Per line text gradient (ala beeline reader)

Question

I've looked at the demo of Beeline reader and would like to test it out on a physical version of my books, which are done in LaTeX. I'm wondering if it would even be possible.

Beeline reader takes online test a puts a slight gradient on the letters for a single line of text. On the next line it reverses the direction of the gradient. The idea being that it helps tracking text and improves speed of reading.

Any hints or suggestions as to how to do this in LaTeX?

What follows is an example taken from http://www.beelinereader.com/

Answer 1

with thanks to

How to put color gradient to "Desired Text Only"

\documentclass{article}
\usepackage{lipsum}


\usepackage{tikz}
\usetikzlibrary{fadings,patterns}
\begin{document}

\newcommand\tikzsection[1]{%
  \begin{tikzfadingfrompicture}[name=tikzsection]
    \node [text=white] {#1};
  \end{tikzfadingfrompicture}
    \begin{tikzpicture}
      \node [text=white,inner sep=0pt,outer sep=0pt] (textnode) {#1};
      \shade[path fading=tikzsection,fit fading=false,left color=\lcolor,right color=\rcolor]
      (textnode.south west) rectangle (textnode.north east);
    \end{tikzpicture}%
}

\makeatletter
\setbox1=\vbox{}


\def\zcolor{red}

\setbox2\vbox{\def\write#1#2{}%
\section{Grr dasadsa}
\lipsum\par


% at this point we are at the end of box 2 now
% loop backwards up the vertical list copying glue and penalties
% but modifying boxesto add the prefix.
\loop
%
% an e-tex primitive that reports the last item in the vertical list.
% type 1 is a box, so remove the box (a lien of text) and then
% put into box 1 a box that is this box together with the prefix which
% was saved in box 0
\ifnum\lastnodetype=1
\ifx\rcolor\zcolor
\gdef\lcolor{red}\gdef\rcolor{blue}%
\else
\gdef\lcolor{blue}\gdef\rcolor{red}%
\fi
\global\setbox3\lastbox
\global\setbox1\vbox{%
\hbox{\tikzsection{\copy\thr@@}}%
\unvbox1}%
\fi
%
% type 11 is glue so remove it from this list and add
% equivalent glue to box 1
\ifnum\lastnodetype=11
\skip0\lastskip\unskip
\global\setbox1\vbox{\vskip\skip0\unvbox1}%
\fi
%
% same for penalty
\ifnum\lastnodetype=13
\count0\lastpenalty\unpenalty
\global\setbox1\vbox{\penalty\count0 \unvbox1}%
\fi
%
% other node types are not handled here: either they won't happen
% or they can't be removed anyway so would break the loop \special
% for example would be bad.
%
% -1 means the vertical list is empty: we have reached the top of the box.
\ifnum\lastnodetype=-1
\else
\repeat}%


% Tip the modified box back onto the main list for the page, unbox
% it so that page breaking may still happen (if it could happen in the
% original context).
\unvbox1




\end{document}

---

A modified colour cycle closer to the image added to the question could be to add

\newcount\colorcycle

to the preamble, then modify the box nodetype code to look like:

\ifnum\lastnodetype=1
\ifcase\colorcycle
\gdef\lcolor{red}\gdef\rcolor{black}\or
\gdef\lcolor{black}\gdef\rcolor{red}\or
\gdef\lcolor{blue}\gdef\rcolor{black}\or
\gdef\lcolor{black}\gdef\rcolor{blue}\global\colorcycle\m@ne\fi
\global\advance\colorcycle\@ne
\global\setbox3\lastbox
\global\setbox1\vbox{%
\hbox{\tikzsection{\copy\thr@@}}%
\unvbox1}%
\fi

Answer 2

That’s a wonderful use case for Luatex! (In fact, when I read the about the bee colors yesterday I was hoping for this question to pop up on TeX-SE.) So, without further ado, here is the necessary code. This requires Luaotfload, which Latex users do not have to load explicitly if they are using the fontspec package.

The beecolors code is split into a TeX and a Lua file. The Lua part (beegradients.lua) implements a node processor for the post_linebreak_filter callback. The principle is quite simple: It scans the horizontal lists that constitute a paragraph for glyphs and ligatures which it surrounds with the appropriate PDF color whatsits, recursing into any vlists an hlists it encounters along the way.

The second file (beegradients.tex) contains wrapper macros for defining gradients and toggling the callback. A gradient group is a list of color expressions and can be defined like this:

\definegradientgroup [<name>][<col1>,<col2>, ... ,<coln>]

E.g.

\definegradientgroup [blackwhite][0x000000, 0xFFFFFF]

installs a gradient list consisting of two colors, black and white. Likewise,

\definegradientgroup [red-green-blue][255*0*0, 0*255*0, 0*0*255]

defines a list of three colors, red, green and blue.

These gradient groups can afterwards be referred to by their names. The callback can be activated by means of an environment beegradients:

\startbeegradients [<name>]
  ... ... ...
\stopbeegradients

Where <name> refers to a previously defined gradient group. E.g., in order to use the groups blackwhite and red-green-blue we defined above:

\startbeegradients [blackwhite]
  \input knuth
\stopbeegradients

\startbeegradients [red-green-blue]
  \input knuth
\stopbeegradients

Without the optional argument, \startbeegradients will choose the last active group.

\startbeegradients [blackwhite]
  \input knuth
\stopbeegradients

foo bar baz       %% <- no gradient

\startbeegradients
  \input knuth    %% <- black and white again
\stopbeegradients

Here is a complete example for the Plain format that shows a couple definitions:

\input luaotfload.sty
\input beegradients.tex

\font \mainfont = file:Iwona-Regular.otf at 10pt
\mainfont

\definegradientgroup [mygradients][
  42*11*242,        %% decimal notation, separated by “*”
  83*242*55,
  0xf00ba7,         %% hex notation
  0x1ec001,
  g:23*b:42*r:133,  %% rgb notation, also separated by “*”
  b:53*g:184*r:10,
]

\definegradientgroup     [blackwhite][0x000000, 0xFFFFFF]
\definegradientgroup [red-green-blue][255*0*0, 0*255*0, 0*0*255]
\definegradientgroup            [red][255*0*0, 0*0*0]
\definegradientgroup          [green][0*100*0, 0*255*0]
\definegradientgroup           [blue][0*0*20, 0*0*210]

\input knuth

\startbeegradients [mygradients]    \input knuth \stopbeegradients
\startbeegradients [blackwhite]     \input knuth \stopbeegradients
\startbeegradients [red-green-blue] \input knuth \stopbeegradients
\startbeegradients [red]            \input knuth \stopbeegradients
\startbeegradients [green]          \input knuth \stopbeegradients
\startbeegradients [blue]           \input knuth \stopbeegradients

\bye

Result:

Of course the code is compatible with Latex, you can load it directly:

\documentclass {scrartcl}
\usepackage {fontspec} %% this loads luaotfload as well
\setmainfont {Antykwa Poltawskiego}
\input beegradients.tex

\definegradientgroup [red-green-blue][255*0*0, 0*255*0, 0*0*255]

\begin {document}
  \startbeegradients [red-green-blue]
    \input knuth
  \stopbeegradients
\end {document}

Which leads to this colorful document:

For texlive 2016 see this post: https://tex.stackexchange.com/a/321962/90087