Go生成的动画GIF在Windows中不起作用

Question

I found an example didn't work properly in Windows. This program demonstrates basic usage of Go's standard image packages, which we'll use to create a sequence of bit-mapped images and then encode the sequence as a GIF animation.

package main

import (
    "image"
    "image/color"
    "image/gif"
    "io"
    "math"
    "math/rand"
    "os"
)

import (
    "log"
    "net/http"
    "time"
)

//!+main

var palette = []color.Color{color.White, color.Black}

const (
    whiteIndex = 0 // first color in palette
    blackIndex = 1 // next color in palette
)

func main() {
    //!-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())

    if len(os.Args) > 1 && os.Args[1] == "web" {
        //!+http
        handler := func(w http.ResponseWriter, r *http.Request) {
            lissajous(w)
        }
        http.HandleFunc("/", handler)
        //!-http
        log.Fatal(http.ListenAndServe("localhost:8000", nil))
        return
    }
    //!+main
    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 < 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
}

The code runs properly in cmd, however, if I run it in Windows Power Shell like:

.\lissajous.exe >out.gif

The out.gif can't be opened and I don't know why.

Answer 1

The GIF file (the gif data) is a binary format, not textual. Attempting to write it to the standard output and redirecting that to a file may suffer transformations. For example, the Windows PowerShell most likely converts some control characters (like " " to " "), so the resulting binary will not be identical to what gif.EncodeAll() writes to the standard output. Apparently cmd.exe does not do such transformations.

I recommend writing to a file directly (you may pass an os.File as the output), or an in-memory buffer which you can dump to a file using ioutil.WriteFile().

Here's how writing directly to a file could look like:

f, err := os.Create("a.gif")
if err != nil {
    panic(err)
}
defer f.Close()
lissajous(f)

Here's how the in-memory solution could look like:

buf := &bytes.Buffer{}
lissajous(buf)
if err := ioutil.WriteFile("a.gif", buf.Bytes(), 0666); err != nil {
    panic(err)
}

See related issue: image/gif: result of EncodeAll not viewable in Eye of GNOME

There is still a chance the result won't be readable by some apps (see above issue), which can be fixed by converting the output image in unix with the following command:

convert original.gif -coalesce unoptimized.gif

Source: Fix animated GIF images which eog can't open, but Firefox and ImageMagick can