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              <p>
    <pre>
            位图:例如jpeg,一般具有自身尺寸和自身比例
            矢量图: 例如svg,一般不需要自身尺寸和自身比例。如果有,那么一定是既有自身尺寸,又有自身比例
            css渐变图:没有自身尺寸和比例
            element: 拥有element元素的尺寸和比例
        </pre>
</p>
<p>下面示例:图片尺寸: 700 x 466 容器尺寸: 800 x 500</p>
<div class="area">
    <p>
        <pre>
                contain: 最大限度填充背景区域,但是不能被裁剪,因此例子中的图片会适应高度放大
                cover: 填满背景容器区域,图片可以被裁剪,因此例子中的图片会适应宽度放大
                auto: 如果图片有自身尺寸,则按照自身尺寸展示,可以看到下面的例子中图片不会撑满容器
                      如果图片没有自身尺寸并且没有自身比例,那么图片会填充整个背景容器,图片可能会被拉伸变形
                      如果图片没有自身尺寸,但是有自身比例,那么按照'contain'来展示图片
                      如果图片只有一个尺寸,并且有自身比例,那么另一个尺寸会通过比例计算出来
                      如果图片只有一个尺寸,没有自身比例,那么另一个尺寸就是图片容器的尺寸
            </pre>
    </p>
    <label>单个值</label><br/>
    <select name="one" id="one">
            <option value="contain">contain</option>
            <option value="cover">cover</option>
            <option value="auto" selected>auto</option>
        </select>
    <div class="display" id="oneDisplay"></div>
</div>
<div class="area">
    <p>根据自身比例来,如果存在自身比例,那么为auto的值会根据给定的值来计算,如果没有自身比例,那么另一个值为图片容器尺寸</p>
    <p>若值为百分比,那么会根据图片容器尺寸先折算成具体尺寸,然后再根据比例计算另一边的尺寸</p>
    <p>如果backgound-attachment为fixed时,图片容器尺寸为当前视图窗口</p>
    <label>两个值(一个为'auto',另一个数值)</label><br/>
    <input type="text" class="two-value" id="twoValue">
    <div class="display" id="twoDisplay1"></div>
</div>
<div class="area">
    <p>按照具体给定的尺寸显示,参看上面解释</p>
    <p>图片可能会被拉伸</p>
    <label>两个值(数值)</label><br/>
    <input type="text" class="two-value" id="twoValue1">
    <input type="text" class="two-value" id="twoValue2">
    <div class="display" id="twoDisplay2"></div>
</div>
            
          
!
            
              pre {
    white-space: pre-line;
}
.area {
    margin-right: 20px;
    margin-bottom: 20px;
}
.display {
    margin-top: 10px;
    border: 1px dotted darkblue;
    width: 800px;
    height: 500px;
    background: url("http://p0xmvstv1.bkt.clouddn.com/css/background/ylj.jpeg") no-repeat;
}

            
          
!
            
              var one = document.querySelector("#one");
var twoValue = document.querySelector("#twoValue");
var twoValue1 = document.querySelector("#twoValue1");
var twoValue2 = document.querySelector("#twoValue2");

var oneDisplay = document.querySelector("#oneDisplay");
var twoDisplay1 = document.querySelector("#twoDisplay1");
var twoDisplay2 = document.querySelector("#twoDisplay2");

one.addEventListener("change", function(e) {
    var value = e.target.value;
    oneDisplay.style.backgroundSize = value;
});

twoValue.addEventListener("change", function(e) {
    var value = e.target.value;
    twoDisplay1.style.backgroundSize = "auto " + value;
});

twoValue1.addEventListener("change", function(e) {
    var value = e.target.value;
    var value2 = twoValue2.value;
    twoDisplay2.style.backgroundSize = value + " " + value2;
});

twoValue2.addEventListener("change", function(e) {
    var value = e.target.value;
    var value1 = twoValue1.value;
    twoDisplay2.style.backgroundSize = value1 + " " + value;
});

            
          
!
999px
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