convert a game maker tilemap to a vertex buffer

tilemap_to_vertex_buffer.gml

function tilemap_to_vertex_buffer(tilemap, format) {
    // vertex format: position, normal, texture
    var vbuff = vertex_create_buffer();
    vertex_begin(vbuff, format);
    var ts_tileset = tilemap_get_tileset(tilemap);
    var ts_tex = tileset_get_texture(ts_tileset);
    var ts_uvs = tileset_get_uvs(ts_tileset);
    var ts_tex_width = texture_get_texel_width(ts_tex);
    var ts_tex_height = texture_get_texel_height(ts_tex);
    var ts_tile_width = tilemap_get_tile_width(tilemap);
    var ts_tile_height = tilemap_get_tile_height(tilemap);
    var ts_tex_tile_width = ts_tile_width * ts_tex_width;
    var ts_tex_tile_height = ts_tile_height * ts_tex_height;
    var ts_horizontal_count = (ts_uvs[2] - ts_uvs[0]) / ts_tex_tile_width;
    
    for (var i = 0; i < 10; i++) {
        for (var j = 0; j < 10; j++) {
            var index = tile_get_index(tilemap_get(tilemap, i, j));
            var index_x = index % ts_horizontal_count;
            var index_y = index div ts_horizontal_count;
            
            var p1u = ts_uvs[0] + ts_tex_tile_width * index_x;
            var p1v = ts_uvs[1] + ts_tex_tile_height * index_y;
            var p2u = ts_uvs[0] + ts_tex_tile_width * (index_x + 1);
            var p2v = ts_uvs[1] + ts_tex_tile_height * index_y;
            var p3u = ts_uvs[0] + ts_tex_tile_width * (index_x + 1);
            var p3v = ts_uvs[1] + ts_tex_tile_height * (index_y + 1);
            var p4u = ts_uvs[0] + ts_tex_tile_width * index_x;
            var p4v = ts_uvs[1] + ts_tex_tile_height * (index_y + 1);
            
            vertex_position_3d(vbuff, 16 * i, 16 * j, 0);
            vertex_normal(vbuff, 0, 0, 1);
            vertex_texcoord(vbuff, p1u, p1v);
            
            vertex_position_3d(vbuff, 16 * (i + 1), 16 * j, 0);
            vertex_normal(vbuff, 0, 0, 1);
            vertex_texcoord(vbuff, p2u, p2v);
            
            vertex_position_3d(vbuff, 16 * (i + 1), 16 * (j + 1), 0);
            vertex_normal(vbuff, 0, 0, 1);
            vertex_texcoord(vbuff, p3u, p3v);
            
            vertex_position_3d(vbuff, 16 * (i + 1), 16 * (j + 1), 0);
            vertex_normal(vbuff, 0, 0, 1);
            vertex_texcoord(vbuff, p3u, p3v);
            
            vertex_position_3d(vbuff, 16 * i, 16 * (j + 1), 0);
            vertex_normal(vbuff, 0, 0, 1);
            vertex_texcoord(vbuff, p4u, p4v);
            
            vertex_position_3d(vbuff, 16 * i, 16 * j, 0);
            vertex_normal(vbuff, 0, 0, 1);
            vertex_texcoord(vbuff, p1u, p1v);
        }
    }
    
    vertex_end(vbuff);
    vertex_freeze(vbuff);
    layer_destroy(tilemap);
    
    return vbuff;
}