VAO索引仅绘制一条线？

Question

大家好我试图绘制一个索引的VAO，但它并没有真正起作用。 它只绘制第一个元素（第一行）。 我用它绘制两条线，只有三个顶点，但只有第一个工作。 VAO创建代码：

GLushort indices[] = {
    0, 1, 1, 2
};

GLfloat pos[] = {
    50, 50,
    200, 100,
    500, 200
};

GLfloat colors[] = {
    1, 1, 1,
    1, 1, 1,
    1, 1, 1
};

GLuint vaoID;
GLuint vboID[2];
GLuint ibo;

trackShader.use();

glGenVertexArrays(1, &vaoID);
glBindVertexArray(vaoID);
glGenBuffers(2, vboID);

glGenBuffers(1, &ibo);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, 4 * sizeof(GLushort), indices, GL_STATIC_DRAW);

trackShader.addAttribute("vertex");
glBindBuffer(GL_ARRAY_BUFFER, vboID[0]);
glBufferData(GL_ARRAY_BUFFER, 6 * sizeof(GLfloat), pos, GL_STATIC_DRAW);
glVertexAttribPointer(trackShader.attribute("vertex"), 2, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(trackShader.attribute("vertex"));

trackShader.addAttribute("color");
glBindBuffer(GL_ARRAY_BUFFER, vboID[1]);
glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(GLfloat), colors, GL_STATIC_DRAW);
glVertexAttribPointer(trackShader.attribute("color"), 3, GL_FLOAT, GL_FALSE, 0, 0);
glEnableVertexAttribArray(trackShader.attribute("color"));

glBindVertexArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
trackShader.disable();

绘图代码：

glBindVertexArray(vaoID);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
trackShader.use();
glDrawElements(GL_LINES, 4, GL_UNSIGNED_SHORT, 0);
trackShader.disable();
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);
glBindVertexArray(0);