使用Go图像库从stdout解码bmp图像

Question

I am trying to decode a bmp image using the image and golang.org/x/image/bmp libraries. The image is output by ffmpeg into stdout. This is the code to get the frame:

cmd := exec.Command("ffmpeg", "-accurate_seek", "-ss", strconv.Itoa(index), "-i",
    filename, "-frames:v", "1", "-hide_banner", "-loglevel", "0", "pipe:.bmp")
var out bytes.Buffer
cmd.Stdout = &out
err := cmd.Run()
if err != nil {
    log.Fatal(err)
}

o := bufio.NewReader(&out)

and then I decode it using img, _, err := image.Decode(o) However this gives an error of "image: unknown format". I have already registered the bmp format in the main method, and I have successfully decoded actual BMP files from disk previously, just not from stdout.

I have tried just using bmp.Decode instead of image.Decode but this just gives the error "EOF".

I thought maybe I was not getting the stdout in the correct way, but if I just write it straight to a file:

o := bufio.NewReader(&out)

outputfile, err := os.Create("test.bmp")
if err != nil {
    log.Fatal(err)
}
defer outputfile.Close()
io.Copy(outputfile, o)

then it works fine and I can open it.

Edit: code

Answer 1

Update: turns out the issue was -ss takes time not frame index.

I tried to reproduce the issue but it seems to work for me. Maybe add cmd.Stderr = os.Stderr to see if ffmpeg output gives come clue. Could you also post a fully runnable example?

package main

import (
    "bytes"
    "image"
    "log"
    "os"
    "os/exec"

    _ "golang.org/x/image/bmp"
)

func main() {
    cmd := exec.Command(
        "docker", "run",
        "--rm",
        "mwader/static-ffmpeg",
        "-ss", "5",
        "-f", "lavfi",
        "-i", "testsrc",
        "-frames:v", "1",
        "-f", "image2",
        "-vcodec", "bmp",
        "pipe:1",
    )
    var out bytes.Buffer
    cmd.Stderr = os.Stderr
    cmd.Stdout = &out
    err := cmd.Run()
    if err != nil {
        log.Fatal(err)
    }

    img, imgFormat, imgErr := image.Decode(&out)

    log.Printf("img.Bounds(): %#+v
", img.Bounds())
    log.Printf("imgFormat: %#+v
", imgFormat)
    log.Printf("imgErr: %#+v
", imgErr)
}