Opening and Closing a Stream

The Stream class interface sends an SRB_OPEN_STREAM request to a Stream class minidriver to open a stream with the selected video format. Information passed in SRB_OPEN_STREAM includes the index of the stream to be open and a pointer to a pointer to a KS_VIDEOINFOHEADER structure. The stream index corresponds to the index of the stream in the array of KS_DATARANGE_VIDEO structures returned by the minidriver in response to an earlier SRB_GET_STREAM_INFO request. For more information about handling SRB_GET_STREAM_INFO, see Stream Categories.

The following example code obtains the stream index, kernel streaming data format, and kernel streaming video info header.


int StreamNumber = pSrb->StreamObject->StreamNumber;
PKS_DATAFORMAT_VIDEOINFOHEADER  pKSDataFormat = 
    (PKS_DATAFORMAT_VIDEOINFOHEADER) pSrb->CommandData.OpenFormat;
PKS_VIDEOINFOHEADER pVideoInfoHdrRequested = 
    &pKSDataFormat->VideoInfoHeader;

Minidrivers should verify that they can support the requested stream format. In particular, the contents of the KS_BITMAPINFOHEADER structure should be verified, along with cropping and scaling information specified by the rcSource and rcTarget members.

If the device hardware cannot support the capture frame rate requested in the AvgTimePerFrame member of KS_VIDEOINFOHEADER, it should always select the next lower frame rate available. For example, if a camera can support a capture frame rate of 7 frames per second (fps) and 15 fps, and a client application attempts to open the stream at a capture frame rate of 10 fps, the camera should create a 7-fps physical stream.

For a ten-second capture in which all 70 available physical frames are captured, the minidriver should report 100 frames captured, 30 frames of which were dropped by the KSPROPERTY_DROPPEDFRAMES_CURRENT property.

Special rules apply when the output buffer is a DirectDraw surface. In this case, the biWidth member of the KS_BITMAPINFOHEADER structure actually represents the stride of the destination DirectDraw surface, which typically is larger than the video image width. The stride of a surface is usually the width of the surface multiplied by its byte-depth. For example, for a surface that is 640 pixels wide with a color depth of 32 bits-per-pixel, the stride would be 2560 bytes.

To determine the requested image width, use the following code example:


if (IsRectEmpty(&pVideoInfoHdrRequested->rcTarget) {
    Width =  pVideoInfoHdrRequested->bmiHeader.biWidth;
    Height = pVideoInfoHdrRequested->bmiHeader.biHeight;
} 
else {
    Width = pVideoInfoHdrRequested->rcTarget.right − 
            pVideoInfoHdrRequested->rcTarget.left;
    Height = pVideoInfoHdrRequested->rcTarget.bottom − 
             pVideoInfoHdrRequested->rcTarget.top;
}

The Stream class interface sends an SRB_CLOSE_STREAM request to the minidriver to close a stream. The minidriver should then return all outstanding stream SRBs to the Stream class interface.

 

 

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