Bitmap Cache for RDP (Windows Embedded CE 6.0)
1/6/2010
A bitmap cache reduces the bitmap data transferred between the Remote Desktop Protocol (RDP) client and the server. The RDP client creates an uncompressed, temporary bitmap cache file that contains bitmaps that are repeatedly sent from the terminal server to the client. The RDP client uses Graphics Device Interface (GDI) calls with these cached bitmaps to draw bitmaps onto the display.
Bitmap Cache Types
The following table shows the two types of bitmap caches.
| Cache type | Description |
|---|---|
Volatile |
A volatile bitmap cache resides in RAM and is valid for the session. This cache is always used. The default minimum size of RAM cache is 1.5 MB, and is configurable. |
Persistent |
A persistent bitmap cache is stored on disk. The check box on the Experience tab in the RDP client user interface (UI) refers to the persistent bitmap cache. |
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Volatile Cache
The destination for the initial bitmap cache is always RAM cache. Because the server is aware of the RAM cache size, it determines how many bitmaps the client can cache.
A volatile cache is useful for caching bitmaps on systems with 8 bits-per-pixel (bpp) color settings. A volatile cache is also useful for systems that have a smaller amount of disk space to store bitmaps. However, a volatile cache is temporary and does not persist data. After a Terminal Services session has ended, the bitmaps in the volatile cache are freed and must be re-transferred to the client computer during the next session. This could affect performance.
Persistent Cache
If the persistent bitmap cache is enabled, depending on the size of the RAM cache, the server may decide to persist some bitmaps to disk, in addition to storing bitmaps in RAM cache. For example, the server sends a bitmap to the RAM cache that it assumes will be used later, with the persistent flag set. The server also stores a copy of the bitmap on disk. If the client needs a bitmap after that bitmap has been evicted from RAM cache, the server instructs the client to use the bitmap copy from the cache on the disk and does not send the bitmap data again.
A persistent cache is useful for caching bitmaps on systems which support 16-bpp, 24-bpp, or 32-bpp color settings.
For information about how to set persistent bitmap caching, see Bitmap and Caching Properties of IMsRdpClientAdvancedSettings and IMsRdpClientAdvancedSettings Methods for Bitmaps and Caching.