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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../index.html">Go语言圣经</a></li><li class="chapter-item expanded affix "><a href="../preface-zh.html">译者序</a></li><li class="chapter-item expanded affix "><a href="../preface.html">前言</a></li><li class="chapter-item expanded "><a href="../ch1/ch1.html"><strong aria-hidden="true">1.</strong> 入门</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch1/ch1-01.html"><strong aria-hidden="true">1.1.</strong> Hello, World</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-02.html"><strong aria-hidden="true">1.2.</strong> 命令行参数</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-03.html"><strong aria-hidden="true">1.3.</strong> 查找重复的行</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-04.html" class="active"><strong aria-hidden="true">1.4.</strong> GIF动画</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-05.html"><strong aria-hidden="true">1.5.</strong> 获取URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-06.html"><strong aria-hidden="true">1.6.</strong> 并发获取多个URL</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-07.html"><strong aria-hidden="true">1.7.</strong> Web服务</a></li><li class="chapter-item expanded "><a href="../ch1/ch1-08.html"><strong aria-hidden="true">1.8.</strong> 本章要点</a></li></ol></li><li class="chapter-item expanded "><a href="../ch2/ch2.html"><strong aria-hidden="true">2.</strong> 程序结构</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch2/ch2-01.html"><strong aria-hidden="true">2.1.</strong> 命名</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-02.html"><strong aria-hidden="true">2.2.</strong> 声明</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-03.html"><strong aria-hidden="true">2.3.</strong> 变量</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-04.html"><strong aria-hidden="true">2.4.</strong> 赋值</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-05.html"><strong aria-hidden="true">2.5.</strong> 类型</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-06.html"><strong aria-hidden="true">2.6.</strong> 包和文件</a></li><li class="chapter-item expanded "><a href="../ch2/ch2-07.html"><strong aria-hidden="true">2.7.</strong> 作用域</a></li></ol></li><li class="chapter-item expanded "><a href="../ch3/ch3.html"><strong aria-hidden="true">3.</strong> 基础数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch3/ch3-01.html"><strong aria-hidden="true">3.1.</strong> 整型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-02.html"><strong aria-hidden="true">3.2.</strong> 浮点数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-03.html"><strong aria-hidden="true">3.3.</strong> 复数</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-04.html"><strong aria-hidden="true">3.4.</strong> 布尔型</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-05.html"><strong aria-hidden="true">3.5.</strong> 字符串</a></li><li class="chapter-item expanded "><a href="../ch3/ch3-06.html"><strong aria-hidden="true">3.6.</strong> 常量</a></li></ol></li><li class="chapter-item expanded "><a href="../ch4/ch4.html"><strong aria-hidden="true">4.</strong> 复合数据类型</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../ch4/ch4-01.html"><strong aria-hidden="true">4.1.</strong> 数组</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-02.html"><strong aria-hidden="true">4.2.</strong> Slice</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-03.html"><strong aria-hidden="true">4.3.</strong> Map</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-04.html"><strong aria-hidden="true">4.4.</strong> 结构体</a></li><li class="chapter-item expanded "><a href="../ch4/ch4-05.html"><strong aria-hidden="true">4.5.<
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<li><em>凹语言(专为 WebAssembly 设计): <a href="https://github.com/wa-lang/wa">https://github.com/wa-lang/wa</a></em></li>
<li><em>KCL 配置语言(Rust): <a href="https://github.com/kcl-lang/kcl">https://github.com/kcl-lang/kcl</a></em></li>
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<h2 id="14-gif动画"><a class="header" href="#14-gif动画">1.4. GIF动画</a></h2>
<p>下面的程序会演示Go语言标准库里的image这个package的用法我们会用这个包来生成一系列的bit-mapped图然后将这些图片编码为一个GIF动画。我们生成的图形名字叫利萨如图形Lissajous figures这种效果是在1960年代的老电影里出现的一种视觉特效。它们是协振子在两个纬度上振动所产生的曲线比如两个sin正弦波分别在x轴和y轴输入会产生的曲线。图1.1是这样的一个例子:</p>
<p><img src="../images/ch1-01.png" alt="" /></p>
<p>译注要看这个程序的结果需要将标准输出重定向到一个GIF图像文件使用 <code>./lissajous &gt; output.gif</code> 命令。下面是GIF图像动画效果</p>
<p><img src="../images/ch1-01.gif" alt="" /></p>
<p>这段代码里我们用了一些新的结构包括const声明struct结构体类型复合声明。和我们举的其它的例子不太一样这一个例子包含了浮点数运算。这些概念我们只在这里简单地说明一下之后的章节会更详细地讲解。</p>
<p><u><i>gopl.io/ch1/lissajous</i></u></p>
<pre><code class="language-go">// Lissajous generates GIF animations of random Lissajous figures.
package main
import (
&quot;image&quot;
&quot;image/color&quot;
&quot;image/gif&quot;
&quot;io&quot;
&quot;math&quot;
&quot;math/rand&quot;
&quot;os&quot;
&quot;time&quot;
)
var palette = []color.Color{color.White, color.Black}
const (
whiteIndex = 0 // first color in palette
blackIndex = 1 // next color in palette
)
func main() {
// The sequence of images is deterministic unless we seed
// the pseudo-random number generator using the current time.
