q-to-print-size-math.js

// The standard chrome points per pixel value
const chromePPI = 96;

function mmToInch(mm) {
  return mm / 25.4;
}

// this is NZZ print specific
const columnGap = 5;
const columnWidth = 54.2;
function colsToMm(cols) {
  // explanation of the math here:
  // 1 column is 54.2mm wide
  // the gap between the columns is 5mm, so for 2 columns, we need to add 1*5 mm for the column gap.
  // For 2.5 columns we need to add 2*5mm for the gap (therefore the Math.ceil)
  // the whole thing gets rounded to 2 positions after the . in the end.
  let mm = Math.round((cols * columnWidth + (Math.ceil(cols) - 1) * columnGap) * 100) / 100;

  // if we have some 1/2 columns, we need to remove 1/2 column gap
  // because these graphics will always be combined with another one with 1/2 column next to it, and there should be some gap between
  if (Math.ceil(cols) - cols === 0.5) {
    mm = mm - columnGap / 2;
  }

  if (cols >= 6) {
    return mm + 24; // Add 24mm if the graphic spans across pages (more than 5 columns)
  }

  return mm;
}

const dpi = 1200;
// dpr is the devicePixelRatio we tell the headless chrome to use
// it is the factor from CSS pixels to physical pixels (physical in this case: what ends up in the PNG)
// we need this to end up with correct font sizes
const dpr = dpi / chromePPI;
const screenshotParameters = {
  dpr,
  //the screenshot width (a CSS pixel value) is the width in inch * target dpi / dpr
  width: Math.round((mmToInch(colsToMm(numberOfColumns)) * dpi) / dpr))
);