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Preview and Overlay Modes
A capture driver can implement two methods for viewing an incoming video
stream: preview mode and overlay mode. If a capture driver implements both methods,
the user can choose which method to use.
Preview mode transfers digitized frames from the capture hardware to system
memory and then displays the digitized frames in the capture window by using
graphics device interface (GDI) functions. Applications might decrease the preview
rate when the parent window loses focus, and increase the preview rate when the
parent window gains focus. This action improves general system responsiveness
because the preview operation is processor intensive.
There are three messages to control the preview operation.
When preview and scaling are both enabled, the captured video frame is
stretched to the dimensions of the capture window. Enabling preview mode automatically
disables overlay mode.
Overlay mode is a hardware function that displays the contents of the capture
buffer on the monitor without using CPU resources. You can enable and disable
overlay mode by sending the WM_CAP_SET_OVERLAY message (or the capOverlay macro) to a capture window. Enabling overlay mode automatically disables
preview mode.
You can also set the scroll position of the video frame within the client area
of the capture window for preview mode or overlay mode by sending the WM_CAP_SET_SCROLL message (or the capSetScrollPos macro) to a capture window.
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Win32 Programmer's Reference (win32.hlp)
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