Example step

Find the mouse position and translate it to scene's coordinates

Goal

Understand the difference between page, canvas and scene coordinates and the related formulas

start from our default scene
open the javascript console, we'll use it for debug
note that we are going to use jQuery, as trying to manage different browsers' events is not our aim and would require some boilerplate

define a new function and name it onMouseMove

function onMouseMove(event) {

   // code here
   
}

now we need to listen for mousemove event on canvas element, passing it the event handler function onMouseMove
```
$(canvas).on('mousemove', onMouseMove);
```
our onMouseEvent function will receive event as argument, which is normalized by jQuery in order to provide consistent data across different environments
```
 $(canvas).on('mousemove', evenHandler);
```
with event we can get the current mouse position related to the page
```
var pageX = event.pageX;
var pageY = event.pageY;
```
then we will get the current canvas' offsets: the distance from the left and the top borders
```
var offset = $(renderer.domElement).offset();
```
first, let's calculate the mouse position related to the canvas: just subtract offset.left from pageX and offset.top from event.pageY
```
var canvasX = pageX - offset.left;
var canvasY = pageY - offset.top;
```
print it to the console or write it into an HTML element in order to monitor it and try to run the page just to see it
now we must translate local coordinates to WebGL context's coordinates: we know that the context is just a cartesian graph with (0,0) in the center of canvas, horizontal x axis and vertical y axis. The limits are (-1,1) for all axis

so just divide the local coordinates per canvas width, multiply it for 2 and subtract 1 to the result: these are the real canvas coordinates!

var mouseX = (localX / renderer.domElement.width) * 2 - 1;
var mouseY = (1 - (localY / renderer.domElement.height)) * 2 - 1;

print or write the canvas coordinates in order to see and compare them with local and page coordinates.