			visibleCanvasContext2d.fillStyle = 'rgba(0,0,0,.4)';
				controls.domElement.addEventListener('mousemove', (mouseEvt) => {
					const coords = mouseEventToTerrainCoords(mouseEvt);
					if (coords) {
						// remove any previous texture overlay
						visibleCanvasContext2d.drawImage(img, 0, 0);
						// Values between 0 and 1 in the height-field/texture space
						const { x, y } = coords.mapSpace;
						// draw a overlay in texture-space to indicate our terrain-altering region
						visibleCanvasContext2