Frame With Solid Left Line Redux

Posted on March 24, 2019

As I had posted earlier, I like the visual effect of placing a solid line on the left side of a block:

I use this for displaying short code snippets in the lecture notes of a course that I teach. Typically these are a few lines of Matlab code but recently I needed to post a bigger code snippet where the frame should break across the page. In principle, this should have been a simple change—replace the framed with a textbackground—but that did not work out of the box. This blog post explains a simple solution.

First, let’s start with the naive solution:

\definecolor[darkgray]  [s=0.5]
\definecolor[lightgray] [s=0.95]

\definetextbackground
  [leftbartext]
  [
    location=paragraph,
    frame=on,
    framecolor=darkgray,
    rulethickness=2ex,
    leftoffset=5.25ex,
    rightoffset=2.25ex,
    topoffset=2.25ex,
    bottomoffset=2.25ex,
    background=color,
    backgroundcolor=lightgray,
  ]

Frame and textbackground handle the offsets differently. Frame adds the rulethickness to the offset, while textbackground does not. So, I have manually set the offset in each direction. The above code gives the following output:

So far, so good. So, all that we need is to replace frame=on with frame=off, leftframe=on. Unfortunately, that does not work.

\definetextbackground
  [leftbartext]
  [
    ...
    frame=off,
    leftframe=on,
    ...
  ]

gives

Notice that the output has no frame! To understand why this happens, we need to dig a bit into how textbackgrounds work. Most of the heavy lifting for drawing the background is done by Metapost. The default setting of textbackground has (in anch-bck.mkvi

\setuptextbackground
  [
    ...
    mp=mpos:region:draw,
    ...
  ]

where mpos:region:draw is defined as

\startuseMPgraphic{mpos:region:draw}
    draw_multi_pars
\stopuseMPgraphic

which simply calls draw_multi_pars defined in mp-abck.mpiv. The reason that we don’t get anything when frame=off is because draw_multi_pars does not include the code to selectively draw the boundary. mp-abck.mpiv also includes another macro called draw_multi_side to draw only the left boundary. So, we can define the left-bar textbackground as follows:

\definetextbackground
  [leftbartext]
  [
    ..
    mp=mpos:region:leftbar,
    frame=off,
    ..
  ]


\startuseMPgraphic{mpos:region:leftbar}
  draw_multi_pars;
  draw_multi_side;
\stopuseMPgraphic

We don’t need to specify leftframe=on because draw_multi_side only draws the leftframe. To conclude, here is the complete code:

\definetextbackground
  [leftbartext]
  [
    location=paragraph,
    mp=mpos:region:leftbar,
    width=broad,
    frame=off,
    framecolor=darkgray,
    rulethickness=2ex,
    leftoffset=5ex,
    rightoffset=2.25ex,
    topoffset=2.25ex,
    bottomoffset=2.25ex,
    background=color,
    backgroundcolor=lightgray,
  ]


\startuseMPgraphic{mpos:region:leftbar}
  draw_multi_pars;
  draw_multi_side;
\stopuseMPgraphic

As in the case of the definition using framedtext, it is straight forward to inherit from leftbartext text background to get backgrounds with different color:

\definecolor[lightred]   [0.95(red,white)]
\definecolor[lightgreen] [0.95(green,white)]
\definecolor[lightblue]  [0.95(blue,white)]

\definetextbackground
  [exampletext]
  [leftbartext]
  [
    framecolor=darkgreen,
    backgroundcolor=lightgreen,
  ]

\definetextbackground
  [alerttext]
  [leftbartext]
  [
    framecolor=darkred,
    backgroundcolor=lightred,
  ]

\definetextbackground
  [blocktext]
  [leftbartext]
  [
    framecolor=darkblue,
    backgroundcolor=lightblue,
  ]

which gives

This entry was posted in Visualization and tagged backgrounds, framed.