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Double Buffering Vertex Data

When you use the vertex array range extension and the shared memory hint, the GPU reads data directly from memory managed by your application. To avoid having the GPU and your application access the data at the same time, you'll need to synchronize access. A simple approach is for your application to operate on the vertex array data, flush it to the GPU, and wait until the GPU is finished before working on the data again. This is what Figure 8-6 shows.

To ensure that the GPU is finished executing commands before the CPU sends more data, you can insert a token into the command stream and use that to determine when the CPU can touch the data again, as described in “Fence Extension.” Figure 8-6 uses the fence extension command glFinishObject to synchronize buffer updates. Notice that when the CPU is processing data, the GPU is waiting. Similarly, when the GPU is processing data, the CPU is waiting. In other words, the application executes synchronously. A more efficient way is for the application to double buffer your data so that you can use the waiting time to process more data.

To double buffer your data, you must supply two sets of data to work on. Notice in Figure 8-7 that while the GPU is operating on one set of vertex array data, the CPU is processing the next. After the initial startup, neither processing unit is idle. Using the glFinishObject function provided by the fence extension, as shown, ensures that buffer updates are synchronized

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