如何从具有H264硬件编码器的网络摄像头访问H264流？

Question

I'm developing a video capture application using DirectShow and I'd like to take advantage of the H264 hardware encoders present on some web-cams.

In particular I'm looking at two cameras: the Creative Connect HD VF0750 and the Logitech C920 which both claim to feature H264 hardware encoders.

Using GraphEdit, so far I've had mixed results. The Logitech C920 camera exposes an H264 stream as a Pin on the DirectShow WDM Capture filter but only on Windows 7. Trying the same on Windows 8 only shows output Pins for MJPEG and uncompressed YUV streams.

On the other hand, the Creative Connect HD VF0750 camera does not appear to expose an H264 Pin at all - even in Windows 7.

Yet, they both claim H264 encoders, and they claim that Skype can take advantage of the hardware encoders.

So the question is - what interface or API does Skype use to obtain the hardware encoded H264 stream from these cameras? Since they're offered by different manufacturers it stands to reason that they use some common API and not just some obscure made-for-Skype custom interface.

I've used code like this to enumerate all pins and media types/sub types for each:

IBaseFilterPtr f = getBaseFilterFromEnum();    // code omitted for brevity

IEnumPinsPtr pEnum;
f->EnumPins(&pEnum);
IPinPtr pin;
while (pEnum->Next(1, &pin, NULL) == S_OK)
{
    PIN_INFO PinInformation;
    pin->QueryPinInfo(&PinInformation);                 

    IKsPropertySetPtr pKsPropertySet = pin;
    if(pKsPropertySet)
    {
        GUID Category;
        DWORD nCategorySize;                

        if(SUCCEEDED(pKsPropertySet->Get(AMPROPSETID_Pin, AMPROPERTY_PIN_CATEGORY, NULL, 0, &Category, sizeof Category, &nCategorySize)))
        {
            // checks category type here

            IAMStreamConfigPtr pAmStreamConfig = pin;
            if(pAmStreamConfig)
            {
                int fmts, sccsz;
                pAmStreamConfig->GetNumberOfCapabilities(&fmts, &sccsz);
                BYTE* scc = new BYTE[sccsz];            

                for (int i = 0; i < fmts; i++)
                {
                    AM_MEDIA_TYPE* mt;
                    pAmStreamConfig->GetStreamCaps(i, &mt, scc);
                    if (mt)
                    {
                        if (mt->pbFormat)
                        {
                            // query here based on mt->formattype

                            //  and based on mt->subtype
                            if(mt->subtype == MEDIASUBTYPE_H264)
                            {
                                // has H264
                            }else if(mt->subtype == MEDIASUBTYPE_RGB24)
                            {
                                // has RGB
                            }else if(mt->subtype == MEDIASUBTYPE_YUY2)
                            {
                                // has YUY2
                            }else if(mt->subtype == MEDIASUBTYPE_MJPG)
                            {
                                // has MJPEG
                            }else
                            {
                                // has some other sub-type
                                //   not getting this on either of my cams
                            }
                            // free mem here
                        }
                        // more free mem here
                    }
                }
                delete[] scc;
            }else{                  
                // Error Getting Formats - this happens for the Video Terminal Pins
            }
        }           
    }
}

This is output I'm getting for the two cameras on Windows 7:

Live! Cam Connect HD VF0750
   Capture (CAPTURE):  
       18 Formats: RGB; YUY2; MJPEG; (VIDEO_INFO; )
   Video Camera Terminal (UNKNOWN):  
       Error Getting Formats
   Still (STILL):  
       3 Formats: RGB; (VIDEO_INFO; )
HD Pro Webcam C920
   Capture (CAPTURE):  
       106 Formats: H264; YUY2; MJPEG; (VIDEO_INFO; VIDEO_INFO2; )
   Video Camera Terminal (UNKNOWN):
       Error Getting Formats

On Windows 8, the output is different. Neither of them come up with H264 sub-types (despite both claiming to support H264 hardware-encoded stream).

At least in Win7 the Logitech camera provides the H264 pin as expected.

Answer 1

Over time I received a few submissions from users for video capture capabilities of Logitech HD Pro Webcam C920 camera in different systems. The data is here . Availability of hardware H.264 video requires specific (most recent?) version of driver, and behavior is different across operating system versions. While in Windows 7 H264 media type is available on the capture pin the usual way, in newer OSes there is a separate additional Capture pin with H264 media types. Apparently, applications capturing H.264 are assumed to be aware of this new layout and connect pins/capture appropriately.

The same applies to Logitech Webcam C930e camera (Windows 7 - no H.264 at all , Windows 8.1 - H.264 on second capture pin ).

The data on the secondary capture pin has standard MEDIASUBTYPE_H264 formatting: