CSS动画实例：升空的气球
1．制作一个气球
设页面中有<div class="balloon"></div>，为. balloon设置样式规则如下：
.balloon
{
height: 96px;
width: 80px;
background: hsla(0, 100%, 50%, 0.8);
border-radius: 50% 50% 50% 50% / 45% 45% 55% 55%;
position: absolute;
top: 30%;
left:50%;
}
.balloon:before
{
content: '';
position: absolute;
width: 20%;
height: 30%;
top: 8%;
left: 16%;
border-radius: 50%;
transform: rotate(40deg);
background: hsla(0, 0%, 100%, 0.75);
}
可在页面中显示如图1所示的图形。
图1 气球
这个图形像一个气球，其中，样式.balloon绘制大的红色填充圆形，.balloon:before绘制里面的小圆形。再为这个气球加上一个手把。在生成气球形状的div中加入一个子层，定义如下：
<div class="balloon">
<div class=" handle"></div>
</div>
为. handle定义样式规则如下：
.handle
{
width: 2%;
height: 60%;
background:hsl(45, 100%, 40%);
top: 100%;
left: 50%;
transform: translate(-50%, 0);
position: absolute;
}
.handle:before,.handle:after
{
content: '';
position: absolute;
height: 5%;
transform: translate(-50%, 0);
border-radius: 25% / 50%;
left: 50%;
}
.handle:before
{
top: 0;
background:hsl(45, 100%, 40%);
width: 500%;
}
.handle:after
{
top: 5%;
background: hsla(0, 100%, 50%, 0.8);
width: 700%;
}
此时可在页面中显示如图2所示的图形。
图2 带手把的气球
为这个气球定义关键帧，使得它进行升空。编写的完整的HTML文件如下。
<!DOCTYPE html> <html> <head> <title>气球升空</title> <style> .container { margin: 0 auto; width: 300px; height:300px; position: relative; display:flex; justify-content:center; align-items:center; background: #fff; overflow: hidden; border: 4px solid rgba(255, 0, 0, 0.9); border-radius: 2%; } .balloon { height: 96px; width: 80px; background: hsla(0, 100%, 50%, 0.8); border-radius: 50% 50% 50% 50% / 45% 45% 55% 55%; position: absolute; top: 30%; left:50%; animation: float 5s infinite linear both; } .balloon:before { content: ''; position: absolute; width: 20%; height: 30%; top: 8%; left: 16%; border-radius: 50%; transform: rotate(40deg); background: hsla(0, 0%, 100%, 0.75); } .handle { width: 2%; height: 60%; background:hsl(45, 100%, 40%); top: 100%; left: 50%; transform: translate(-50%, 0); position: absolute; } .handle:before,.handle:after { content: ''; position: absolute; height: 5%; transform: translate(-50%, 0); border-radius: 25% / 50%; left: 50%; } .handle:before { top: 0; background:hsl(45, 100%, 40%); width: 500%; } .handle:after { top: 5%; background: hsla(0, 100%, 50%, 0.8); width: 700%; } @keyframes float { from { transform: translate(0, 300px); } to { transform: translate(0, -300px); } } </style> </head> <body> <div class="container"> <div class="balloon"> <div class="handle"></div> </div> </div> </body> </html>
在浏览器中打开包含这段HTML代码的html文件，可以呈现出如图1所示的动画效果。
图1 一个升空的气球
2．红绿蓝3个升空的气球
一个红色气球升空太孤单了，加两个气球（一个绿色、一个蓝色）陪着升空。
由生成气球图形的CSS定义可知，要生成不同颜色（用hsl函数表示颜色时的色相）、不同水平位置（升空总是从底部向上，垂直位置忽略好了）和不同大小的气球，可以在<div class="balloon"></div>定义中加上style来描述反应这三个特征的自定义变量—hue、--left和—size。然后适当修改CSS样式中与之相关的定义。
直接给出完整的HTML文件内容如下，读者自己体会与1个气球升空代码的差别。
<!DOCTYPE html> <html> <head> <title>气球升空</title> <style> .container { margin: 0 auto; width: 300px; height:300px; position: relative; display:flex; justify-content:center; align-items:center; background: #fff; overflow: hidden; border: 4px solid rgba(255, 0, 0, 0.9); border-radius: 2%; } .balloon { height: calc(1.2 * var(--size)); width: var(--size); background: hsla(var(--hue), 100%, 50%, 0.8); border-radius: 50% 50% 50% 50% / 45% 45% 55% 55%; position: absolute; top: 30%; left:var(--left, 50%); animation: float 5s infinite linear both; } .balloon:before { content: ''; position: absolute; width: 20%; height: 30%; top: 8%; left: 16%; border-radius: 50%; transform: rotate(40deg); background: hsla(0, 0%, 100%, 0.75); } .handle { width: 2%; height: 60%; background:hsl(45, 100%, 40%); top: 100%; left: 50%; transform: translate(-50%, 0); position: absolute; } .handle:before,.handle:after { content: ''; position: absolute; height: 5%; transform: translate(-50%, 0); border-radius: 25% / 50%; left: 50%; } .handle:before { top: 0; background:hsl(45, 100%, 40%); width: 500%; } .handle:after { top: 5%; background: hsla(var(--hue), 100%, 50%, 0.8) width: 700%; } @keyframes float { from { transform: translate(-50%, -50%) translate(0, 300px); } to { transform: translate(-50%, -50%) translate(0, -300px); } } </style> </head> <body> <div class="container"> <div class="balloon" style="--left: 20%; --hue: 0; --size: 80px;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 50%; --hue: 120; --size: 60px;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 70%; --hue: 240; --size: 100px;"> <div class="handle"></div> </div> </div> </body> </html>
在浏览器中打开包含这段HTML代码的html文件，可以呈现出如图2所示的动画效果。
图2 三个升空的气球
3．满屏都是气球在升空
由图2看出，三个升空的气球速度相同，垂直方向位置也相同，能否让它们有所不同呢？可以通过为气球定义动画完成时间和延迟时间来实现，再加上两个自定义变量—rate和—delay。
