Falling Leaves

Make the falling thing read as a leaf. Reach for ambient-section-particles when you want a bounded atmosphere of generic motes. Reach for this when the shape has to be recognisable.

Build the tumble first

Turn each leaf about its own long axis so it shows its face, thins to nothing edge-on, then opens out on the other side. That instant of near-disappearance is what the eye reads as "leaf". A sprite that only spins in the picture plane reads as confetti, a coin, or a paper scrap, however good the artwork is.

On 2-D canvas the tumble is a horizontal scale that crosses zero:

ctx.save();
ctx.translate(l.x, l.y);
ctx.rotate(l.roll);              // long axis drifting in-plane
ctx.scale(Math.cos(l.spin), 1);  // the tumble: cos crosses 0, edge-on
ctx.globalAlpha = l.alpha;
ctx.drawImage(sprite, -w / 2, -h / 2, w, h);
ctx.restore();