[日常] Go语言圣经-GIF动画练习语法

1.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。
2.常量声明和变量声明一般都会出现在包级别
3.[]color.Color{...}生成的是一个slice切片和gif.GIF{...}生成的是一个struct结构体这两个表达式就是我们说的复合声明
4.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，struct内部的变量可以以一个点(.)来进行访问
5.import了一个包路径包含有多个单词的package时，比如image/color（image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

练习 1.5：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color.RGBA{0xRR, 0xGG, 0xBB, 0xff}来得到#RRGGBB这个色值，三个十六进制的字符串分别代表红、绿、蓝像素。

package main
 
import (
        "image"
        "image/color"
        "image/gif"
        "io"
        "math"
        "math/rand"
        "os"
        "time"
)
 
//定义一个slice切片变量,复合声明
var palette = []color.Color{color.White, color.RGBA{0, 255, 68, 255}}
 
//声明一组常量
const (
        whiteIndex = 0 
        blackIndex = 1 
)
 
func main() {
        rand.Seed(time.Now().UTC().UnixNano())
        lissajous(os.Stdout)
}
 
//定义函数
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 < nframes; i++ {
                rect := image.Rect(0, 0, 2*size+1, 2*size+1)
                img := image.NewPaletted(rect, palette)
                for t := 0.0; t < 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, &anim) // NOTE: ignoring encoding errors
 
}

练习 1.6：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。

package main
 
import (
        "image"
        "image/color"
        "image/gif"
        "io"
        "math"
        "math/rand"
        "os"
        "time"
)
 
//定义一个slice切片变量,复合声明
var palette = []color.Color{color.White, color.RGBA{0, 255, 68, 255},color.RGBA{26, 0, 255, 255}}
 
//声明一组常量
const (
        whiteIndex = 0
        blackIndex = 1
)
 
func main() {
        rand.Seed(time.Now().UTC().UnixNano())
        lissajous(os.Stdout)
}
 
//定义函数
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 < nframes; i++ {
                rect := image.Rect(0, 0, 2*size+1, 2*size+1)
                img := image.NewPaletted(rect, palette)
                for t := 0.0; t < 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, &anim) // NOTE: ignoring encoding errors
 
}