Static vs Dynamic Geometry

A buffer created without the D3DUSAGE_DYNAMIC flag is calleda static buffer. Static buffers are generally placed in video memorywhere its contents can be processed most efficiently. However, thetrade-off of making a buffer static is that writing to and reading fromthe memory of a static buffer is inherently slow because accessingvideo memory is slow. For this reason we use static buffers to holdstatic data (data that will not need to be changed (accessed) very frequently).Terrains and city buildings are examples of good candidatesfor static buffers because the terrain and building geometry will usuallynot change during the course of the application. Static buffers shouldbe filled with geometry at application initialization time and never atrun time.A buffer created with the D3DUSAGE_DYNAMIC flag is called adynamic buffer. Dynamic buffers are generally placed in AGP memorywhere its memory can be updated quickly. Dynamic buffers are notprocessed as quickly as static buffers because the data must be transferredto video memory before rendering, but the benefit of dynamicbuffers is that they can be updated reasonably fast (fast CPU writes).Therefore, if you need to update the contents of a buffer frequently, itshould be made dynamic. Particle systems are good candidates fordynamic buffers because they are animated, and thus their geometryis usually updated every frame.