Zoom e drag de imagem com Canvas - Advanced Method

[jsmoraes]

Destaque para a captura do canvas através de getElementByTagName('canvas')[0] em vez de getElementById
O código permite mover e zoom.


Código Fonte:

<nabble_embed><canvas></canvas></nabble_embed>

<script type="text/javascript" charset="utf-8">
	var canvas = document.getElementsByTagName('canvas')[0];

	var gkhead = new Image;
	gkhead.src = 'https://www.dropbox.com/s/r6axpeyohisjl62/n5189-21052015.jpg?dl=1';
canvas.width =900; canvas.height = 580;

	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,50,50);
		}
		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://astronomia-e-astrofotos.1069742.n5.nabble.com/file/n1016/Luarbarlow_190613_191541_k5.jpg';

//canvas.width = gkhead.width + 100; canvas.height = gkhead.height + 100;
	
	// 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>
<style>
 canvas { display:block; margin:1em auto; background:#fff; border:2px solid #ff0000 }
</style>