Line ends at wrong location in picture environment

Question

I draw a yellow square inside a grey square, exactly in the middle. Unexpectedly, the yellow square is not aligned horizontally but instead moved to the right:

enter image description here

Why? If I understand correctly, the yellow square should end at x=37, and the grey at x=38.

\documentclass{article}
\usepackage{xcolor}
\begin{document}
\setlength{\unitlength}{1mm}
\hrule\noindent

\begin{picture}(40,40)
\linethickness{36mm}
\color{gray}
\put(2,20){\line(1,0){36}}

\linethickness{34mm}
\color{yellow}
\put(3,20){\line(1,0){34}}
\end{picture}

\hrule
\end{document}

You need to terminate your lines with % to prevent spurious white space. In particular, putting one after \linethickness{34mm} will correct the alignment of the yellow and grey squares, but the other lines should also be terminated with %, otherwise both rectangles move too far to the right. — Jake, Oct 23 '13 at 08:04
@Jake Can you explain this further or give a link to an example/explanation? — Jost, Oct 23 '13 at 08:29
@Jost: What is the use of percent signs (%) at the end of lines? — Jake, Oct 23 '13 at 08:42
Thanks you, somehow I completely did not expect, that an image-environment could be whitespace-sensitive. — olpa, Oct 23 '13 at 09:00

David Carlisle · Accepted Answer · 2020-03-13T21:31:44.883

The picture mode commands make some attempt to ignore white space between the instructions but \color defeats that attempt so you are seeing the word space after \color (and other places). Note that TeX is designed as a text typesetting language promarily and white space is not syntactic decoration as in most programming languages, it is (usually) an instruction to add stretchable inter-word space at that point.

It is best to avoid typesetting anything in picture mode that is not inside \put so

\color{yellow}
\put(3,20){\line(1,0){34}}

not

\put(3,20){\color{yellow}\line(1,0){34}}

Although \color is in fact safe outside \put as long as you avoid adding space around it

\color{yellow}%

The definition of \linethickness has been changed (2020 release) to avoid this extra space.

https://github.com/latex3/latex2e/issues/274

Line ends at wrong location in picture environment

1 Answers1