# Fil/d3-inertia

Dragging stuff with inertia
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# Inertia

An extension to d3-drag that continues the mouse movement with some inertia (by default, 5 seconds).

The `inertia` object exposes a `position` and a `velocity`, that correspond to the mouse coordinates relative to the target of the `d3.drag` method. During the drag gesture, the position just follows the mouse, and the velocity accumulates the movement. When the drag gesture ends, the `render()` method is called repeatedly with a tweening argument `t` that goes from 0 to 1. It is up to you to know what to do with these vectors.

The most simple case is: `currentPosition = last position + t * velocity`.

This example demonstrates a more creative use of the parameters. As you can see, `d3.inertia` can be used on its own, but you will need to manage quite a few parameters.

(TODO: a helping function for common use cases).

# Dragging the globe

The naïve method to rotate a globe uses `mouse.x` and `mouse.y` as proxies for longitude and latitude. It works when the rotation is small, but try to put the globe "upside-down" and suddenly moving the mouse to the left rotates the globe to the right, and vice versa.

The correct solution is to track the spherical coordinates of the point that is under the mouse, and apply a rotation to the globe that will move the initial point to the current mouse position. Computing that rotation involves quaternions.

This method, introduced by Jason Davies and Mike Bostock, is called versor dragging.

d3-inertia packages this method.

Include the scripts in your page with

``````<script src="https://unpkg.com/versor"></script>
<script src="https://unpkg.com/d3-inertia"></script>
``````

Then, define a `render()` function that redraws the globe, and call:

``````var inertia = d3.geoInertiaDrag(canvas, function() { render(); }, [projection], [opt]);
``````

SVG example + code

canvas is the target container (it can also be svg)

projection is an object with two methods:

• projection.rotate([r]) gets and sets the rotation.

• projection.invert([x,y]) transforms screen coordinates into spherical coordinates lambda, phi; most projections have an exact invert, but for the few which do not, this doesn't have to be very accurate to give an “almost correct” movement.

If a projection is defined globally, and not specified in the function call, the plugin will use it.

opt is an array of options, which can include:

• start, move, end: callback functions on the corresponding events

• time: reference time for the interia (in ms, default 5000)

# Credits

Thanks to Jason Davies, Mike Bostock and Widi Harsojo.

The quaternion & versor functions are taken from the versor package.