为什么我的行进立方体算法这么慢？

Question

i have somewhat implemented marching cubes in unity/c# (you dont need to know unity to help me though) and i cant stop feeling like i have made a big mistake in my code because it is so slow. i am already running it on a separate thread but it just takes ages to complete. please help me optimize my code.

private void _UpdateChunk()
{
    lock (this)
    {
        // clear the tri, vert and uv lists
        ClearMeshData();

        // Loop through each "cube" in the terrain.
        for (int x = 0; x < width; x++)
        {
            for (int y = 0; y < height; y++)
            {
                for (int z = 0; z < width; z++)
                {

                    // Create an array of floats representing each corner of a cube and get the value from our terrainMap.
                    float[] cube = new float[8];
                    float[] strengths = new float[8];
                    for (int i = 0; i < 8; i++)
                    {

                        Vector3Int corner = new Vector3Int(x, y, z) + gamedata.CornerTable[i];
                        cube[i] = terrainMap[corner.x, corner.y, corner.z].BlockType;
                        strengths[i] = terrainMap[corner.x, corner.y, corner.z].Strength;

                    }

                    // Pass the value into the MarchCube function.
                    MarchCube(new Vector3(x, y, z), cube, strengths);

                }
            }
        }
    }
}
void MarchCube(Vector3 position, float[] cube, float[] strengths)
{

    // Get the configuration index of this cube.
    int configIndex = GetCubeConfiguration(cube);

    // If the configuration of this cube is 0 or 255 (completely inside the terrain or completely outside of it) we don't need to do anything.
    if (configIndex == 0 || configIndex == 255)
        return;

    // Loop through the triangles. There are never more than 5 triangles to a cube and only three vertices to a triangle.
    int edgeIndex = 0;
    Vector3 vert1 = new Vector3();
    Vector3 vert2 = new Vector3();
    float vert1sample = 0;
    float vert2sample = 0;
    float lerp = 0;
    int indice = 0;
    for (int i = 0; i < 5; i++)
    {
        for (int p = 0; p < 3; p++)
        {
                
            // Get the current indice. We increment triangleIndex through each loop.
            indice = gamedata.TriangleTable[configIndex, edgeIndex];

            // If the current edgeIndex is -1, there are no more indices and we can exit the function.
            if (indice == -1)
                return;
            // Get the vertices for the start and end of this edge.
            vert1 = position + gamedata.EdgeTable[indice, 0];
            vert2 = position + gamedata.EdgeTable[indice, 1];
            vert1sample = strengths[gamedata.EdgeIndexTable[indice, 0]];
            vert2sample = strengths[gamedata.EdgeIndexTable[indice, 1]];
                
            // Get the midpoint of this edge.
            lerp = Mathf.Abs(vert1sample) / (Mathf.Abs(vert2sample) + Mathf.Abs(vert1sample));

                

            Vector3 vertPosition = Vector3.Lerp(vert1, vert2, lerp);
            
                
            // Add to our vertices and triangles list and incremement the edgeIndex.

            vertices.Add(vertPosition);

            triangles.Add(vertices.Count - 1);
            if (getChunkVoxel(vert1 + chunkPosition) != 0)
            {
                uvs.Add(new Vector2(getChunkVoxel(vert1 + chunkPosition) - 1, 0));
            }
            else
            {
                uvs.Add(new Vector2(getChunkVoxel(vert2 + chunkPosition) - 1, getChunkVoxel(vert2 + chunkPosition) - 1));
            }
                
                
                
                edgeIndex++;
        }
    }
}
int GetCubeConfiguration(float[] cube)
{

    // Starting with a configuration of zero, loop through each point in the cube and check if it is below the terrain surface.
    int configurationIndex = 0;
    for (int i = 0; i < 8; i++)
    {

        // If it is, use bit-magic to the set the corresponding bit to 1. So if only the 3rd point    in the cube was below
        // the surface, the bit would look like 00100000, which represents the integer value 32.
        if (cube[i] < terrainSurface)
                configurationIndex |= 1 << i;

    }

    return configurationIndex;

}

it appears that this is the part that slows my game down, help would be appreciated

i already made it faster by changing terrainpoint from a class to a struct but it is still very slow.

Answer 1

One main reason it is slow is that there is a lot of allocations in the loop putting a lot of pressure on the garbadge collector . There is currently 11 allocation per "cube" in the terrain in _UpdateChunk and up to 17 in MarchCube (possibly even more if the expressions like position + gamedata.EdgeTable[indice, 0] allocates a new vector). This is not reasonable. Many allocation are not needed . For example cube and strengths can be preallocated once for all the cubes in the beginning of _UpdateChunk . You do not need to allocate the vector in the expression to compute corner : you can just compute the components separately manually (or you can possibly preallocate the vector and reset its component when needed). The same thing applies for the new Vector3(x, y, z) can can be preallocated and set in the loop. Such an algorithm is computationally intensive so you should get away any overhead like virtual method calls and allocations/GC -- only low-level arrays accesses and mathematical operations should remains.

Note that some computations can be optimized. For example GetCubeConfiguration can be modified so to be branchless. Mathf.Abs(vert1sample) can be precomputed so not to compute it twice (though the compiler may already do that). I am also wondering if the expression like vertices.Add are efficient but this is dependent of the type of container which is not provided here.