Understanding OpenGL glPushMatrix and glPopMatrix

Question

I'm new to OpenGL and trying to render two objects, but only one of them should rotate. I learned from here that I can use glPushMatrix and glPopMatrix to apply the roration on only one object, but mine does not work.

Here are my codes:

void renderObjs() {
    //glPushMatrix();
    drawPyramid(0, 0.7, 0, 0.289, 0, 0.167, -0.289, 0, 0.167, 0, 0, -0.33); // a function to draw triangles
    //glPopMatrix();

    glPushMatrix();
    glRotatef(0.3f, 1.f, 1.f, 1.f);
    drawPyramid(0 - 0.5, 0.7 - 0.5, 0 - 0.5, 0.289 - 0.5, 0 - 0.5, 0.167 - 0.5, -0.289 - 0.5, 0 - 0.5, 0.167 - 0.5, 0 - 0.5, 0 - 0.5, -0.33 - 0.5);
    glPopMatrix();
}

int main() {
    // some initialization
    ...codes here...

    while (!glfwWindowShouldClose(window))
    {
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
        glMatrixMode(GL_MODELVIEW);

        renderObjs();

        glfwSwapBuffers(window);                      
        glfwPollEvents();
    }

    // other useful functions
    ...codes here...
}

But neither of my pyramids rotates. Why is this happening? Am I using the glPushMatrix and glPopMatrix in the wrong way?

Answer 1

The matrix transformation operations like glRotatef, specify a matrix and multiply the current matrix by the new matrix. glPushMatrix pushes the current matrix onto the matrix stack. glPopMatrix pops a matrix from the matrix stack and sets the current matrix through the popped matrix. The unit of the angle of glRotate is degrees:

glPushMatrix();
glRotatef(30.0f, 1.f, 1.f, 1.f);
drawPyramid(0 - 0.5, 0.7 - 0.5, 0 - 0.5, 0.289 - 0.5, 0 - 0.5, 0.167 - 0.5, -0.289 - 0.5, 0 - 0.5, 0.167 - 0.5, 0 - 0.5, 0 - 0.5, -0.33 - 0.5);
glPopMatrix();

---

If you want to rotate the object continuously, you need to increase the rotation angle in each image:

float angle = 0.0f;

void renderObjs() {
    //glPushMatrix();
    drawPyramid(0, 0.7, 0, 0.289, 0, 0.167, -0.289, 0, 0.167, 0, 0, -0.33); // a function to draw triangles
    //glPopMatrix();

    glPushMatrix();
   
    glRotatef(angle, 1.f, 1.f, 1.f);
    angle += 0.1f   

    drawPyramid(0 - 0.5, 0.7 - 0.5, 0 - 0.5, 0.289 - 0.5, 0 - 0.5, 0.167 - 0.5, -0.289 - 0.5, 0 - 0.5, 0.167 - 0.5, 0 - 0.5, 0 - 0.5, -0.33 - 0.5);
    glPopMatrix();
}