Question

我正在尝试将3D几何体写入二进制STL文件。以下是主程序的工作原理：

ParseSTL stl();
//generate the 3D model
std::string outfname = "test.stl";
std::ofstream outf(outfname, std::ios_base::out & std::ios_base::binary);
stl.writeBinarySTL(outf);

和STL作者是：

void writeBinarySTL(std::ofstream &outf)
{
    char attr[2] = { 0 };
    char header[80] = { 0 };
    int count = 0;
    outf.write(header, 80);
    //count += 80;
    //std::cout << "after header: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
    //if ((int)outf.tellp() != count)
    //    std::cout << "something is wrong!\n";
    outf.write(reinterpret_cast<char *>(&facecount), sizeof(int));
    //count += sizeof(int);
    //std::cout << "after number of faces: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
    //if ((int)outf.tellp() != count)
    //    std::cout << "something is wrong!\n";
    for (int i = 0; i < facecount; i++)
    {
        struct face{
            float n[3];
            float v[3][3];
        } f;
        for (int j = 0; j < 3; ++j)
            f.n[j] = (float) normals[i][j];
        for (int j = 0; j < 3; ++j)
            for (int k = 0; k < 3; ++k)
                f.v[j][k] = (float)vertices[faces[i][j]][k];
        outf.write(reinterpret_cast<char *>(&f), sizeof(f));
        //count += sizeof(f);
        //std::cout << "after struct: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
        //if ((int)outf.tellp() != count)
        //{
        //    std::cout << "something is wrong!\n";
        //    break;
        //}
        outf.write(attr, 2);
        //count += 2;
        //std::cout << "after attributes: file position: " << outf.tellp() << "  bytes written: " << count << "\n";
        //if ((int)outf.tellp() != count)
        //{
        //    std::cout << "something is wrong!\n";
        //    break;
        //}
    }
    //std::cout << "Bytes written: " << count << "\n";
}

注释部分仅用于调试，facecount是存储三角形数量的类ParseSTL的成员，normals是存储所有法线的3D向量的向量faces vertices是存储所有顶点坐标的3D矢量矢量，faces是3个整数的矢量，用于存储属于三角形的vrtices的索引。现在，如果我运行程序，这是我得到的输出：

Vertex count = 4243
Face count = 3168
Edge count = 0
after header: file position: 80  bytes written: 80
after number of faces: file position: 84  bytes written: 84
after struct: file position: 132  bytes written: 132
after attributes: file position: 134  bytes written: 134
...
after struct: file position: 532  bytes written: 532
after attributes: file position: 534  bytes written: 534
after struct: file position: 583  bytes written: 582
something is wrong!

因此，似乎行outf.write(reinterpret_cast<char *>(&f), sizeof(f));正在写入比它应该多1个字节的行。这种漂移偶尔发生一次，最后，我得到一个158,484字节的文件，而不是158,896字节的文件。如果我在编写结构后添加outf.seekp(count)，则最终文件大小是正确的，但是某些浮点数没有正确写入（因此漂移并不总是在结构的末尾）。

我想知道我做错了什么导致这个额外的字节被写入文件。哦，我正在使用Microsoft Visual Stusio Express 2013。

Answer 1

这一行

std::ofstream outf(outfname, std::ios_base::out & std::ios_base::binary);

需要

std::ofstream outf(outfname, std::ios_base::out | std::ios_base::binary);
                                              ^^^^^ The difference

可以通过省略std::ios_base::out标志来简化（感谢@Blastfurnace提示）。它还减少了这种错误蔓延的可能性。

std::ofstream outf(outfname, std::ios_base::binary);

当写入二进制文件时，`std :: ofstream :: write`有时会写入比它应该多的字节

1 个答案: