So I want to **grab a frame** from a video **at a specific time** using **libav** for the use as a thumbnail.

What I'm using is the following code. It compiles and works fine (in regards to retrieving a picture at all), yet I'm having a hard time getting it to **retrieve the right picture**. 

I simply can't get my head around the all but clear logic behind libav's apparent use of multiple time-bases per video. Specifically figuring out which functions expect/return which type of time-base.

The docs were of basically no help whatsoever, unfortunately. SO to the rescue?

```
#define ABORT(x) do {fprintf(stderr, x); exit(1);} while(0)

av_register_all();

AVFormatContext *format_context = ...;
AVCodec *codec = ...;
AVStream *stream = ...;
AVCodecContext *codec_context = ...;
int stream_index = ...;

// open codec_context, etc.

AVRational stream_time_base = stream->time_base;
AVRational codec_time_base = codec_context->time_base;

printf("stream_time_base: %d / %d = %.5f\n", stream_time_base.num, stream_time_base.den, av_q2d(stream_time_base));
printf("codec_time_base: %d / %d = %.5f\n\n", codec_time_base.num, codec_time_base.den, av_q2d(codec_time_base));

AVFrame *frame = avcodec_alloc_frame();

printf("duration: %lld @ %d/sec (%.2f sec)\n", format_context->duration, AV_TIME_BASE, (double)format_context->duration / AV_TIME_BASE);
printf("duration: %lld @ %d/sec (stream time base)\n\n", format_context->duration / AV_TIME_BASE * stream_time_base.den, stream_time_base.den);
printf("duration: %lld @ %d/sec (codec time base)\n", format_context->duration / AV_TIME_BASE * codec_time_base.den, codec_time_base.den);

double request_time = 10.0; // 10 seconds. Video's total duration is ~20sec
int64_t request_timestamp = request_time / av_q2d(stream_time_base);
printf("requested: %.2f (sec)\t-> %2lld (pts)\n", request_time, request_timestamp);

av_seek_frame(format_context, stream_index, request_timestamp, 0);

AVPacket packet;
int frame_finished;
do {
    if (av_read_frame(format_context, &packet) < 0) {
        break;
    } else if (packet.stream_index != stream_index) {
        av_free_packet(&packet);
        continue;
    }
    avcodec_decode_video2(codec_context, frame, &frame_finished, &packet);
} while (!frame_finished);

// do something with frame

int64_t received_timestamp = frame->pkt_pts;
double received_time = received_timestamp * av_q2d(stream_time_base);
printf("received:  %.2f (sec)\t-> %2lld (pts)\n\n", received_time, received_timestamp);

```

Running this with a test movie file I get this output:

```
    stream_time_base: 1 / 30000 = 0.00003
    codec_time_base: 50 / 2997 = 0.01668

    duration: 20062041 @ 1000000/sec (20.06 sec)
    duration: 600000 @ 30000/sec (stream time base)
    duration: 59940 @ 2997/sec (codec time base)

    requested: 10.00 (sec)  -> 300000 (pts)
    received:  0.07 (sec)   -> 2002 (pts)

```

**The times don't match. What's going on here? What am I doing wrong?**

---

While searching for clues I stumbled upon this [this statement](http://libav-users.943685.n4.nabble.com/How-do-I-convert-dts-pts-values-into-seconds-tt945776.html#a945779) from the libav-users mailing list…

> 
> [...] **packet PTS/DTS** are in units of the **format context's time\_base**,  
> 
>  where the **AVFrame->pts** value is in units of the **codec context's time\_base**.
> 
> 
> In other words, the container can have (and usually does) a different
>  time\_base than the codec. Most libav players don't bother using the
>  codec's time\_base or pts since not all codecs have one, but most
>  containers do. (This is why the dranger tutorial says to ignore AVFrame->pts)
> 
> 
> 

…which confused me even more, given that I couldn't find any such mention in the official docs.

Anyway, I replaced…

```
double received_time = received_timestamp * av_q2d(stream_time_base);

```

…with…

```
double received_time = received_timestamp * av_q2d(codec_time_base);

```

…and the output changed to this…

```
...

requested: 10.00 (sec)  -> 300000 (pts)
received:  33.40 (sec)  -> 2002 (pts)

```

**Still no match.** What's wrong?