// Thanks to Randall McPherson for pointing out the omission.
rand.Seed(time.Now().UTC().UnixNano())
lissajous(os.Stdout)
}
func lissajous(out io.Writer) {
const (
cycles = 5 // number of complete x oscillator revolutions
res = 0.001 // angular resolution
size = 100 // image canvas covers [-size..+size]
nframes = 64 // number of animation frames
delay = 8 // delay between frames in 10ms units
)
freq := rand.Float64() * 3.0 // relative frequency of y oscillator
anim := gif.GIF{LoopCount: nframes}
phase := 0.0 // phase difference
for i := 0; i &lt; nframes; i++ {
rect := image.Rect(0, 0, 2*size+1, 2*size+1)
img := image.NewPaletted(rect, palette)
for t := 0.0; t &lt; cycles*2*math.Pi; t += res {
x := math.Sin(t)
y := math.Sin(t*freq + phase)
img.SetColorIndex(size+int(x*size+0.5), size+int(y*size+0.5),
blackIndex)
}
phase += 0.1
anim.Delay = append(anim.Delay, delay)
anim.Image = append(anim.Image, img)
}
gif.EncodeAll(out, &amp;anim) // NOTE: ignoring encoding errors
}
</code></pre>
<p>当我们import了一个包路径包含有多个单词的package时比如image/colorimage和color两个单词通常我们只需要用最后那个单词表示这个包就可以。所以当我们写color.White时这个变量指向的是image/color包里的变量同理gif.GIF是属于image/gif包里的变量。</p>
<p>这个程序里的常量声明给出了一系列的常量值常量是指在程序编译后运行时始终都不会变化的值比如圈数、帧数、延迟值。常量声明和变量声明一般都会出现在包级别所以这些常量在整个包中都是可以共享的或者你也可以把常量声明定义在函数体内部那么这种常量就只能在函数体内用。目前常量声明的值必须是一个数字值、字符串或者一个固定的boolean值。</p>
<p>[]color.Color{...}和gif.GIF{...}这两个表达式就是我们说的复合声明4.2和4.4.1节有说明。这是实例化Go语言里的复合类型的一种写法。这里的前者生成的是一个slice切片后者生成的是一个struct结构体。</p>
<p>gif.GIF是一个struct类型参考4.4节。struct是一组值或者叫字段的集合不同的类型集合在一个struct可以让我们以一个统一的单元进行处理。anim是一个gif.GIF类型的struct变量。这种写法会生成一个struct变量并且其内部变量LoopCount字段会被设置为nframes而其它的字段会被设置为各自类型默认的零值。struct内部的变量可以以一个点.来进行访问就像在最后两个赋值语句中显式地更新了anim这个struct的Delay和Image字段。</p>
<p>lissajous函数内部有两层嵌套的for循环。外层循环会循环64次每一次都会生成一个单独的动画帧。它生成了一个包含两种颜色的201*201大小的图片白色和黑色。所有像素点都会被默认设置为其零值也就是调色板palette里的第0个值这里我们设置的是白色。每次外层循环都会生成一张新图片并将一些像素设置为黑色。其结果会append到之前结果之后。这里我们用到了append(参考4.2.1)内置函数将结果append到anim中的帧列表末尾并设置一个默认的80ms的延迟值。循环结束后所有的延迟值被编码进了GIF图片中并将结果写入到输出流。out这个变量是io.Writer类型这个类型支持把输出结果写到很多目标很快我们就可以看到例子。</p>
<p>内层循环设置两个偏振值。x轴偏振使用sin函数。y轴偏振也是正弦波但其相对x轴的偏振是一个0-3的随机值初始偏振值是一个零值随着动画的每一帧逐渐增加。循环会一直跑到x轴完成五次完整的循环。每一步它都会调用SetColorIndex来为(x,y)点来染黑色。</p>
<p>main函数调用lissajous函数用它来向标准输出流打印信息所以下面这个命令会像图1.1中产生一个GIF动画。</p>
<pre><code>$ go build gopl.io/ch1/lissajous
$ ./lissajous &gt;out.gif
</code></pre>
<p><strong>练习 1.5</strong> 修改前面的Lissajous程序里的调色板由黑色改为绿色。我们可以用<code>color.RGBA{0xRR, 0xGG, 0xBB, 0xff}</code>来得到<code>#RRGGBB</code>这个色值,三个十六进制的字符串分别代表红、绿、蓝像素。</p>
<p><strong>练习 1.6</strong> 修改Lissajous程序修改其调色板来生成更丰富的颜色然后修改SetColorIndex的第三个参数看看显示结果吧。</p>
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