Detecting a column split is not the same as locating it

TLDR

Layout detectors frequently identify column split existence accurately while placing split coordinates off by $15\text{ to }32\text{px}$ due to ragged-right text clustering. Loosening spatial containment tolerances to absorb coordinate errors causes centered titles and email headers to be absorbed into columns. A bounded valley scan post-pass snaps detected split coordinates to true whitespace channels using coverage histograms without altering split counts.

Detector Step	Technical Responsibility	Coordinate Accuracy	Output Result
Pass 1: Split Detection	Confirms 2-column existence	Off by $32\text{px}$ ($X=566\text{px}$)	Body lines misclassified as full-width
Pass 2: Valley Scan Relocation	Snaps $X$ to local whitespace valley	Exact ($X=598\text{px}$)	Clean 2-column body reflow

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Technical problem analysis

In a 16-page two-column document, column detection reported a split at $X = 566\text{px}$. The actual physical gutter center was $X = 598\text{px}$.

Because $566\text{px}$ fell inside the left column text region, 13 body lines spanning $144\text{px}$ to $581\text{px}$ straddled the boundary by $15\text{px}$, causing them to fall out of their column and render as full-width blocks across the page center.

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Technical solution: bounded valley scan post-pass

Perform a localized histogram scan bounded to 3 body-font heights around the initial split coordinate:

export function snapSplitToWhitespaceValley(detectedSplitX, textItems, bodyFontHeightPx = 12) {
  const scanWindowPx = bodyFontHeightPx * 3; // Bounded search window radius
  const lo = detectedSplitX - scanWindowPx;
  const hi = detectedSplitX + scanWindowPx;
// Compute coverage histogram stepping 2px across search window   const coverageHistogram = computeCoverageHistogram(textItems, lo, hi, 2);   const minCoverage = Math.min(...coverageHistogram.counts);
// Valley threshold tolerates full-page footer lines crossing the center   const valleyThreshold = minCoverage + Math.max(2, Math.ceil(minCoverage * 0.5));   const valleyRun = findWidestRunBelowThreshold(coverageHistogram, valleyThreshold);
// Safety Guards: Ignore runs touching window edges or narrower than 0.75x font size   if (valleyRun.widthPx < bodyFontHeightPx * 0.75 || valleyRun.touchesWindowEdge) {     return detectedSplitX; // Fail-safe: keep original coordinate   }
return valleyRun.centerX; }

Verification results

• Before Valley Scan: Split coordinate $X = 566\text{px} \rightarrow 13$ intro paragraph lines rendered as full-width breaks.
• After Valley Scan: Split snapped to $X = 598.5\text{px} \rightarrow$ All body lines correctly assigned to column 1.

Rule of thumb: Separate column existence detection from coordinate localization, using bounded valley scans to snap split coordinates to physical whitespace channels.