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The assignment says "Your task is to write an HTML file that contains JavaScript that will randomly display one of the images above. If the page is refreshed in the browser, you should get another random image." so I did it.
Now it says "When the user clicks anywhere on the image, display an alert window that shows the X and Y position of where the click occurred relative to the image". Here is my code:
<html>
<head>
<title>Assignment 2</title>
<script type="text/javascript">
var imageURLs = [
"p1.jpg"
, "p2.jpg"
, "p3.jpg"
, "p4.jpg"
];
function getImageTag() {
var img = '<img src=\"';
var randomIndex = Math.floor(Math.random() * imageURLs.length);
img += imageURLs[randomIndex];
img += '\" alt=\"Some alt text\"/>';
return img;
}
</script>
</head>
<body>
<script type="text/javascript">
document.write(getImageTag());
</script>
</body>
</html>
941
The position of x-coordinate of the mouse click is found by subtracting the event's x position with the bounding rectangle's x position.
You can actually use HTML for this. The image tag has an attribute known as ismap.
What this attribute does is specify that an image is part of a server-side image map. When clicking on such map, the click coordinates are sent to the server as a url query string.
Images must be nested under links for this to work. Here is an example
<a href="http://www.google.com">
<img src="myimage.png" alt="My Image" ismap">
</a>
If you can't use image maps for this, here is a javascript/jquery solution
First, you need to include the jQuery library at the bottom of your body tag.
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.3/jquery.min.js"></script>
$(document).ready(function() {
$("img").on("click", function(event) {
var x = event.pageX - this.offsetLeft;
var y = event.pageY - this.offsetTop;
alert("X Coordinate: " + x + " Y Coordinate: " + y);
});
});<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<img src="http://vignette1.wikia.nocookie.net/seraphina/images/b/b2/Dragonseraphina.jpg/revision/latest?cb=20160103194957" height="200" width="200" alt="dragon">You listen for the click event, and pass in event as the parameter.
The event.pageX property returns the position of the mouse pointer, relative to the left edge of the document.
98
The map solution will only give you the client side pixel coordinates. If you'd like to get the pixel coordinate relative to the original image you need the naturalHeight and naturalWidth information which has height and width of the original image.
code:
// https://stackoverflow.com/questions/34867066/javascript-mouse-click-coordinates-for-image
document.getElementById(imageid).addEventListener('click', function (event) {
// https://stackoverflow.com/a/288731/1497139
bounds=this.getBoundingClientRect();
var left=bounds.left;
var top=bounds.top;
var x = event.pageX - left;
var y = event.pageY - top;
var cw=this.clientWidth
var ch=this.clientHeight
var iw=this.naturalWidth
var ih=this.naturalHeight
var px=x/cw*iw
var py=y/ch*ih
alert("click on "+this.tagName+" at pixel ("+px+","+py+") mouse pos ("+x+"," + y+ ") relative to boundingClientRect at ("+left+","+top+") client image size: "+cw+" x "+ch+" natural image size: "+iw+" x "+ih );
});
$(document).ready(function() {
$("img").on("click", function(event) {
bounds=this.getBoundingClientRect();
var left=bounds.left;
var top=bounds.top;
var x = event.pageX - left;
var y = event.pageY - top;
var cw=this.clientWidth
var ch=this.clientHeight
var iw=this.naturalWidth
var ih=this.naturalHeight
var px=x/cw*iw
var py=y/ch*ih
alert("click on "+this.tagName+" at pixel ("+px+","+py+") mouse pos ("+x+"," + y+ ") relative to boundingClientRect at ("+left+","+top+") client image size: "+cw+" x "+ch+" natural image size: "+iw+" x "+ih );
});
});<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<img src="https://upload.wikimedia.org/wikipedia/commons/7/77/Avatar_cat.png" height="256" width="256" alt="kitten">example
click on IMG at pixel (445.5,334.125) mouse pos (148.5,49) relative to boundingClientRect at (483.5,64) client image size: 640 x 480 natural image size: 1920 x 1080
$(document).ready(function () {
$("img").on("click", function (event) {
$('#img_coordinate').html("X Coordinate: " + (event.pageX - this.offsetLeft) + "<br/> Y Coordinate: " + (event.pageY - this.offsetTop));
});
});
html
<img src="img/sample.gif" />
<p id="img_coordinate"></p>
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