Streaming Capture from an MCI Device
MCI devices augment the capture operation in real-time capture and step-frame
capture. You can specify the MCI device, such as a videodisc or video-cassette
recorder (VCR), acting as the video source for your capture operation by using
the
WM_CAP_SET_MCI_DEVICE message (or the
capSetMCIDeviceName macro) and specifying the name of the device. You can also retrieve the
device name currently set by using the
WM_CAP_GET_MCI_DEVICE message (or the
capGetMCIDeviceName macro).
In real-time capture, the capture window synchronizes the capture operation
and compensates for delays associated with positioning the MCI video source and
initializing the resources (such as capture buffers) required for capturing
data. The capture window expects a valid MCI video device to be installed in the
system for capturing data this way.
Specifications for controlling an MCI device are stored in the members of the
CAPTUREPARMS structure. MCI-compatible video sources include VCRs and laserdiscs. If the
fMCIControl member of this structure is set to TRUE, the capture window coordinates MCI
operation. The capture window uses the parameters specified in the
dwMCIStartTime and
dwMCIStopTime members to obtain the starting and stopping positions, in milliseconds, of
the sequence. If the value of
fMCIControl is FALSE, the video source is not treated as an MCI device and the contents
of
dwMCIStartTime and
dwMCIStopTime are ignored.
You can use Media Player to quickly verify that an MCI video device is
properly connected to the system. Playing a device with Media Player verifies that the
MCI configuration for the device is correct. If an image appears on the video
display, the video source is connected properly to the capture hardware.
In step-frame capture, the capture window synchronizes the capture operation
and compensates for the delays associated with positioning the MCI video source
and initializing the resources required for capturing data. In addition, the
capture window ensures that no frames are dropped; it steps through the video
frames individually, ensuring that the frame is captured and stored before
capturing the next frame in the video stream.
Specifications for controlling step-frame capture are stored in the members of
the
CAPTUREPARMS structure. Step-frame capture uses the following members in addition to the
members used for real-time capture:
fStepMCIDevice,
fStepCaptureAt2x, and
wStepCaptureAverageFrames. If the
fStepMCIDevice member is set to TRUE, the capture window coordinates step-frame capture. The
capture window uses the parameters specified in the
dwMCIStartTime and
dwMCIStopTime members for the starting and stopping positions, in milliseconds, of the
sequence. The capture window uses
fStepCaptureAt2x to determine if the capture hardware should capture video frames at twice the
normal resolution and uses
wStepCaptureAverageFrames to specify the number of times each frame in the capture operation is sampled.
If
fStepMCIDevice is FALSE, real-time capture is used instead of step-frame capture and the
contents of
fStepCaptureAt2x, and
wStepCaptureAverageFrames are ignored.
If a step-frame capture is specified and
fStepCaptureAt2x is set to TRUE, the capture hardware captures at twice the specified
resolution. (The resolutions of both the height and width are doubled.) The software
interpolates the pixels in the higher resolution image to produce the image at
the specified resolution. This form of averaging can improve the edge definition
of images in a frame. You can enable this option if the hardware does not
support hardware-based decimation and you are capturing in the RGB format.
Note If your hardware supports hardware-based decimation, it can capture samples
at a higher rate than specified and use these additional samples to obtain
color definitions that are more consistent with the original image. The additional
samples are discarded after they are used, and the hardware passes samples to
the capture driver at the specified rate.
If a step-frame capture is specified, the
wStepCaptureAverageFrames member specifies the number of times a frame is sampled when creating a frame
based on the average sample. This averaging technique reduces the random
digitization noise appearing in a frame. A typical value for the number of averages
is 5.
For more information about MCI, see
MCI.
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