使用GL_LINE_LOOP绘制3D立方体失败，求解决方案

我尝试用OpenGL的GL_LINE_LOOP绘制3D立方体，但无法得到正确的立方体形状，当前输出效果如下：

我的实现代码：

void cube1()
{
    glBegin(GL_LINE_LOOP);

    //Cube1
    //Face1
    glColor3f(1.0, 0.0, 0.0);
    glVertex3f(-0.1, 0, 0);
    glVertex3f(0.4, 0, 0);
    glVertex3f(0.4, -0.3, 0);
    glVertex3f(-0.1, -0.3, 0);

    //Face2
    glColor3f(1.0, 1.0, 0.0);
    glVertex3f(-0.1, -0.3, 0);
    glVertex3f(-0.1, -0.3, 1);
    glVertex3f(-0.1, 0, 1);
    glVertex3f(-0.1, 0, 0);

    //Face3
    glColor3f(1.0, 0.0, 1.0);
    glVertex3f(-0.1, 0, 0);
    glVertex3f(-0.1, 0, 1);
    glVertex3f(0.4, 0, 1);
    glVertex3f(0.4, 0, 0);

    //Face4
    glColor3f(0.0, 1.0, 1.0);
    glVertex3f(0.4, 0, 0);
    glVertex3f(0.4, 0, 1);
    glVertex3f(0.4, -0.3, 1);
    glVertex3f(0.4, -0.3, 0);

    //Face5
    glColor3f(0.0, 1.0, 0.0);
    glVertex3f(0.4, -0.3, 0);
    glVertex3f(-0.1, -0.3, 0);
    glVertex3f(-0.1, -0.3, 1);
    glVertex3f(0.4, -0.3, 1);

    //Face6
    glColor3f(0.0, 0.0, 1.0);
    glVertex3f(0.4, -0.3, 1);
    glVertex3f(-0.1, -0.3, 1);
    glVertex3f(-0.1, 0, 1);
    glVertex3f(0.4, 0, 1);

    glEnd();
}

问题原因 #

GL_LINE_LOOP的作用是将所有连续传入的顶点连接成一个单一的闭合折线环。你现在把6个面的所有顶点都放在同一个glBegin(GL_LINE_LOOP)和glEnd()之间，导致OpenGL会把第一个面的最后一个顶点和第二个面的顶点继续连接，最终形成的是一个混乱的长线环，而不是每个面独立的闭合线框。

解决方法 #

给每个立方体的面单独创建一个GL_LINE_LOOP，也就是每个面都用glBegin(GL_LINE_LOOP)和glEnd()包裹起来，让每个面都是独立的闭合线框。修改后的代码如下：

void cube1()
{
    //Face1
    glBegin(GL_LINE_LOOP);
    glColor3f(1.0, 0.0, 0.0);
    glVertex3f(-0.1, 0, 0);
    glVertex3f(0.4, 0, 0);
    glVertex3f(0.4, -0.3, 0);
    glVertex3f(-0.1, -0.3, 0);
    glEnd();

    //Face2
    glBegin(GL_LINE_LOOP);
    glColor3f(1.0, 1.0, 0.0);
    glVertex3f(-0.1, -0.3, 0);
    glVertex3f(-0.1, -0.3, 1);
    glVertex3f(-0.1, 0, 1);
    glVertex3f(-0.1, 0, 0);
    glEnd();

    //Face3
    glBegin(GL_LINE_LOOP);
    glColor3f(1.0, 0.0, 1.0);
    glVertex3f(-0.1, 0, 0);
    glVertex3f(-0.1, 0, 1);
    glVertex3f(0.4, 0, 1);
    glVertex3f(0.4, 0, 0);
    glEnd();

    //Face4
    glBegin(GL_LINE_LOOP);
    glColor3f(0.0, 1.0, 1.0);
    glVertex3f(0.4, 0, 0);
    glVertex3f(0.4, 0, 1);
    glVertex3f(0.4, -0.3, 1);
    glVertex3f(0.4, -0.3, 0);
    glEnd();

    //Face5
    glBegin(GL_LINE_LOOP);
    glColor3f(0.0, 1.0, 0.0);
    glVertex3f(0.4, -0.3, 0);
    glVertex3f(-0.1, -0.3, 0);
    glVertex3f(-0.1, -0.3, 1);
    glVertex3f(0.4, -0.3, 1);
    glEnd();

    //Face6
    glBegin(GL_LINE_LOOP);
    glColor3f(0.0, 0.0, 1.0);
    glVertex3f(0.4, -0.3, 1);
    glVertex3f(-0.1, -0.3, 1);
    glVertex3f(-0.1, 0, 1);
    glVertex3f(0.4, 0, 1);
    glEnd();
}

另外，也可以考虑使用GL_LINES来绘制立方体的所有棱边，但每个棱需要两个顶点，不过用GL_LINE_LOOP给每个面单独绘制是更直观的方式。

内容的提问来源于stack exchange，提问作者Alexander