Decoding bmp images from stdout using go image library - image

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