VCM is an intermediary between an application and compression and decompression drivers. The compression and decompression drivers compress and decompress individual frames of data.
When an application makes a call to VCM, VCM translates the call into a message. The message is sent by using the ICSendMessage function to the appropriate compressor or decompressor, which compresses or decompresses the data. VCM receives the return value from the compression or decompression driver and then returns control to the application.
If a macro is defined for a message, the macro expands to an ICSendMessage function call supplying appropriate parameters for that message. If a macro is defined for a message, your application should use it rather than the message. In this overview, these macros follow messages in parentheses.