Directshow returns wrong Frame rate FPS

2
votes

I want to have the frame rate of a media file using DirectShow.

Currently, I use the following methods which seem inaccurate in some cases:

I add a SourceFilter to my graph, enum its pins, then call a pPin->ConnectionMediaType(&compressedMediaFormat) and extract AvgTimePerFrame out from it. As far as I understand it is the average time per frame expressed in 100 nanoseconds. So, I just divide 10,000,000 / AvgTimePerFrame to get the average FPS of the file.

For those media files which have almost the same frame time for all frames, I get a correct FPS. But for those, which have different frame times for different frames this method returns very inaccurate results.

A correct way to get that would be to get the duration and frame count of the file and calculate the average FPS out of it (frameCount / duration). This is a costly operation however as I understand because calculating the exact number of frames requires passing through the whole file.

I wonder if there is a way to get that frame rate information more accurately?

1
c++winapidirectshowframe-rate

1 Answers

2
votes

The media files don't have to be of fixed frame rate, in general - there might be variable frame rate. The metadata of the file still has some frame rate related information which, in this case, might be inaccurate. When you start accessing the file, you have the quickly available metadata information about the frame rate. Indeed, to get the full picture you are supposed to read all frames and process their time stamps.

Even though in many it is technically possible to quickly read just time stamps of frames without reading the actual data, DirectShow demultiplexers/parsers have no method defined to obtain the information, so you would have to read and count the frames to get the accurate information.

You don't need to decompress video for that though, and you can also remove clock from the filter graph when doing this, so that counting frames does not require streaming data in real time (frames would be streamed at maximal rate in that case).