点击filter方法没反应，下载后的源码同样点击了也没反应
<a href="javascirpt: filter()">Filter</a>
function filter(){
var imageData = contexta.getImageData( 0 , 0 , canvasa.width , canvasa.height )
contextb.putImageData( imageData , 0 , 0 , 0 , 0 , canvasb.width , canvasb.height )
}
