EchoFrame
class EchoFrame(val zoomX: Float = 1.0f, val zoomY: Float = zoomX, val spin: Float = 0.0f, val driftX: Float = 0.0f, val driftY: Float = 0.0f, val centreX: Float = 0.5f, val centreY: Float = 0.5f, val copy: EchoCopy? = null)
How the last frame comes back this frame: zoomed (zoomX, zoomY per sixtieth of a second), turned (spin, radians a second) about a centre that can move, and drifted (shares of the screen a second). copy adds a second, fainter, turned or mirrored copy.