{"id":397,"date":"2021-06-05T18:21:10","date_gmt":"2021-06-05T18:21:10","guid":{"rendered":"https:\/\/bronsonzgeb.com\/?p=397"},"modified":"2021-06-05T18:21:11","modified_gmt":"2021-06-05T18:21:11","slug":"gpu-mesh-voxelizer-part-3-render-tons-of-voxels-with-drawmeshinstancedindirect","status":"publish","type":"post","link":"https:\/\/bronsonzgeb.com\/index.php\/2021\/06\/05\/gpu-mesh-voxelizer-part-3-render-tons-of-voxels-with-drawmeshinstancedindirect\/","title":{"rendered":"GPU Mesh Voxelizer Part 3: Render tons of voxels with DrawMeshInstancedIndirect"},"content":{"rendered":"\n

In this article, we render all the voxels from our voxelization process. If you haven’t read the previous parts, you can find part 1 here<\/a> and part 2 here<\/a>. We’ll generate a mesh and draw it via the `DrawMeshInstancedIndirect` API. Doing so requires us to write a custom shader as well. Let’s get started.<\/p>\n\n\n\n

Drawing tons of meshes with DrawMeshInstancedIndirect<\/h2>\n\n\n\n

Our goal is to draw millions of voxels. I chose to approach this using the DrawMeshInstancedIndirect<\/code> API, which draws a copy of a single mesh in a zillion different places. It works by uploading a single copy of a mesh and an array of positions to the GPU. Then in the shader, we grab the relevant position and use it to place and draw an instance of the mesh. Here’s what the drawing code looks like.<\/p>\n\n\n\n

if (_drawBlocks)\n{\n    _blocksArgsBuffer.SetData(new[] {_voxelMesh.triangles.Length, _gridPointCount, 0, 0, 0});\n    _blocksMaterial.SetBuffer(\"_Positions\", _voxelPointsBuffer);\n    Graphics.DrawMeshInstancedIndirect(_voxelMesh, 0, _blocksMaterial, _meshCollider.bounds, _blocksArgsBuffer);\n}<\/code><\/pre>\n\n\n\n
The `DrawMeshInstancedIndirect` API takes five arguments here:<\/p>\n\n\n\n
  1. The mesh<\/li>
  2. The submesh index (we won’t have submeshes, so 0 for us)<\/li>
  3. A material<\/li>
  4. The bounds<\/li>
  5. An args buffer<\/li><\/ol>\n\n\n\n
    Let’s tackle them in order, starting with the voxel mesh.<\/p>\n\n\n\n
    Generating a Voxel Mesh<\/h2>\n\n\n\n
    The plan is to create a small cube mesh that’s the size of one voxel, then draw as many instances of that mesh as we need. Alternatively, we could use a constant size cube and scale it, but I figure that creating a new mesh one time (or every time we change the voxel size) is cheaper than scaling every voxel every time we render. Realistically, the difference is probably negligible, but this is the choice I made.<\/p>\n\n\n\n
    So, first thing’s first, we need to create the vertices. As you may know, a cube has eight unique vertices. However, if we want our cubes to have a nice flat look, we need to duplicate several vertices. Why is that? Because each vertex holds a single normal, but each vertex shares three different faces, each with its own normal. And so, if we reuse the same vertex across multiple faces, the normal will get averaged across the three faces. The result is that the lighting will be smooth across the sharp corners of the cube. To illustrate, here’s what that looks like in Blender.<\/p>\n\n\n\n