reading binary file

[Cplusc]

I am trying to read an stl binary file using following code but just a few numbers of data are read correctly, the rest are either very large or small. the data look like 1e+161 or nan or 1e-38. I build the geometry in comsol and exported the stl binary file. To check whether the problem is coming from geometry or not, I converted the stl to off file and read the off file and the data were as expected. here is the code,

 struct Triangle {
    float x1, y1, z1;
    float x2, y2, z2;
    float x3, y3, z3;
};

std::vector<Triangle> readSTL(std::string filename) {
    std::vector<Triangle> triangles;

    std::ifstream file(filename, std::ios::in | std::ios::binary);
    if (!file) {
        throw std::runtime_error("Failed to open file: " + filename);
    }

    char header[80];
    file.read(header, 80);

    uint32_t num_triangles;
    file.read(reinterpret_cast<char*>(&num_triangles), sizeof(num_triangles));

    for (uint32_t i = 0; i < num_triangles; i++) {
        // Read vertex coordinates
        float v1x, v1y, v1z, v2x, v2y, v2z, v3x, v3y, v3z;
        file.read(reinterpret_cast<char*>(&v1x), sizeof(float));
        file.read(reinterpret_cast<char*>(&v1y), sizeof(float));
        file.read(reinterpret_cast<char*>(&v1z), sizeof(float));
        file.read(reinterpret_cast<char*>(&v2x), sizeof(float));
        file.read(reinterpret_cast<char*>(&v2y), sizeof(float));
        file.read(reinterpret_cast<char*>(&v2z), sizeof(float));
        file.read(reinterpret_cast<char*>(&v3x), sizeof(float));
        file.read(reinterpret_cast<char*>(&v3y), sizeof(float));
        file.read(reinterpret_cast<char*>(&v3z), sizeof(float));

        uint16_t attr_byte_count;
        file.read(reinterpret_cast<char*>(&attr_byte_count), sizeof(attr_byte_count));

        // Create Triangle object
        Triangle t = { v1x, v1y, v1z, v2x, v2y, v2z, v3x, v3y, v3z };
        triangles.push_back(t);
    }

    file.close();

    for (const auto& t : triangles) {
        std::cout << "Triangle vertex coordinates: "
            << "(" << t.x1 << ", " << t.y1 << ", " << t.z1 << ") "
            << "(" << t.x2 << ", " << t.y2 << ", " << t.z2 << ") "
            << "(" << t.x3 << ", " << t.y3 << ", " << t.z3 << ")" << std::endl;
    }

    return triangles;
}

[mbozzi]

According to https://en.wikipedia.org/wiki/STL_(file_format) there is a normal vector you have to read out with every triangle.

[Cplusc]

@mbozzi thank you. that was the issue. but how skipping normals can cause the data become of such large or very small order. here is the code for future readers

std::vector<Triangle> readSTL(std::string filename) {
    std::vector<Triangle> triangles;

    std::ifstream file(filename, std::ios::in | std::ios::binary);
    if (!file) {
        throw std::runtime_error("Failed to open file: " + filename);
    }

    char header[80];
    file.read(header, 80);

    uint32_t num_triangles;
    file.read(reinterpret_cast<char*>(&num_triangles), sizeof(num_triangles));

    for (uint32_t i = 0; i < num_triangles; i++) {
        // Read normal vector
        float nx, ny, nz;
        file.read(reinterpret_cast<char*>(&nx), sizeof(float));
        file.read(reinterpret_cast<char*>(&ny), sizeof(float));
        file.read(reinterpret_cast<char*>(&nz), sizeof(float));

        // Read vertex coordinates
        float v1x, v1y, v1z, v2x, v2y, v2z, v3x, v3y, v3z;
        file.read(reinterpret_cast<char*>(&v1x), sizeof(float));
        file.read(reinterpret_cast<char*>(&v1y), sizeof(float));
        file.read(reinterpret_cast<char*>(&v1z), sizeof(float));
        file.read(reinterpret_cast<char*>(&v2x), sizeof(float));
        file.read(reinterpret_cast<char*>(&v2y), sizeof(float));
        file.read(reinterpret_cast<char*>(&v2z), sizeof(float));
        file.read(reinterpret_cast<char*>(&v3x), sizeof(float));
        file.read(reinterpret_cast<char*>(&v3y), sizeof(float));
        file.read(reinterpret_cast<char*>(&v3z), sizeof(float));

        uint16_t attr_byte_count;
        file.read(reinterpret_cast<char*>(&attr_byte_count), sizeof(attr_byte_count));

        // Create Triangle object
        Triangle t = { v1x, v1y, v1z, v2x, v2y, v2z, v3x, v3y, v3z };
        triangles.push_back(t);
    }

    file.close();

    for (const auto& t : triangles) {
        std::cout << "Triangle vertex coordinates: "
            << "(" << t.x1 << ", " << t.y1 << ", " << t.z1 << ") "
            << "(" << t.x2 << ", " << t.y2 << ", " << t.z2 << ") "
            << "(" << t.x3 << ", " << t.y3 << ", " << t.z3 << ")" << std::endl;
    }

    return triangles;
} 

[mbozzi]

It's because of that 2-byte chunk. Since you read out the 2-byte value too early, it actually got filled with half of a 4-byte float. After that your subsequent reads would start at the wrong location.

[Cplusc]

That's right. Thank you