We have determined that our terrain rendering system needs some work to minimize the number of batches being transferred to the GPU in order to improve performance. I'm looking for suggestions on how best to improve what we're trying to accomplish.
We logically split our terrain mesh into smaller grid cells which are 32x32 world units. Each cell has meta data that dictates the four 256x256 textures that are used for spatting along with the alpha blend data, shadow, and light mappings. Each cell contains 81 vertices in a 9x9 grid.
Presently, we examine each cell and determine the four textures that are being used to spat the cell. We combine that geometry with any other cell that perhaps uses the same four textures regardless of spat order. If the spat order for a cell differs, the blend map is adjusted so that the spat order is maintained the same as other like cells and blending happens in the right order too.
But even with this batching approach, it isn't uncommon when looking out across an area of open terrain to have between 1200-1700 batch count depending upon how frequently textures differ or have different texture blends are between cells. We are only doing frustum culling presently.
So using texture spatting, are there other alternatives that can reduce the batch count and allow rendering to be extremely performance-friendly even under DirectX9c?
We considered using texture atlases since we're targeting DirectX 9c & older OpenGL platforms but trying to repeat textures using atlases and shaders result in seam artifacts which we haven't been able to eliminate with the exception of disabling mipmapping. Disabling mipmapping results in poor quality textures from a distance.
How have others batched together terrain geometry such that one could spat terrain using various textures, minimizing batch count and texture state switches so that rendering performance isn't negatively impacted?