Multisample Support through StretchBlt
Although not the recommended mechanism for supporting multisampling, the driver can implement multisampling support by rendering to a large back buffer and performing a stretch blt to resample the large back buffer to the lower resolution primary. However, if this is the mechanism by which the driver supports multisampling, the driver must set the new capability bit D3DPRASTERCAPS_STRETCHBLTMULTISAMPLE in the RasterCaps member of the D3D8CAPS structure. For a description of D3DCAPS8, see the DirectX 8.0 SDK documentation.
When the driver sets the D3DPRASTERCAPS_STRETCHBLTMULTISAMPLE bit, it indicates that it:
Fails requests from applications to enable and disable full-scene anti-aliasing while the same scene is being rendered. That is, it fails requests to turn on and off the BOOL value of the D3DRS_MULTISAMPLEANTIALIAS device render state (D3DRENDERSTATETYPE) during the rendering of a single scene. Note that requests to change the BOOL value of D3DRS_MULTISAMPLEANTIALIAS must not fail for a different scene. That is, if D3DRS_MULTISAMPLEANTIALIAS is TRUE for one scene, it could be FALSE for another scene.
Is nonresponsive to requests from applications to modify samples in a multisample render target. That is, it does not respond to setting the bitmask of the D3DRS_MULTISAMPLEMASK device render state (D3DRENDERSTATETYPE).
It is important to note that if the driver uses a stretch blt to perform a page flip in fullscreen mode, the driver should specify the supported sample counts in the wFlipMSTypes member of the DDPIXELFORMAT's MultiSampleCaps structure and not the wBltMSTypes member as a flip is being performed.