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Creating Device-Specific Components for Image Acquisition APIs

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Image acquisition APIs, such as TWAIN, typically require device-specific components, such as TWAIN data sources. These device-specific components should use the IStillImage COM Interface and the IStiDevice COM Interface to communicate with user-mode still image device drivers and the event monitor.

Image acquisition APIs can call IStillImage::GetDeviceValue and IStillImage::SetDeviceValue to read and write Registry Entries for Still Image Devices. For example, the name of each still image device's TWAIN data source is stored in the registry.

Because the TWAIN API does not allow an application to specify the active device when calling a data source, the data source will typically call IStillImage::GetDeviceList to obtain a list of all still image devices, and then will search the list to find the correct device, usually based on the manufacturer and model names. The manufacturer and model text names are obtained from the setup information (INF) file. Because TWAIN has a 32-character limit for data source names, and because WIA appends "WIA-" to strings to construct the compatible names, the text in the INF file should not be longer than 28 characters. Otherwise, TWAIN-compatible applications that perform a comparison on the entire string, and not just the first 32 characters, might not be able to automatically find the device that caused the application to launch.

To access a device, image acquisition software calls IStillImage::CreateDevice to create an instance of the COM object that defines the IStiDevice interface. The IStiDevice interface provides several methods for performing device I/O operations. When creating the object instance, image acquisition software should specify the "data" Transfer Modes.

Image acquisition software can call IStiDevice::Subscribe to request the event monitor to deliver notification of Still Image Device Events. Once notification is received, IStiDevice::GetLastNotificationData can be called to determine the type of event. IStiDevice::UnSubscribe should be called when notifications are no longer needed.

When the image acquisition software has finished using the IStiDevice interface, it must call IStiDevice::Release.