rem: var delta_x = viewPortPx.x - (size.w / 2); var delta_y = viewPortPx.y - (size.h / 2); 
add: var delta_x = viewPortPx.x - Math.ceil(size.w / 2); var delta_y = viewPortPx.y - Math.ceil(size.h / 2); 
context:     getLonLatFromViewPortPx: function (viewPortPx) {        var lonlat = null;        if (viewPortPx != null) {            var size = this.map.getSize();            var center = this.map.getCenter();            var res  = this.map.getResolution();                    var delta_x = viewPortPx.x - (size.w / 2);            var delta_y = viewPortPx.y - (size.h / 2);                        lonlat = new OpenLayers.LonLat(center.lon + delta_x * res ,                                         center.lat - delta_y * res);         }        return lonlat;    },
meta: 1027	/local1/tlutelli/issta_data/temp/all_js/javascript/2006_temp/2006/1027/bef4a7d3a95909b1b1fceca1ae4806379ff8d401/Layer.js/clean/lib/OpenLayers/Layer.js