static const HWAccel *get_hwaccel(enum AVPixelFormat pix_fmt, enum HWAccelID selected_hwaccel_id)
{
    int i;
    for (i = 0; hwaccels[i].name; i++)
        if (hwaccels[i].pix_fmt == pix_fmt &&
            (!selected_hwaccel_id || selected_hwaccel_id == HWACCEL_AUTO || hwaccels[i].id == selected_hwaccel_id))
            return &hwaccels[i];
    return NULL;
}