// mark point of interest in an animated way
svoid g22markPointAnimation(ImageSurface is, Pt pt) {
  is.addOverlay(g -> {
    var p = scalePt(pt, is.getZoomX(), is.getZoomY());
    double blend = zeroToOneSine(frac(sysSeconds()));
    drawSquareAround(g, p, 1, blendColor(blend, Color.green, Color.red));
    drawSquareAround(g, p, 2, blendColor(blend, Color.red, Color.green));
    drawSquareAround(g, p, 3, blendColor(blend, Color.white, Color.black));
  });
  repaintEvery(is, 100);
}