I'm playing about with OpenGL ES 2.0. If I'm working with a simple 2D projection, if I have a large 2D grid of vertices which are pretty much static (think map tiles), of which only a small proportion are visible at any one time, would it be better to...
- Work out in the CPU which vertices are visible, and and create a VBO to draw just those triangles that make up the visible tiles in each frame?
or
- Keep a static VBO with the entire tiled grid, and then just rely on the graphics card (RPi, in my case) to clip out the off-screen triangles?
Or perhaps some combination of the two (like sets of overlapping pre-computed grids)? How big does the grid have to be before the latter option becomes unworkable?
Edit I decided to make several calls to glDrawElements(), drawing sub-ranges of the index buffer that I knew would overlap the viewport. At the scale I'm working at it doesn't seem to make any difference to the speed over drawing the entire element array, even on a Pi Zero.
However, this approach would require more computation to determine which ranges of elements needed to be rendered if there was any rotation of the grid involved - effectively rasterising my own quad. I'm interested to hear if this is a reasonable approach.
There are some other options like a more exotic structure for breaking up the plane into sub areas, I guess. Still not sure if any of this is really necessary, though.
Thanks!
Please note: I don't want to discuss drawing tiles in the fragment shader, I'm more interested in the correct way to work with the vertex shader than actually solving the described problem.
If that's a regular grid, I'd split it in large chunks, so the screen width (larger side) would fit 2-3 such chunks. They don't need to overlap if it's regular grid.
Checking one chunk's visibility is trivial and cheap, as well as finding/selecting those few that must be drawn. And the wasted/clipped area is small enough to not worry about it. You don't have to go crazy and trim every single vertex that's outside of the screen.
Each chunk would have own VBO, and it would be weakly cached when it goes fully outside of screen, so you don't have to rebuild/reload resources needed to draw that chunk if you quickly return to this part of the map.
Splitting in chunks minimizes the memory requirements and speeds up the level loading. You spend time only loading the part of the screen that user will see immediately. This also allows quite huge maps, since you can prefetch the areas that you're going towards to.