如何使用javascript预先缓存图像以便快速查看？ [英] How do I pre-cache images for quick viewing with javascript?

问题描述

我有一个网页，我希望用户在将鼠标放在图像的某个部分上时看到新图像。我使用了图像映射。

I have a webpage where I want the user to see a new image when they put thier mouse over a certain part of the image. I used an image map.

<img src="pic.jpg" usemap="#picmap" />
<map id="picmap" name="picmap"><area shape="rect" coords ="10,20,30,40"
onMouseOver="mouse_on_write('mouse is on spot')"
onMouseOut="mouse_off('mouse is off spot')"
href="http://www....html" target="_blank" />
</map>
<p id="desc"></p>

在标题中我定义了这些函数：

Where in the header I defined these functions:

 <script type="text/javascript">
 function mouse_off(txt)
    {
    document.getElementById("desc").innerHTML=txt;
    document.p1.src="pic.jpg";
    }
 function mouse_on_write(txt)
    {
    document.getElementById("desc").innerHTML=txt;
    document.p1.src="pic2.jpg";
  </script>

它有效，但速度很慢。将鼠标放在第二张图像上时，需要几秒钟才能显示;我的临时解决方案是大幅减小图像的大小，因为它们很大（现在它们快速切换为2.5mb，但仍然不是无缝的）。 如何在不降低图像质量的情况下使图像切换更加无缝？
第二个想法我意识到我也可以只显示小尺寸和大尺寸的图像，鼠标悬停时，他们会切换位置;我该怎么做？这会减少滞后吗？

It works, but it is slow. When the mouse is put over the second image it takes some few seconds to appear; my temporary solution was to drastically reduce the size of the images because they were huge (at 2.5mb they switch fast now, but still not seamless). How can I make the image switching more seamless without reduction in picture quality? On second thought I realize that I could also just have both images displayed, at a small and a large scale, and on mouse over they would switch places; How would I do this? Would this reduce lag?

推荐答案

您不需要创建任何页面元素，它们都可以使用JavaScript预加载：

You don't need to create any page elements, it can all be preloaded using JavaScript:

tempImg = new Image()
tempImg.src="pic2.jpg"

编辑：

如果您有很多图像，可以使用穷人的多种图像preloader：

If you have a lot of images, you can use the poor-man's multi-preloader:

preloads = "red.gif,green.gif,blue.gif".split(",")
var tempImg = []

for(var x=0;x<preloads.length;x++) {
    tempImg[x] = new Image()
    tempImg[x].src = preloads[x]
}

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