<p>我有240x157 png，和div大小为120x88</p><p>所以，我制作了240x157的画布并粘贴png。</p><p>div的大小比png小，但是div有滚动条，我可以用拖拽条看到整个图片。这是完美的。</p><p><code></code><code></code></p> <p><br /></p> <pre class="brush:js;toolbar:false;">var ctx = $('canvas')[0].getContext('2d'); console.log("test") var chara = new Image(); chara.src = "https://dl.dropbox.com/s/yr8ehzbdwm0csc7/Madeira_-_Entrance_to_Town%2C_c._1900.jpg?dl=0"; chara.onload = () => { ctx.clearRect(0, 0,240,157); ctx.drawImage(chara,0,0,240,157); }</pre> <pre class="brush:css;toolbar:false;">#whole{ border:1px solid; width:120px; height:78px; overflow:scroll; }</pre> <pre class="brush:html;toolbar:false;"><script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <div id="whole"> <canvas id="test" width="240px" height="157px"></canvas> </div></pre> <p><br /></p> <p>我的最终目标是通过用户改变比例，当图片大于div时，会出现滚动条，如photoshop等。</p><p>所以，我测试了1/2的比例，看到整个图片在129 x78</p><p>看起来成功了，但是仍然有滚动条，它没有滚动必要的区域。</p><p>为什么会这样呢?</p><p>在我的理解中，当画布尺寸为240x157，比例为1/2时，应该出现120x78，为什么只有图片收缩而不是画布本身?</p><p><code></code><code></code></p> <p><br /></p> <pre class="brush:js;toolbar:false;">var ctx = $('canvas')[0].getContext('2d'); console.log("test") var chara = new Image(); chara.src = "https://dl.dropbox.com/s/yr8ehzbdwm0csc7/Madeira_-_Entrance_to_Town%2C_c._1900.jpg?dl=0"; ctx.scale(1/2,1/2) // adding here. chara.onload = () => { ctx.clearRect(0, 0,240,157); ctx.drawImage(chara,0,0,240,157); }</pre> <pre class="brush:css;toolbar:false;">#whole{ border:1px solid; width:120px; height:78px; overflow:scroll; }</pre> <pre class="brush:html;toolbar:false;"><script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script> <div id="whole"> <canvas id="test" width="240px" height="157px"></canvas> </div></pre> <p><br /></p>

const canvas = $('canvas')[0];

var ctx = canvas.getContext('2d');
console.log("test")

var chara = new Image();
chara.src = "https://dl.dropbox.com/s/yr8ehzbdwm0csc7/Madeira_-_Entrance_to_Town%2C_c._1900.jpg?dl=0";

$zoom.addEventListener('input', () =>{

  const zoom = $zoom.value * .5;

  canvas.width = 240 * zoom;
  canvas.height = 157 * zoom;

  ctx.scale(zoom,zoom) // adding here.

  ctx.clearRect(0, 0,240,157);
  ctx.drawImage(chara,0,0,240,157);


});


chara.onload = () => { 
      ctx.clearRect(0, 0,240,157);
      ctx.drawImage(chara,0,0,240,157);

 }

#whole{
  border:1px solid;
  width:120px;
  height:78px;
  overflow:auto;
  
}
canvas{
margin:0;
display:block;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input id="$zoom" type="range" min="1" max="10" value="2">
<div id="whole">
<canvas id="test" width="240px" height="157px"></canvas>
</div>