stone.json

{
    "render": { "mode": "solid_box" },
    "faces": {
        "north": {
            "culls_neighbours": true,
            "culled_by_relatives": true,
            "textures": [
                "/textures/stone_0.png",
                "/textures/stone_1.png"
            ]
        },
        "south":    { "copy": "north" },
        "east":     { "copy": "north" },
        "west":     { "copy": "north" },
        "top":      { "copy": "north" },
        "bottom":   { "copy": "north" }
    }
}

voxel_mesher.cpp

...

static bool shouldCullFace(const chunk_pos_t &cpos, const local_pos_t &lpos, voxel_t voxel, VoxelFace face, VoxelFace neighbour_face)
{
    // Sometimes local position can go off limits
    // (we can check voxels in neighbouring chunks),
    // so to avoid any accesses to non-existent areas
    // we have to pack it into voxel_pos_t and unpack it back.
    const voxel_pos_t vpos = convert::toVoxelPosition(cpos, lpos);
    const chunk_pos_t cpos_p = convert::toChunkPosition(vpos);
    const local_pos_t lpos_p = convert::toLocalPosition(vpos);

    const auto chunk = voxel_pool.find(convert::toChunkPosition(vpos));
    if(chunk != voxel_pool.cend()) {
        const voxel_t neighbour = chunk->second.at(convert::toVoxelIndex(lpos_p));

        if(neighbour != voxel) {
            if(const VoxelInfo *info = voxels::find(voxel)) {
                const auto info_face = info->faces.find(neighbour_face);
                if(info_face != info->faces.cend())
                    return info_face->second.culls_neighbours;
                return false;
            }

            // Assume NULL_VOXEL
            return false;
        }

        if(const VoxelInfo *info = voxels::find(voxel)) {
            const auto info_face = info->faces.find(face);
            if(info_face != info->faces.cend())
                return info_face->second.culled_by_relatives;
            return true;
        }

        return true;
    }

    // Assume NULL_VOXEL
    return false;
}

...