Tile tile = tileCache.get(tileRequest);
BufferedImage image;
switch (renderRequest.layer) {
case DIFFERENCE :
image = tile.linearCombination(1, surfA, -1, surfB, 0, renderRequest);
break;
case HAGERSTRAND :
long elapsed = Math.abs(surfB.dateTime - surfA.dateTime);
image = tile.linearCombination(-1, surfA, -1, surfB, elapsed/60, renderRequest);
break;