setInstancedMeshPositions.js

function setInstancedMeshPositions(mesh, section) {
  for ( var i = 0; i < mesh.count; i ++ ) {
    var xStaticPosition = -sectionWidth * (i - 1);
		var xSectionPosition = sectionWidth * section;
		var x = xStaticPosition + xSectionPosition;
		dummy.position.set(x, 0, 0);
		dummy.updateMatrix();
    mesh.setMatrixAt( i, dummy.matrix );
  }
  mesh.instanceMatrix.needsUpdate = true;
}