2.6.3. Draw mode

For large meshes, each vertex is typically included in several triangles. The number of times that such a vertex is processed depends on the function that performs the draw operation.

Using the API function glDrawElements, the attributes of one vertex have to be processed only once. The alternative function, glDrawArrays, causes each set of vertex data to be transferred and processed each time it is used in a triangle. For this reason, the most efficient way to pass geometry data when using typical meshes, is usually with indexed mode, using glDrawElements.

Alternatively, restructure the mesh into triangle fans or triangle strips, as this reduces the number of times each vertex occurs in the vertex array. You can also use triangle fans or triangle strips to reduce the number of indices transferred when glDrawElements is used.

Note

By drawing a mesh with triangle fans or strips, it typically requires more draw calls, than when compared to using individual triangles. Because each draw call has some amount of processing overhead, splitting the mesh you could easily move the processing bottleneck from vertex processing to draw call overhead, eliminating the advantage of performing less vertex processing. See Identifying problems in applications for more information about bottlenecks.

Store vertex data tightly and in the order that it is to be used. This improves the effectiveness of the vertex cache, by minimizing the amount of data transferred from RAM to the vertex cache.

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