• Announcements

    • khawk

      Download the Game Design and Indie Game Marketing Freebook   07/19/17

      GameDev.net and CRC Press have teamed up to bring a free ebook of content curated from top titles published by CRC Press. The freebook, Practices of Game Design & Indie Game Marketing, includes chapters from The Art of Game Design: A Book of Lenses, A Practical Guide to Indie Game Marketing, and An Architectural Approach to Level Design. The GameDev.net FreeBook is relevant to game designers, developers, and those interested in learning more about the challenges in game development. We know game development can be a tough discipline and business, so we picked several chapters from CRC Press titles that we thought would be of interest to you, the GameDev.net audience, in your journey to design, develop, and market your next game. The free ebook is available through CRC Press by clicking here. The Curated Books The Art of Game Design: A Book of Lenses, Second Edition, by Jesse Schell Presents 100+ sets of questions, or different lenses, for viewing a game’s design, encompassing diverse fields such as psychology, architecture, music, film, software engineering, theme park design, mathematics, anthropology, and more. Written by one of the world's top game designers, this book describes the deepest and most fundamental principles of game design, demonstrating how tactics used in board, card, and athletic games also work in video games. It provides practical instruction on creating world-class games that will be played again and again. View it here. A Practical Guide to Indie Game Marketing, by Joel Dreskin Marketing is an essential but too frequently overlooked or minimized component of the release plan for indie games. A Practical Guide to Indie Game Marketing provides you with the tools needed to build visibility and sell your indie games. With special focus on those developers with small budgets and limited staff and resources, this book is packed with tangible recommendations and techniques that you can put to use immediately. As a seasoned professional of the indie game arena, author Joel Dreskin gives you insight into practical, real-world experiences of marketing numerous successful games and also provides stories of the failures. View it here. An Architectural Approach to Level Design This is one of the first books to integrate architectural and spatial design theory with the field of level design. The book presents architectural techniques and theories for level designers to use in their own work. It connects architecture and level design in different ways that address the practical elements of how designers construct space and the experiential elements of how and why humans interact with this space. Throughout the text, readers learn skills for spatial layout, evoking emotion through gamespaces, and creating better levels through architectural theory. View it here. Learn more and download the ebook by clicking here. Did you know? GameDev.net and CRC Press also recently teamed up to bring GDNet+ Members up to a 20% discount on all CRC Press books. Learn more about this and other benefits here.
Sign in to follow this  
Followers 0
staticVoid2

FFmpeg: calculating PTS of decoded video frames

2 posts in this topic

Hi there,

 

I'm making a basic video player in c++ using libffmpeg and SDL and I'm having some problems getting the correct Presentation Time Stamp (PTS) of each decoded frame in the stream.

 

I've read through the entire tutorial here: http://dranger.com/ffmpeg/ and my code is fairly similar although more object orientated.

 

I stream the packets into two separate queues: one for audio and one for video. A thread (the video thread) then picks up the packets in the video queue, decodes them and adds the result images to a display queue. Each of the images in the display has a value describing the number of seconds along the timeline when the frame is to be displayed (PTS). The problem I'm having is calculating this value!

 

For the most part the video player works and audio seems to be completely fine. here is the code in the video thread:

 

 

while( mVideoThreadRunning )
{
AVPacket packet;
if( mVideoPacketQueue.Pop( &packet ) )
{
int frameFinished = 0;
 
if( avcodec_decode_video2( mVideoCodecContext, mFrame, &frameFinished, &packet ) == -1 )
{
exit(0);
}
 
assert( packet.dts != AV_NOPTS_VALUE );
// use the packet decode timestamp as pts
double pts = (double)packet.dts;
 
// multiply by time base to get the time offset
pts *= av_q2d( mVideoCodecContext->time_base );
 
if( frameFinished )
{
// wait until the display queue frees up
while( mDisplayQueue.Size() >= 100 )
{
SDL_Delay(10);
}
 
// allocate a video frame to be displayed at 'pts' time
VideoFrame frame( mVideoCodecContext->width, mVideoCodecContext->height, pts );
 
// convert the frame data to mOutput (rgb24)
sws_scale(mSwsContext, mFrame->data, mFrame->linesize, 0, mVideoCodecContext->height, &frame.mData, &frame.mPitch);
 
cout << frame.mPts << endl;
// add the frame to the display queue
mDisplayQueue.PushSort( frame, [](const VideoFrame& a, const VideoFrame& b) -> bool
{
return a.mPts < b.mPts;
});
}
}
}
 

 

when I print out the pts of the first 6 frames I get this:

 

334

417.5

501

584.501

668.001

751.501

 

and the time base is 1001/48000 (AVRational) which is just under 48fps (~0.02), so for example when I calculate the pts for the first frame (334) it gives:

 

334 * av_q2d( time_base )[0.02] = 6.68 so the first frame only shows up after 6.68 seconds in the timeline.

 

This code works for roughly half the videos I've tested on it but this problem keeps creeping up. Is there any other calculations I need to perform to get the correct PTS every time? I wish there was more documentation on this.

 

Cheers.

Edited by staticVoid2
0

Share this post


Link to post
Share on other sites
  • Despite the fact that the first video frame doesn't show up until 6.68 seconds into playing the video, are the audio and video in sync?
  • Does the audio also have a delay, or does it start immediately?
  • Are you taking into account AVFormatContext.start_time? If the start_time is > 0, I would suggest subtracting start_time from all PTS (otherwise there's a delay).

 

Note that AVPacket.dts is in AVStream.time_base units, not AVCodecContext.time_base units.

 

PTS/DTS issues are my primary source of anger when dealing with video. I have found the method used by dranger's tutorial not to be sufficient. I used a combination of methods: checking AVFrame.best_effort_timestamp, checking AVPacket.dts and pts, and maybe a few other things I can't remember. If all of those failed (i.e. were AV_NOPTS_VALUE), I gave the frame a duration of 1/fps.

 

Also, what files are you struggling with? I typically found MPGs to be the most problematic.

2

Share this post


Link to post
Share on other sites

Thank you so much Cornstalks!

 

It was because I was using the mVideoCodecContext->time_base instead of the stream :)

 

As for the implementation; I currently sync the video with a master clock but the audio plays out of sync from this (using SDL_AudioSpec) I'm going to change this in the future so that the audio also syncs with the master clock.

 

there was an audio delay on some of the videos I tested but I also wasn't talking into account the start_time you mentioned so that could have been the problem.

 

I've not tested with many different formats so far, mainly just mp4's

 

Again, thanks so much!

0

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0