HTML5 Canvas Zoom and Pan Image

HTML5 Canvas Zoom and Pan Image.markdown

HTML5 Canvas Zoom and Pan Image

Pure JavaScript Zoom and Pan, taken from http://phrogz.net/tmp/canvas_zoom_to_cursor.html

A Pen by TechSlides on CodePen.

License.

index.html

<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>

<canvas></canvas>

script.js

	var canvas = document.getElementsByTagName('canvas')[0];
	canvas.width = 800;
canvas.height = 600;

	var gkhead = 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);

          ctx.save();
          ctx.setTransform(1,0,0,1,0,0);
          ctx.clearRect(0,0,canvas.width,canvas.height);
          ctx.restore();

          ctx.drawImage(gkhead,0,0);

        }
        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';
	
	// 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());
      }
	}

style.css

		body { background:#eee; margin:1em; text-align:center; }
		canvas { display:block; margin:1em auto; background:#fff; border:1px solid #ccc }