Similar to decompressing video data, your driver will receive a series of messages when it is used to compress data. The client-application will send messages to your driver to coordinate the following activities:
The following messages are used by video compression drivers:
| Message | Description |
|---|---|
| ICM_COMPRESS | Compresses a frame of data into the buffer provided by the client-application. |
| ICM_COMPRESS_BEGIN | Prepares for compressing data. |
| ICM_COMPRESS_END | Cleans up after compressing. |
| ICM_COMPRESS_GET_FORMAT | Obtains the driver's suggest the output format of the compressed data. |
| ICM_COMPRESS_GET_SIZE | Obtains the maximum size of one frame of data when it is compressed in the output format. |
| ICM_COMPRESS_QUERY | Determines if a driver can compress a specific input format. |
The video compressed with these messages is returned to the client-application. When compressing data, your driver can use either software or hardware to do the compression.
Note When MSVideo recompresses a file, each frame is decompressed to a full frame before it is passed to the compressor.