画像を高品質に拡大縮小する「resampling.js」の使い方[公式]

<script src="resampling.js"></script>
<button onclick="double();" style="font-size:32px;">size * 2</button> 
<button onclick="half();" style="font-size:32px;">size * 0.5(50%)</button>
<p>[元画像]</p>
<img id="SrcImage" src="test.png">
<p>[拡大縮小後]</p>
<canvas id="DstCanvas"></canvas>
<script>
  var src_img = document.getElementById("SrcImage");
  var dst_canvas = document.getElementById("DstCanvas");
  var dst_ctx = dst_canvas.getContext("2d");    
  
  function double(){  
        
    // 元画像の描画
    dst_canvas.width  = src_img.width;
    dst_canvas.height = src_img.height;        
    dst_ctx.drawImage(src_img,0,0); 
    
    // ImageDataの生成
    var source = dst_ctx.getImageData(0,0,dst_canvas.width,dst_canvas.height); 
    dst_canvas.width  = (src_img.width) * 2; 
    dst_canvas.height = (src_img.height)* 2;          
    var destination = dst_ctx.createImageData(dst_canvas.width,dst_canvas.height); 
    
    // 拡大縮小の実行
    EffectResampling(source,destination,BiCubic_Filter,false);
    
    // canvasへ描画
    dst_ctx.putImageData(destination,0,0);
  }  
  
  function half(){  
        
    // 元画像の描画
    dst_canvas.width  = src_img.width;
    dst_canvas.height = src_img.height;        
    dst_ctx.drawImage(src_img,0,0); 
    
    // ImageDataの生成
    var source = dst_ctx.getImageData(0,0,dst_canvas.width,dst_canvas.height); 
    dst_canvas.width  = Math.round((src_img.width)  * 0.5); 
    dst_canvas.height = Math.round((src_img.height) * 0.5);          
    var destination = dst_ctx.createImageData(dst_canvas.width,dst_canvas.height); 
    
    // 拡大縮小の実行
    EffectResampling(source,destination,BiCubic_Filter,true);
    
    // canvasへ描画
    dst_ctx.putImageData(destination,0,0);
  }    
</script>