staticVoid2

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.

Hi there,

I'm using Google maps static API and I need to know how to get the world-size dimensions of the returned image.

I've used the formula they give in their online documentation here: https://developers.google.com/maps/documentation/staticmaps/ but the values returned are roughly 2.5x what they should be.

the formula is:

resolution = 256 * pow(2, zoom_level)

I then put this over the size of the world circumference (i.e. 40,075km) and then multiple by the resolution of the returned image.

size = (40,075,000 / (256 * pow(2, zoom_level)) * image_res

In my example the zoom level is 15 and image res is 1024. The value returned is 4891.96.. but it should really be closer to 1750,00 (judging by the features in the image).

Any ideas?
Thanks.