d3js: Use view port center for zooming focal point - dictionary

Question

We are trying to implement zoom buttons on top of a map created in D3 - essentially as it works on Google maps. The zoom event can be dispatched programmatically using
d3ZoomBehavior.scale(myNewScale);
d3ZoomBehavior.event(myContainer);
and the map will zoom using the current translation for the view. If using zoom buttons the focal point (zoom center) is no longer the translation but the center of the view port. For zoom using the scroll wheel we have the option of using zoom.center - but this apparently have no effect when dispatching your own event.
I'm confused as to how a calculate the next translation taking the new scaling factor and the view port center into account.
Given that I know the current scale, the next scale, the current translation and the dimensions of the map view port how do I calculate the next translation, so that the center of the view port do not change?

Answer 1

I've recently had to do the same thing, and I've got a working example up here http://bl.ocks.org/linssen/7352810. Essentially it uses a tween to smoothly zoom to the desired target scale as well as translating across by calculating the required difference after zooming to centre.
I've included the gist of it below, but it's probably worth looking at the working example to get the full effect.
html
<button id="zoom_in">+</button>
<button id="zoom_out">-</button>
js
var zoom = d3.behavior.zoom().scaleExtent([1, 8]).on("zoom", zoomed);
function zoomed() {
svg.attr("transform",
"translate(" + zoom.translate() + ")" +
"scale(" + zoom.scale() + ")"
);
}
function interpolateZoom (translate, scale) {
var self = this;
return d3.transition().duration(350).tween("zoom", function () {
var iTranslate = d3.interpolate(zoom.translate(), translate),
iScale = d3.interpolate(zoom.scale(), scale);
return function (t) {
zoom
.scale(iScale(t))
.translate(iTranslate(t));
zoomed();
};
});
}
function zoomClick() {
var clicked = d3.event.target,
direction = 1,
factor = 0.2,
target_zoom = 1,
center = [width / 2, height / 2],
extent = zoom.scaleExtent(),
translate = zoom.translate(),
translate0 = [],
l = [],
view = {x: translate[0], y: translate[1], k: zoom.scale()};
d3.event.preventDefault();
direction = (this.id === 'zoom_in') ? 1 : -1;
target_zoom = zoom.scale() * (1 + factor * direction);
if (target_zoom < extent[0] || target_zoom > extent[1]) { return false; }
translate0 = [(center[0] - view.x) / view.k, (center[1] - view.y) / view.k];
view.k = target_zoom;
l = [translate0[0] * view.k + view.x, translate0[1] * view.k + view.y];
view.x += center[0] - l[0];
view.y += center[1] - l[1];
interpolateZoom([view.x, view.y], view.k);
}
d3.selectAll('button').on('click', zoomClick);

Answer 2

A more succinct version of Wil's solution:
var vis = d3.select('.vis');
var zoom = d3.behavior.zoom()...
var width = .., height = ..;
function zoomByFactor(factor) {
var scale = zoom.scale();
var extent = zoom.scaleExtent();
var newScale = scale * factor;
if (extent[0] <= newScale && newScale <= extent[1]) {
var t = zoom.translate();
var c = [width / 2, height / 2];
zoom
.scale(newScale)
.translate(
[c[0] + (t[0] - c[0]) / scale * newScale,
c[1] + (t[1] - c[1]) / scale * newScale])
.event(vis.transition().duration(350));
}
};
function zoomIn() { zoomByFactor(1.2); }
function zoomOut() { zoomByFactor(0.8); }

Answer 3

I've found this to be quite difficult to do in practice. The approach I've taken here is to simply create a mouse event that triggers the zoom when the zoom buttons are used. This event is created at the center of the map.
Here's the relevant code:
.on("click", function() {
var evt = document.createEvent("MouseEvents");
evt.initMouseEvent(
'dblclick', // in DOMString typeArg,
true, // in boolean canBubbleArg,
true, // in boolean cancelableArg,
window,// in views::AbstractView viewArg,
120, // in long detailArg,
width/2, // in long screenXArg,
height/2, // in long screenYArg,
width/2, // in long clientXArg,
height/2, // in long clientYArg,
0, // in boolean ctrlKeyArg,
0, // in boolean altKeyArg,
(by > 0 ? 0 : 1), // in boolean shiftKeyArg,
0, // in boolean metaKeyArg,
0, // in unsigned short buttonArg,
null // in EventTarget relatedTargetArg
);
this.dispatchEvent(evt);
});
The whole thing is a bit of a hack, but it works in practice and I've found this much easier than to calculate the correct center for every offset/zoom.