javascript - this is a program dealing with canvas and java script -


bellow code program suppose lower blinking ball bottom of screen , stay blinking @ bottom. process suppose take place gradually (incrementing factor of 10) happens suddenly. should do? 

        <!doctype html>  <html> <head> <meta charset="iso-8859-1"> <title>animationdemocanvas.html &copy; kari laitinen</title>   <script type="text/javascript">  var ball_center_point_x = 300 ; var ball_center_point_y = 240 ;  var current_ball_color = "cyan" ;  var ball_must_be_shown  =  true ;   function draw_on_canvas() {    var canvas = document.getelementbyid( "animation_demo_canvas" ) ;    var context = canvas.getcontext("2d") ;      context.fillstyle = "rgb( 255, 255, 210 )" ;      context.fillrect( 0, 0, canvas.width, canvas.height ) ;     if ( ball_must_be_shown  ==  true )    {       context.fillstyle = current_ball_color ;        context.beginpath() ;       ball_center_point_y +=10;       context.arc(  ball_center_point_x, ball_center_point_y,                    50, 0, 2 * math.pi, true )        context.closepath(ball_center_point_y = 429) ;       context.stroke() ;       context.fill() ;        ball_must_be_shown  =  false ;    }    else    {       ball_must_be_shown  =  true ;    }      settimeout( function () { draw_on_canvas(); }, 100 ) ; }  </script>  <style type="text/css">     #centered    {       width: 600px;       height: 500px;       margin: 30px auto;            border: 1px solid black;    }  </style> </head>  <body onload="draw_on_canvas()">     <div id=centered>       <canvas id=animation_demo_canvas          width=600 height=500>       </canvas>     </div>  </body> </html>

here's working fiddle you.

var ball_center_point_x = 300 ; var ball_center_point_y = 240 ;  var current_ball_color = "cyan" ;  var ball_must_be_shown  =  true ; draw_on_canvas();  function draw_on_canvas() {    var canvas = document.getelementbyid( "animation_demo_canvas" ) ;    var context = canvas.getcontext("2d") ;     context.fillstyle = "rgb( 255, 255, 210 )" ;      context.fillrect( 0, 0, canvas.width, canvas.height ) ; //here part in question:     if ( ball_must_be_shown  ==  true )    {       context.fillstyle = current_ball_color ;        context.beginpath() ;       if(ball_center_point_y + 50 < canvas.height)           ball_center_point_y +=10;       context.arc(  ball_center_point_x, ball_center_point_y,                    50, 0, 2 * math.pi, true )        context.closepath() ;       context.stroke() ;       context.fill() ;        ball_must_be_shown  =  false ;    }    else    {       ball_must_be_shown  =  true ;    }      settimeout( draw_on_canvas, 100 ) ; }

basically problems lies in settimeout(), reference function directly, instead of using string. don't settimeout("draw_on_canvas()", 100)

update: if(ball_center_point_y + 50 < canvas.height) ball_center_point_y +=10;

this portion stops circle going through bottom. since diameter dictated in context.arc() 50 add circle's y-axis(ball_center_point_y), have compared against height of canvas. if condition evaluates true won't update y-axis.


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