javascript - Correct Mouseposition on element while using CSS aspect ratio -


i have canvas scales 16:9 aspect ratio. mouse not take offset (black background) account.

the dotted green line marks canvas. @ edge mouse should display: mouse : 0% | 23% rather mouse : 7% | 23%.

enter image description here

the problem seems occur here:

var x = math.floor( mouse.x / canvas.width * 100 );  var y = math.floor( mouse.y / canvas.height * 100 );

it works fine when go fullscreen mode. (because there no offset) otherwise not take offset account.

additional code:

css:

#aspectoverlay {     /*   16:9    */     width: 100vw;      height: 56.25vw; /* 100/56.25 = 1.778 */     border: 1px #afff00 dashed;     max-height: 100vh;     max-width: 177.78vh; /* 16/9 = 1.778 */     margin: auto;     position: absolute;     top:0;bottom:0; /* vertical center */     left:0;right:0; /* horizontal center */     z-index: 100;     opacity: 1; }

js:

var canvas = document.getelementbyid('aspectoverlay');     canvas.width = window.innerwidth;     canvas.height = window.innerheight; var ctx = canvas.getcontext('2d');  window.addeventlistener('resize', resizecanvas, false); function resizecanvas () {     canvas.width = window.innerwidth;     canvas.height = window.innerheight; } canvas.addeventlistener('mousemove', function(e){     mouse.x = e.x;      mouse.y = e.y; }); var x = math.floor( mouse.x / canvas.width * 100 );  var y = math.floor( mouse.y / canvas.height * 100 ); ctx.filltext("mouse : " + x + "% | " + y + "%", 50,50);

you need take offsets of element account so

mouse.x = e.x - this.offsetleft; mouse.y = e.y - this.offsettop;

and since using css resize , setting canvas innerheight , width, can't use canvas width , height calculations anymore need use offsetwidth , offsetheight

var x = math.floor(mouse.x / canvas.offsetwidth * 100); var y = math.floor(mouse.y / canvas.offsetheight * 100);

live demo

full screen

var canvas = document.getelementbyid('aspectoverlay'); canvas.width = window.innerwidth; canvas.height = window.innerheight;  var ctx = canvas.getcontext('2d'),     mouse = {         x: 0,         y: 0     };  ctx.fillstyle = "#fff"; window.addeventlistener('resize', resizecanvas, false);  function resizecanvas() {     canvas.width = window.innerwidth;     canvas.height = window.innerheight;     ctx.fillstyle = "#fff"; }  canvas.addeventlistener('mousemove', function (e) {     ctx.clearrect(0, 0, canvas.width, canvas.height);     mouse.x = e.x - this.offsetleft;     mouse.y = e.y - this.offsettop;      var x = math.floor(mouse.x / canvas.offsetwidth * 100);     var y = math.floor(mouse.y / canvas.offsetheight * 100);     ctx.filltext("mouse : " + x + "% | " + y + "%", 50, 50); });

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