-a};   
\end{axis}
\end{tikzpicture}
\end{document}

I do not know if this answer or Torbjørn's is better. Both seem to require some manual tuning of the colorbar width, heigth, and x/y min/max values etc. as far as I can see it.

The output looks OK, as it does for the other answers.

Answer 2

Can't you just make a new (normal) axis and repeat the use of \addplot graphics? A groupplot makes aligning them easy. You just need to make sure that ymin/ymax is set correctly.

(The whitespace between colorbar and axis is from colorbar.pdf, so better cropping should fix that.)

\documentclass[border=5mm]{standalone}
\usepackage{pgfplots} % loads tikz which loads graphicx
\pgfplotsset{compat=1.16} % it's recommended to set an explicit version
\usepgfplotslibrary{groupplots}

\begin{document}
\begin{tikzpicture}
\begin{groupplot}[
             group style={
                group size=2 by 1,
                horizontal sep=5mm
             },
             height=5cm,
             width=7cm,
             scale only axis,
             xmin=0,xmax=1,
             ymin=0,ymax=1,
            ]

\nextgroupplot
\addplot graphics [xmin=0, xmax=1, ymin=0, ymax=1] {example-image-a};

\nextgroupplot[width=1cm,xtick=\empty,ytick pos=right]
\addplot graphics[xmin=0,xmax=1,ymin=0,ymax=1] {colorbar};

\end{groupplot}
\end{tikzpicture}
\end{document}