Background

A line is being drawn using MetaPost on an overlay, which is used as the background for a framed layer. The layer is embedded within a setups so that a dot's position on the timeline is recalculated each time. The calculation is based on the value of the current section's title text.

Problem

The horizontal bars at the each end of the timeline shifts a slight amount. Compare:

When shown overlapped, the difference is apparent, albeit subtle:

Code

The minimal working example is reasonably straightforward. The bulk of the code is taken up by parsing the number from the section title within startuseMPgraphic.

definepapersize[BookPaperSize][
 width=8in,
 height=6in,
]

setuppapersize[BookPaperSize]

% Specify the width and height here because the inline figures need to
% maintain aspect ratio while setting a pleasing width.
startsetups[BookIllustrationSetups]
 setlayerframed[BookIllustrationLayer][
 frame=off,
 x=zeropoint,
 y=1.25in,
 width=makeupwidth,
 background=BookTimelineOverlay,
 ]
stopsetups

setupbackgrounds[page][
 background=BookIllustrationLayer,
 setups=BookIllustrationSetups
]

definelayer[BookIllustrationLayer][
 width=paperwidth,
 height=paperheight,
]

defineoverlay[BookTimelineOverlay][useMPgraphicBookTimelineGraphic]

startuseMPgraphicBookTimelineGraphic
 % Define constants
 numeric EVENT_BEGAN, TIME_OFFSET, PATH_THICKNESS;
 EVENT_BEGAN := 13799;
 TIME_OFFSET := 1.25in;
 PATH_THICKNESS := 2pt;

 % The book timeline title macro contains a number representing when the
 % event transpired and additional information.
 string sectionTitle, sectionTitleDigits;
 sectionTitle := "getspecificstructuretitle3";
 sectionTitleDigits := "";

 % Extract only digits and decimals from the book timeline title macro.
 for i = 0 upto length( sectionTitle ):
 string sectionChar;
 sectionChar := substring( i, i + 1 ) of sectionTitle;

 % A space indicates that the number has no more digits.
 if sectionChar = " ":
 break;
 fi;

 % Concatentate all the digits together.
 if ((sectionChar >= "0") and (sectionChar <= "9")) or (sectionChar = "."):
 sectionTitleDigits := sectionTitleDigits & sectionChar;
 fi;
 endfor;

 % Convert the digits from a string to a numeric value.
 numeric eventTime;
 eventTime := 0;

 for i = scantokens( sectionTitleDigits ):
 eventTime := i;
 endfor;

 numeric eventX;
 eventX := 1 - (eventTime / EVENT_BEGAN);

 % Draw the line.
 draw (TIME_OFFSET, .5*overlayheight) -- (overlaywidth, .5*overlayheight) 
 withpen pencircle scaled PATH_THICKNESS;

 % Draw starting |.
 draw (TIME_OFFSET, 0) -- (TIME_OFFSET, overlayheight) 
 withpen pencircle scaled PATH_THICKNESS;

 % Draw ending |.
 draw (overlaywidth, 0) -- (overlaywidth, overlayheight) 
 withpen pencircle scaled PATH_THICKNESS;

 % Draw the timeline's dot relative to the event on the timeline.
 filldraw fullcircle scaled 9 
 shifted( 
 TIME_OFFSET + eventX * (overlaywidth - TIME_OFFSET), .5*overlayheight 
 );
stopuseMPgraphic

starttext
 chapter[title=Inflation Theory,reference=inflation-theory]
 section[title=13,799 ± 0.021,reference=section]
 input ward

 chapter[title=First Stars,reference=first-stars]
 section[title=13,689 ± 70,reference=section-1]
 input ward
stoptext

Questions

I'm wondering:

• Why does the timeline bar shift slightly?


• How can the shifting be eliminated such that the timeline bars precisely overlap regardless of the timeline's dot's location?

Bonus question:

• How can the code be simplified, especially parsing the title?

The placement of the dot modifies the bounding box. The most ad-hoc solution to this problem is

1. Save the bounding box before drawing the dot


2. Draw the dot


3. Restore the bounding box without dot

The dot will then leak out of the bounding box but that is only a tiny bit.

definepapersize[BookPaperSize][
 width=8in,
 height=6in,
]

setuppapersize[BookPaperSize]

% Specify the width and height here because the inline figures need to
% maintain aspect ratio while setting a pleasing width.
startsetups[BookIllustrationSetups]
 setlayerframed[BookIllustrationLayer][
 frame=off,
 x=zeropoint,
 y=1.25in,
 width=makeupwidth,
 background=BookTimelineOverlay,
 ]
stopsetups

setupbackgrounds[page][
 background=BookIllustrationLayer,
 setups=BookIllustrationSetups
]

definelayer[BookIllustrationLayer][
 width=paperwidth,
 height=paperheight,
]

defineoverlay[BookTimelineOverlay][useMPgraphicBookTimelineGraphic]

startuseMPgraphicBookTimelineGraphic
 % Define constants
 numeric EVENT_BEGAN, TIME_OFFSET, PATH_THICKNESS;
 EVENT_BEGAN := 13799;
 TIME_OFFSET := 1.25in;
 PATH_THICKNESS := 2pt;

