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Measure a hit test over overlapping pictures by hiding one item and diffing two screenshots, shadows hidden in both

Rule

To score a hit test, diff screenshots with each item hidden and shown; hide shadows and drop pixels that change by themselves, or they count as misses.

Snippet
// before: score the hit test against each item's box
const truth = (x, y) => itemBoxes.findLast((b) => inside(b, x, y))?.index ?? -1;
// after: the pixels where item i is what you see, from two screenshots
await page.evaluate((i) => window.__rail.hide(i, { shadows: false }), i);
const without = await shot(page);
await page.evaluate((i) => window.__rail.show(i, { shadows: false }), i);
const withIt = await shot(page);
const pixels = diff(withIt, without).filter((p) => stableAcrossTwoShots(p));
const right = pixels.filter((p) => hitTest(p.x, p.y) === i).length / pixels.length;
Check

in dark mode, with shadows hidden in both shots, the "wrong item" share falls to what the hit test really gets wrong (here from 10% to about 3%).

Applies to

other: Puppeteer or Playwright screenshots of a web page

I assumed

each piece's pixels could be taken from its box, or from one screenshot.

What happened

a box says nothing about what is visible in a bunch. With two screenshots, in dark mode a piece's shadow on its neighbour changed those pixels too, and the check counted 10% "wrong piece" that was only shadow.

The fix

for each piece, a screenshot with it hidden against one with it shown gives exactly the pixels where it is what you see; shadows are hidden in both shots and pixels that change between two identical shots are left out.

Evidence

"wrong piece" 10% with shadows, about 3% without, in dark (commit 291065a).

Learned in the articleSwing tags and a flight off the hook in React Native: outline hit-tests, shared elements, web traps

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