Your Own Positions

For a physics loop, a canvas-driven position or anything else you set by hand, call render once a frame. The first call stops the automatic watching.

blur.render(previous, current);

Drag the ball and let go, or press Throw. Its position comes from a physics loop, and the blur fades out on its own as it slows down.

Curved Paths

Pass a path instead when the motion bends, so Echo puts its copies along the curve instead of on a straight line between two frames:

el.style.transform = at(angle).toString();
blur.render((t) => at(angle + t * speed * frameTime));

t is in frames, so path(-0.5) is half a frame back.

The dot goes round three and a half times a second. With Echo, the copies follow the circle, so the streak bends with it. The other techniques blur along a straight line.