 % The book timeline title macro contains a number representing when the
 % event transpired and additional information.
 string sectionTitle, sectionTitleDigits;
 sectionTitle := "getspecificstructuretitle3";
 sectionTitleDigits := "";

 % Extract only digits and decimals from the book timeline title macro.
 for i = 0 upto length( sectionTitle ):
 string sectionChar;
 sectionChar := substring( i, i + 1 ) of sectionTitle;

 % A space indicates that the number has no more digits.
 if sectionChar = " ":
 break;
 fi;

 % Concatentate all the digits together.
 if ((sectionChar >= "0") and (sectionChar <= "9")) or (sectionChar = "."):
 sectionTitleDigits := sectionTitleDigits & sectionChar;
 fi;
 endfor;

 % Convert the digits from a string to a numeric value.
 numeric eventTime;
 eventTime := 0;

 for i = scantokens( sectionTitleDigits ):
 eventTime := i;
 endfor;

 numeric eventX;
 eventX := 1 - (eventTime / EVENT_BEGAN);

 % Draw the line.
 draw (TIME_OFFSET, .5*overlayheight) -- (overlaywidth, .5*overlayheight)
 withpen pencircle scaled PATH_THICKNESS;

 % Draw starting |.
 draw (TIME_OFFSET, 0) -- (TIME_OFFSET, overlayheight)
 withpen pencircle scaled PATH_THICKNESS;

 % Draw ending |.
 draw (overlaywidth, 0) -- (overlaywidth, overlayheight)
 withpen pencircle scaled PATH_THICKNESS;

 % The dot will change the size of bounding box, so we will just save the
 % bounding box without the dot for later
 path p ; p := bbox currentpicture ;

 % Draw the timeline's dot relative to the event on the timeline.
 filldraw fullcircle scaled 9
 shifted(
 TIME_OFFSET + eventX * (overlaywidth - TIME_OFFSET), .5*overlayheight
 );

 % Restore the bounding box without dot
 setbounds currentpicture to p ;
stopuseMPgraphic

starttext
 chapter[title=Inflation Theory,reference=inflation-theory]
 section[title=13,799 ± 0.021,reference=section]
 input ward

 chapter[title=First Stars,reference=first-stars]
 section[title=13,689 ± 70,reference=section-1]
 input ward
stoptext

Here is a little shorter version of your code, using Lua to parse the chapter title and using the MetaPost drawing as a background for the text layer of the page directly.

definepapersize
 [BookPaperSize]
 [width=8in,
 height=6in]

setuppapersize[BookPaperSize]

defineoverlay
 [BookTimelineOverlay]
 [useMPgraphicBookTimelineGraphic]

setupbackgrounds
 [text]
 [background=BookTimelineOverlay]


startluacode
function userdata.parsetitle(title)
 local a, b = string.match(title, "(%d+),(%d+)")
 return table.concat( a, b , "")
end
stopluacode

startuseMPgraphicBookTimelineGraphic
 % Define constants
 numeric EVENT_BEGAN;
 EVENT_BEGAN := 13799;

 % Convert the digits from a string to a numeric value.
 numeric eventX;
 eventX := lua("mp.print(userdata.parsetitle('getspecificstructuretitle3'))") ;
 eventX := 1 - (eventX / EVENT_BEGAN);

 numeric wb, hb ; wb := 3.33in ; hb := .33in ;
 pickup pencircle scaled 2pt;
 picture p ; p := image(
 % Draw the line.
 draw (0, .5*hb) -- (wb, .5*hb) ;
 % Draw starting |.
 draw (0, 0) -- (0, hb) ;
 % Draw ending |.
 draw (wb, 0) -- (wb, hb) ;
 % Draw the timeline's dot relative to the event on the timeline.
 filldraw fullcircle scaled 9 shifted (eventX * wb, .5*hb);
 );

 numeric h ; h := OverlayHeight ;
 draw p shifted (-.33in, h - hb - .33in);

 setbounds currentpicture to OverlayBox ;
stopuseMPgraphic

starttext
 chapter[title=Inflation Theory,reference=inflation-theory]
 section[title=13,799 ± 0.021,reference=section]
 input ward

 chapter[title=First Stars,reference=first-stars]
 section[title=13,689 ± 70,reference=section-1]
 input ward
stoptext