Differences between console output stream and file stream for media

I encountered a problem while trying to render a GIF in Go. The output GIF file cannot be opened when following a certain method, but works fine with another approach. I found Rendering .gif in Go, but it doesn't address my specific issue.

Here is problematic code:

package mainimport ("bufio""fmt""image""image/color""image/gif""io""math""math/rand""os""time")var palette = []color.Color{color.White, color.Black}const (    whiteIndex = 0    blackIndex = 1)func main() {    w := bufio.NewWriter(os.Stdout)    lissajous(w)}func lissajous(out io.Writer) {    const (        cycles  = 5        res     = 0.001        size    = 100        nframes = 64        delay   = 8    )    rand.Seed(time.Now().UTC().UnixNano())    freq := rand.Float64() * 3.0    anim := gif.GIF{LoopCount: nframes}    phase := 0.0    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)    }    err := gif.EncodeAll(out, &anim)    if err != nil {        return    } else {        fmt.Println(err)    }}

Here are commands:

go build main.go

main > out.gif

Then, out.gif cannot be opened. However, this method works fine:

func main() {    fileName := "out.gif"    f, err3 := os.Create(fileName)    if err3 != nil {        fmt.Println("create file fail")    }    w := bufio.NewWriter(f)     lissajous(w)    w.Flush()    f.Close()}

I'm puzzled about why the first approach fails to create a functional GIF file, while the second approach works. Does it have to do with the way Go handles file writing or buffering?