适当修改.container的定义，使得呈现动画效果的容器大小与浏览器窗口大小相同。并定义多个气球，编写的HTML文件如下。
<!DOCTYPE html> <html> <head> <title>气球升空</title> <style> .container { margin: 0 auto; min-height:100vh; position: relative; display:flex; justify-content:center; align-items:center; background: hsl(180, 100%, 95%); overflow: hidden; border: 4px solid rgba(255, 0, 0, 0.9); border-radius: 2%; } .balloon { --width: calc(var(--size, 10) * 1vmin); height: calc(1.2 * var(--width)); width: var(--width); background: hsla(var(--hue), 100%, 50%, 0.8); border-radius: 50% 50% 50% 50% / 45% 45% 55% 55%; position: absolute; top: 30%; left:var(--left, 50%); animation: float calc(var(--rate, 1) * 1s) calc(var(--delay, 0) * -1s) infinite linear both; } .balloon:before { content: ''; position: absolute; width: 20%; height: 30%; top: 8%; left: 16%; border-radius: 50%; transform: rotate(40deg); background: hsla(0, 0%, 100%, 0.75); } .handle { width: 2%; height: 60%; background:hsl(45, 100%, 40%); top: 100%; left: 50%; transform: translate(-50%, 0); position: absolute; } .handle:before,.handle:after { content: ''; position: absolute; height: 5%; transform: translate(-50%, 0); border-radius: 25% / 50%; left: 50%; } .handle:before { top: 0; background:hsl(45, 100%, 40%); width: 500%; } .handle:after { top: 5%; background: hsla(var(--hue), 100%, 50%, 0.8) width: 700%; } @keyframes float { from { transform: translate(-50%, -50%) translate(0, 100vh); } to { transform: translate(-50%, -50%) translate(0, -100vh); } } </style> </head> <body> <div class="container"> <div class="balloon" style="--left: 20%; --hue: 0; --size: 20;--rate: 1; --delay: 1;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 50%; --hue: 120; --size: 15;--rate: 3; --delay: 3;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 70%; --hue: 240; --size: 12;--rate: 5; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 35%; --hue: 0; --size: 18;--rate: 1; --delay: 1;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 50%; --hue: 120; --size: 21;--rate: 3; --delay: 3;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 89%; --hue: 240; --size: 22;--rate: 5; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 94%; --hue: 57; --size: 38; --rate: 3; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 90%; --hue: 124; --size: 36; --rate: 5; --delay: 7;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 46%; --hue: 228; --size: 24; --rate: 7; --delay: 10;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 59%; --hue: 196; --size: 20; --rate: 2; --delay: 8;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 62%; --hue: 225; --size: 24; --rate: 9; --delay: 9;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 16%; --hue: 190; --size: 18; --rate: 4; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 67%; --hue: 105; --size: 28; --rate: 1; --delay: 10;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 23%; 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--size: 20; --rate: 6; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 20%; --hue: 338; --size: 37; --rate: 6; --delay: 6;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 55%; --hue: 336; --size: 36; --rate: 2; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 81%; --hue: 10; --size: 17; --rate: 2; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 69%; --hue: 314; --size: 20; --rate: 10; --delay: 1;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 59%; --hue: 169; --size: 22; --rate: 7; --delay: 10;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 93%; --hue: 165; --size: 38; --rate: 7; --delay: 2;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 7%; --hue: 271; --size: 33; --rate: 1; --delay: 3;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 21%; --hue: 349; --size: 36; --rate: 8; --delay: 7;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 13%; --hue: 38; --size: 35; --rate: 2; --delay: 9;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 55%; --hue: 164; --size: 27; --rate: 9; --delay: 7;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 70%; --hue: 66; --size: 37; --rate: 5; --delay: 0;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 5%; --hue: 264; --size: 15; --rate: 4; --delay: 9;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 24%; --hue: 265; --size: 40; --rate: 7; --delay: 6;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 11%; --hue: 101; --size: 26; --rate: 4; --delay: 5;"> <div class="handle"></div> </div> <div class="balloon" style="--left: 25%; --hue: 207; --size: 40; --rate: 3; --delay: 0;"> <div class="handle"></div> </div> </div> </body> </html>
在浏览器中打开包含这段HTML代码的html文件，可以呈现出如图3所示的动画效果。
图3 满屏升空的气球
