Interactive Image Maps

We can build interactive image maps in Javascript using the HTML canvas element. The canvas element is a container which permits Javascript to draw on the page. The canvas element supports paths, boxes, circles and other shapes drawn by Javascript running in the browser.

The HTML map tag is used to define an image map. An image map consists of a set of area elements. An image map can be defined as follows:

<map name="map">
  <area shape="poly" id="a" coords="0,0,1,1,2,2,0,0" />
  <area shape="poly" id="b" coords="3,3,4,4,5,5,3,3" />

The coords attribute is a series of coordinate pairs. For the area with id="a", the polygon is defined by the coordinates (0,0), (1,1), (2,2), (0,0).

Define area poygons

Let’s say we want to create an interactive map of the human body. We could pull an image with different muscle groups outlined.

We could use an image editing program to get the coordinates of different polygons within the image. For example, we could use two polygons to define the pectoral muscles. Using GIMP or another program, we could take the coordinates of those polygons and create an image map HTML element.

<map name="map">
  <area shape="poly" alt="Left pec" coords="181,131,182,171,188,178..." />
  <area shape="poly" alt="Right pec" coords="175,131,175,171,172,176..." />

Draw polygons

If we want to highlight the polygons when the user clicks one of the pectoral muscles, we will need to use some Javascript to draw on the canvas element. We can use the function getElementsByTagName to select all of the area elements in our image map.

function drawAllShapes() {
  var elements = document.getElementsByTagName("area");

  for (let element of elements) {
    var coords = element.coords;

Now that we have the coordinates of the polygon, we need to draw them on the canvas. We will use the CanvasRenderingContext2D to draw on the surface of the canvas element.

function drawShape(coords) {
  var can = document.getElementById('myCanvas');
  var hdc = can.getContext('2d');

  // parse coordinates
  var mCoords = coords.split(',');

  // draw on context
  hdc.moveTo(mCoords[0], mCoords[1]);
  for (i=2; i<mCoords.length; i+=2)
    hdc.lineTo(mCoords[i], mCoords[i+1]);

  hdc.lineTo(mCoords[0], mCoords[1]);

In this way a user action can create a drawing on top of an image. The canvas element and its APIs are a hidden gem and are quite powerful!