IMediaSample returned by Sample Grabber has unexpected buffer size

Question

I was working on a audio/video capture library for Windows using Media Foundation. However, I faced the issue described in this post for some webcams on Windows 8.1. I have thus decided to have another implementation using Directshow to support in my app the webcams for which the drivers haven't been updated yet.

The library works quite well, but I have noticed a problem with some webcams for which the sample (IMediaSample) returned has not the expected size according to the format that was set before starting the camera.

For example, I have the case where the format set has a subtype of MEDIASUBTYPE_RGB24 (3 bytes per pixels) and the frame size is 640x480. The biSizeImage (from BITMAPINFOHEADER) is well 640*480*3 = 921600 when applying the format. The IAMStreamConfig::SetFormat() method succeed to apply the format.

hr = pStreamConfig->SetFormat(pmt);

I also set the format to the Sample Grabber Interface as follow:

hr = pSampleGrabberInterface->SetMediaType(pmt);

I applied the format before starting the graph.

However, in the callback (ISampleGrabberCB::SampleCB), I'm receiving a sample of size 230400 (which might be a buffer for a frame of size 320x240 (320*240*3=230400)).

HRESULT MyClass::SampleCB(double sampleTime, IMediaSample *pSample)
{
   unsigned char* pBuffer= 0;
   HRESULT hr = pSample->GetPointer((unsigned char**) &pBuffer);
   if(SUCCEEDED(hr) {
      long bufSize = pSample->GetSize();
      //bufSize = 230400
   }
}

I tried to investigate the media type returned using the IMediaSample::GetMediaType() method, but the media type is NULL, which means, according to the documentation of the GetMediaType method that the media type has not changed (so I guess, it's still the media type I've applied successfully using the IAMStreamConfig::SetFormat() function).

HRESULT hr = pSample->GetMediaType(&pType);

if(SUCCEEDED(hr)) {
   if(pType==NULL) {
     //it enters here => the media type has not changed!
   } 
}

Why the sample buffer size returned is not the expected size in this case? How can I solve this issue?

Thanks in advance!

You are right Wimmel, I fixed it pBuffer->GetSize() -> pSample->GetSize() — skuallpa

Roman R. Roman R. · Accepted Answer · 2015-04-03T13:24:04

Sample Grabber callback will always return "correct" size in terms that it matches actual data size and formats used in streaming pipeline.

If you are seeing the mismatch, it means that your filter graph topology differs from what you expect it to be. You need to review the graph (esp. using remote connection by GraphEdit), inspect media types and check why it was built incorrectly. For example, you could be applying format of your interest after connecting pins, which is too late.

IMediaSample returned by Sample Grabber has unexpected buffer size

1 Answers