javascript - How to draw a rectangle on canvas like we do on paint? -


say want draw rectangle on canvas. want able co-ordinates user's mouse. ideal scenario user clicks @ point , drags down end rectangles draw using paint. how can draw rectangle in paint dragging mouse? (how co-ordinates of mouse when clicks mouse , leaves at?)

here's outline of how drag-draw rectangle on canvas:

in mousedown:

  • save starting mouse position
  • set flag indicating drag has begun

in mousemove:

  • clear canvas of previous rectangle
  • calculate rectangle width/height based on starting vs current mouse position
  • draw rectangle starting xy current mouse position

in mouseup:

  • clear dragging flag because drag over

here's example code , demo: http://jsfiddle.net/m1erickson/6e2yd/

<!doctype html> <html> <head> <link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css --> <script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>  <style>     body{ background-color: ivory; }     #canvas{border:1px solid red;} </style>  <script> $(function(){      // references canvas , context     var canvas=document.getelementbyid("canvas");     var ctx=canvas.getcontext("2d");      // style context     ctx.strokestyle = "blue";     ctx.linewidth=3;      // calculate canvas on window     // (used calculate mousex/mousey)     var $canvas=$("#canvas");     var canvasoffset=$canvas.offset();     var offsetx=canvasoffset.left;     var offsety=canvasoffset.top;     var scrollx=$canvas.scrollleft();     var scrolly=$canvas.scrolltop();      // flage true when user dragging mouse     var isdown=false;      // these vars hold starting mouse position     var startx;     var starty;       function handlemousedown(e){       e.preventdefault();       e.stoppropagation();        // save starting x/y of rectangle       startx=parseint(e.clientx-offsetx);       starty=parseint(e.clienty-offsety);        // set flag indicating drag has begun       isdown=true;     }      function handlemouseup(e){       e.preventdefault();       e.stoppropagation();        // drag over, clear dragging flag       isdown=false;     }      function handlemouseout(e){       e.preventdefault();       e.stoppropagation();        // drag over, clear dragging flag       isdown=false;     }      function handlemousemove(e){       e.preventdefault();       e.stoppropagation();        // if we're not dragging, return       if(!isdown){return;}        // current mouse position       mousex=parseint(e.clientx-offsetx);       mousey=parseint(e.clienty-offsety);        // put mousemove stuff here        // clear canvas       ctx.clearrect(0,0,canvas.width,canvas.height);        // calculate rectangle width/height based       // on starting vs current mouse position       var width=mousex-startx;       var height=mousey-starty;        // draw new rect start position        // current mouse position       ctx.strokerect(startx,starty,width,height);      }      // listen mouse events     $("#canvas").mousedown(function(e){handlemousedown(e);});     $("#canvas").mousemove(function(e){handlemousemove(e);});     $("#canvas").mouseup(function(e){handlemouseup(e);});     $("#canvas").mouseout(function(e){handlemouseout(e);});  }); // end $(function(){}); </script> </head> <body>     <h4>drag mouse create rectangle</h4>     <canvas id="canvas" width=300 height=300></canvas> </body> </html>
  • clear

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