# Changing a pyramid to tetrahedron in opengl

First of all, I am very new to openGL so I don’t know all the “basic” stuff yet. I am doing this exercise in a book and I have been able to draw a pyramid but I want to draw a tetrahedron now. But I can’t seem to get my head around it. I always end up with weird or incomplete shapes. If I am also not doing anything else right, let me know. This is the code. Any help is appreciated.

```    float pt1 = 1.0f;
float pt2 = 0.0f;
glBegin(GL_TRIANGLES);
// Front
glColor3f(1.0f, 0.0f, 0.0f);     // Red
glVertex3f(pt2, pt1, pt2);
glVertex3f(-pt1, -pt1, pt1);
glVertex3f(pt1, -pt1, pt1);

// Right
glColor3f(0.0f, 0.0f, 1.0f);     // Blue
glVertex3f(pt2, pt1, pt2);
glVertex3f(pt1, -pt1, pt1);
glVertex3f(pt1, -pt1, -pt1);

// Back
glColor3f(0.0f, 1.0f, 0.0f);     // Green
glVertex3f(pt2, pt1, pt2);
glVertex3f(pt1, -pt1, -pt1);
glVertex3f(-pt1, -pt1, -pt1);

// Left
glColor3f(1.0f, 1.0f, 0.0f);       // Yellow
glVertex3f(pt2, pt1, pt2);
glVertex3f(-pt1, -pt1, -pt1);
glVertex3f(-pt1, -pt1, pt1);
glEnd();
```

A Tetrahedron consist of 4 vertices:

```const float s_8_9 = sqrt(0.8f / 0.9f); // = 0.9428f
const float s_2_9 = sqrt(0.2f / 0.9f); // = 0.4714f
const float s_2_3 = sqrt(0.2f / 0.3f); // = 0.8165f

float vertices[4][3] = {
{ 0.0f,   0.0f,   1.0f },
{ s_8_9,  0.0f,  -1.0f / 3.0f},
{-s_2_9,  s_2_3, -1.0f / 3.0f},
{-s_2_9, -s_2_3, -1.0f / 3.0f},
};
```

The vertices form 4 triangular faces:

```int indices[4][3] = {{0, 2, 1},  {0, 3, 2},  {1, 3, 0}, {1, 2, 3}};
```

Use the `vertices` and `indices` to draw the Tetrahedron:

```glBegin(GL_TRIANGLES);

// Front
glColor3f(1.0f, 0.0f, 0.0f);     // Red
for (int i = 0; i < 3; i++)
glVertex3fv(vertices[indices[0][i]]);

// Right
glColor3f(0.0f, 0.0f, 1.0f);     // Blue
for (int i = 0; i < 3; i++)
glVertex3fv(vertices[indices[1][i]]);

// Back
glColor3f(0.0f, 1.0f, 0.0f);     // Green
for (int i = 0; i < 3; i++)
glVertex3fv(vertices[indices[2][i]]);

// Left
glColor3f(1.0f, 1.0f, 0.0f);     // Yellow
for (int i = 0; i < 3; i++)
glVertex3fv(vertices[indices[3][i]]);
```