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Double Buffering Texture Data

When you use any technique that allows the GPU to access your texture data directly, such as the texture range extension, it's possible for the GPU and CPU to access the data at the same time. To avoid such a collision, you must synchronize the GPU and the CPU. The simplest way is shown in Figure 9-10. Your application works on the data, flushes it to the GPU and waits until the GPU is finished before working on the data again.

One technique for ensuring that the GPU is finished executing commands before your application sends more data is to 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 9-10 uses the fence extension command glFinishObject to synchronize buffer updates for a stream of single-buffered texture data. Notice that when the CPU is processing texture data, the GPU is idle. Similarly, when the GPU is processing texture data, the CPU is idle. It's much more efficient for the GPU and CPU to work asynchronously than to work synchronously. Double buffering data is a technique that allows you to process data asynchronously, as shown in Figure 9-11.

To double buffer data, you must supply two sets of data to work on. Note in Figure 9-11 that while the GPU is rendering one frame of data, the CPU processes the next. After the initial startup, neither processing unit is idle. Using the glFinishObject function provided by the fence extension ensures that buffer updating is synchronized.

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