long elapsed = Math.abs(surfB.dateTime - surfA.dateTime);
image = tile.linearCombination(-1, surfA, -1, surfB, elapsed/60, renderRequest);
break;
case TRAVELTIME :
default :
image = tile.generateImage(surfA, renderRequest);
}
// add a timestamp to the image if requested.
// of course this will make it useless as a raster for analysis, but it's good for animations.
if (renderRequest.timestamp) {