The client application uses the ICM_GETDEFAULTKEYFRAMERATE message to obtain the driver’s recommendation for the key-frame spacing for compressing data. (A key frame is a frame in a video sequence that does not require information from a previous frame for decompression.) If the client application does not specify another value, this value determines how frequently the client application sends an uncompressed image to your driver with the ICCOMPRESS_KEYFRAME flag set. If your driver supports this option, it should specify the key-frame rate in the DWORD pointed to by dwParam1 and return ICERR_OK. If it does not support this option, return ICERR_UNSUPPORTED.
The client application uses ICM_GETBUFFERSWANTED to determine if your driver will maintain a queue of buffers. Your driver might maintain a queue of buffers if it renders the decompressed data and is designed to keep its hardware pipelines full. If your driver supports this option, it should specify the number of buffers in the DWORD pointed to by dwParam1 and return ICERR_OK. If it does not support this option, return ICERR_UNSUPPORTED.