Skip to content

Instantly share code, notes, and snippets.

@ghostcode

ghostcode/zoom.html

Forked from huawww/zoom.html
Created August 10, 2022 16:20
Show Gist options
  • Save ghostcode/4ada69e1cca519aad483b8e60c161621 to your computer and use it in GitHub Desktop.
Save ghostcode/4ada69e1cca519aad483b8e60c161621 to your computer and use it in GitHub Desktop.
Download ZIP
Raw
zoom.html
<!DOCTYPE HTML>
<html lang="en"><head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Zooming via HTML5 Canvas Context</title>
<style type="text/css" media="screen">
body { background:#eee; margin:1em; text-align:center; }
canvas { display:block; margin:1em auto; background:#fff; border:1px solid #ccc }
</style>
</head><body>
<p>Showing how to use transform methods on the HTML5 Canvas Context to selectively zoom in and out. Drag to pan. Click to zoom at that location. Shift-click to zoom out. Mousewheel up/down over the canvas to zoom in to/out from that location.</p>
<p>By redrawing the canvas at different scales the strokes remain smooth during zooming. For example, zoom in on the earring or circle on the forehead to see fine detail.</p>
<canvas></canvas>
<p id="footer">Copyright © 2011 <a href="mailto:[email protected]">Gavin Kistner</a>. Written to support <a href="http://stackoverflow.com/questions/5189968/zoom-to-cursor-calculations/5526721#5526721">this Stack Overflow answer</a>.</p>
<script type="text/javascript" charset="utf-8">
var canvas = document.getElementsByTagName('canvas')[0];
canvas.width = 800; canvas.height = 600;
var gkhead = new Image;
var ball = new Image;
window.onload = function(){
var ctx = canvas.getContext('2d');
trackTransforms(ctx);
function redraw(){
// Clear the entire canvas
var p1 = ctx.transformedPoint(0,0);
var p2 = ctx.transformedPoint(canvas.width,canvas.height);
ctx.clearRect(p1.x,p1.y,p2.x-p1.x,p2.y-p1.y);
// Alternatively:
// ctx.save();
// ctx.setTransform(1,0,0,1,0,0);
// ctx.clearRect(0,0,canvas.width,canvas.height);
// ctx.restore();
ctx.drawImage(gkhead,200,50);
ctx.beginPath();
ctx.lineWidth = 6;
ctx.moveTo(399,250);
ctx.lineTo(474,256);
ctx.stroke();
ctx.save();
ctx.translate(4,2);
ctx.beginPath();
ctx.lineWidth = 1;
ctx.moveTo(436,253);
ctx.lineTo(437.5,233);
ctx.stroke();
ctx.save();
ctx.translate(438.5,223);
ctx.strokeStyle = '#06c';
ctx.beginPath();
ctx.lineWidth = 0.05;
for (var i=0;i<60;++i){
ctx.rotate(6*i*Math.PI/180);
ctx.moveTo(9,0);
ctx.lineTo(10,0);
ctx.rotate(-6*i*Math.PI/180);
}
ctx.stroke();
ctx.restore();
ctx.beginPath();
ctx.lineWidth = 0.2;
ctx.arc(438.5,223,10,0,Math.PI*2);
ctx.stroke();
ctx.restore();
ctx.drawImage(ball,379,233,40,40);
ctx.drawImage(ball,454,239,40,40);
ctx.drawImage(ball,310,295,20,20);
ctx.drawImage(ball,314.5,296.5,5,5);
ctx.drawImage(ball,319,297.2,5,5);
}
redraw();
var lastX=canvas.width/2, lastY=canvas.height/2;
var dragStart,dragged;
canvas.addEventListener('mousedown',function(evt){
document.body.style.mozUserSelect = document.body.style.webkitUserSelect = document.body.style.userSelect = 'none';
lastX = evt.offsetX || (evt.pageX - canvas.offsetLeft);
lastY = evt.offsetY || (evt.pageY - canvas.offsetTop);
dragStart = ctx.transformedPoint(lastX,lastY);
dragged = false;
},false);
canvas.addEventListener('mousemove',function(evt){
lastX = evt.offsetX || (evt.pageX - canvas.offsetLeft);
lastY = evt.offsetY || (evt.pageY - canvas.offsetTop);
dragged = true;
if (dragStart){
var pt = ctx.transformedPoint(lastX,lastY);
ctx.translate(pt.x-dragStart.x,pt.y-dragStart.y);
redraw();
}
},false);
canvas.addEventListener('mouseup',function(evt){
dragStart = null;
if (!dragged) zoom(evt.shiftKey ? -1 : 1 );
},false);
var scaleFactor = 1.1;
var zoom = function(clicks){
var pt = ctx.transformedPoint(lastX,lastY);
ctx.translate(pt.x,pt.y);
var factor = Math.pow(scaleFactor,clicks);
ctx.scale(factor,factor);
ctx.translate(-pt.x,-pt.y);
redraw();
}
var handleScroll = function(evt){
var delta = evt.wheelDelta ? evt.wheelDelta/40 : evt.detail ? -evt.detail : 0;
if (delta) zoom(delta);
return evt.preventDefault() && false;
};
canvas.addEventListener('DOMMouseScroll',handleScroll,false);
canvas.addEventListener('mousewheel',handleScroll,false);
};
gkhead.src = 'http://phrogz.net/tmp/gkhead.jpg';
ball.src = 'http://phrogz.net/tmp/alphaball.png';
// Adds ctx.getTransform() - returns an SVGMatrix
// Adds ctx.transformedPoint(x,y) - returns an SVGPoint
function trackTransforms(ctx){
var svg = document.createElementNS("http://www.w3.org/2000/svg",'svg');
var xform = svg.createSVGMatrix();
ctx.getTransform = function(){ return xform; };
var savedTransforms = [];
var save = ctx.save;
ctx.save = function(){
savedTransforms.push(xform.translate(0,0));
return save.call(ctx);
};
var restore = ctx.restore;
ctx.restore = function(){
xform = savedTransforms.pop();
return restore.call(ctx);
};
var scale = ctx.scale;
ctx.scale = function(sx,sy){
xform = xform.scaleNonUniform(sx,sy);
return scale.call(ctx,sx,sy);
};
var rotate = ctx.rotate;
ctx.rotate = function(radians){
xform = xform.rotate(radians*180/Math.PI);
return rotate.call(ctx,radians);
};
var translate = ctx.translate;
ctx.translate = function(dx,dy){
xform = xform.translate(dx,dy);
return translate.call(ctx,dx,dy);
};
var transform = ctx.transform;
ctx.transform = function(a,b,c,d,e,f){
var m2 = svg.createSVGMatrix();
m2.a=a; m2.b=b; m2.c=c; m2.d=d; m2.e=e; m2.f=f;
xform = xform.multiply(m2);
return transform.call(ctx,a,b,c,d,e,f);
};
var setTransform = ctx.setTransform;
ctx.setTransform = function(a,b,c,d,e,f){
xform.a = a;
xform.b = b;
xform.c = c;
xform.d = d;
xform.e = e;
xform.f = f;
return setTransform.call(ctx,a,b,c,d,e,f);
};
var pt = svg.createSVGPoint();
ctx.transformedPoint = function(x,y){
pt.x=x; pt.y=y;
return pt.matrixTransform(xform.inverse());
}
}
</script>
</body></html>
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment