White line at edge of black 16:9 PDF

Question

If I create a PDF with a 16:9 aspect ratio (e.g. for viewing via video projector) with a black background, I get a phantom white line at its bottom edge when I convert it to an image.

Here is minimal code to see this:

\documentclass{article}
\usepackage[papersize={160mm,90mm}]{geometry}
\usepackage{color}
\pagecolor{black}
\begin{document}
\null
\end{document}

Compile this via pdflatex or XeTeX, then:

pdftoppm test.pdf img

The result is a black image white a one pixel white line at the bottom.

I can get rid of this line by setting a slightly different papersize, e.g.:

\usepackage[papersize={159.8mm,89.85mm}]{geometry}

I also can get rid of this if I use a 4:3 aspect ratio, e.g.:

\usepackage[screen]{geometry}

Is there a way to get rid of this line besides fudging papersize, asking pdftoppm to crop the page, or cropping afterward in Imagemagick? E.g. via making a black background rectangle on each page which is slightly larger than the paper size? And is the underlying problem a TeX rounding error or something to do with PDF output? Is there a way to directly set the background of a PDF document to black, not white, rather than working via pagecolor?

The goal here is to generate 1920x1080 viewable images from TeX via, say:

pdftoppm -scale-to-x 1920 -scale-to-y -1 -png test.pdf img

I'd also be happy to learn a better way to produce screen output for pages with dark backgrounds. I got into this because when I view 16:9 PDFs generated this way in Preview via Slideshow mode, I see a distracting white border around dark pages. (I don't see this in Acrobat.) In trying to work around that by creating an image for each page, I ran to the pdftoppm problem.

The only related question I can find: Beamer frames show a white line on top

Answer 1

The page color is a rectangle drawn on the page. It is already a bit larger than the mediabox but only to the right and the top. At the left and bottom it begins like the media box exactly at zero. And the conversion then seems to introduce some rounding error which gives a white line.

I don't see any reason why the rectangle shouldn't be a bit larger to the other sides too. This here seems to work fine (with pdflatex!) and the white line disappears. If it works for you you could make a feature request.

\documentclass{article}
\usepackage[papersize={160mm,90mm}]{geometry}
\usepackage{color}
\makeatletter %copied from pdftex.def:
\begingroup\expandafter\expandafter\expandafter\endgroup
\expandafter\ifx\csname currentgrouplevel\endcsname\relax
  \catcode`X=9  % ignore
  \catcode`E=14 % comment
\else
  \catcode`X=14 % comment
  \catcode`E=9  % ignore
\fi

\def\set@page@color{%
  \global\GPT@pagecolortrue
  \global\let\current@page@color\current@color
  \@ifundefined{GPT@outputbox}{%
    % \newbox is outer in plain
    \csname newbox\endcsname\GPT@outputbox
  }{}%
  \@ifundefined{GPTorg@shipout}{%
    \global\let\GPTorg@shipout\shipout
    \gdef\shipout{%
X     % ensure \GPT@outputbox is really void
X     \begingroup
X       \setbox\GPT@outputbox=\box\GPT@outputbox
X     \endgroup
E     \edef\GPT@saved@grouplevel{\number\currentgrouplevel}%
      \afterassignment\GPT@shipout
      \global\setbox\GPT@outputbox=%
    }%
    \gdef\GPT@shipout{%
X     \ifvoid\GPT@outputbox\relax
E     \ifnum\GPT@saved@grouplevel=\currentgrouplevel
E     \else
        \expandafter\aftergroup
      \fi
      \GPT@@shipout
    }%
    \gdef\GPT@@shipout{%
      \ifvoid\GPT@outputbox\relax
        \PackageInfo{pdftex.def}{Ignoring void output box}%
      \else
        \setbox\GPT@outputbox=\vbox{%
          \GPT@pageliteral
          \box\GPT@outputbox
        }%
        \GPTorg@shipout\box\GPT@outputbox\relax
      \fi
    }%
    \gdef\GPT@pageliteral{%
      \ifGPT@pagecolor
        \pdfliteral page{%
          q % gsave
          \current@page@color\GPT@space
          n % newpath
          -1 -1 \strip@pt\pdfpagewidth\GPT@space
          \strip@pt\pdfpageheight\GPT@space re % rectangle
          % there is no need to convert to bp
          f % fill
          Q% grestore
        }%
      \fi
    }%
  }{}%
}
\catcode`\X=11 %
\catcode`\E=11 %

\pagecolor{black}

\begin{document}
\null 
\end{document}