opengl (python) Camera rotate vs Cube Rotate gluLookAt() 's position in display code

Question

This code is a display part.

def display(self):
    
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
        glLoadIdentity()
        gluLookAt(5.0, 5.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0)
        if (self.axis == 0):
            glRotatef(self.pitch, 1.0, 0.0, 0.0)      
        elif (self.axis == 1):
            glRotatef(self.yaw, 0.0, 1.0, 0.0)
        elif (self.axis == 2):
            glRotatef(self.roll, 0.0, 0.0, 1.0)
        
        
        self.colorcube()
        glFlush()
        glutSwapBuffers()

) figured it out that cube rotates when I put the code gluLookAt() before glRotate() and camera rotates when I put the code gluLookAt() after glRotate(). but I don't know why. why does it happen?

Answer 1

OpenGL is a state engine. The current transformation matrix is a global state. Both functions gluLookAt and glRotatef define a matrix and multiply the current transformation matrix with the new defined matrix. The matrix multiplication is not commutative, the order matters:

matrix1 * matrix2 != matrix2 * matrix1

Therfore:

gluLookAt(5.0, 5.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0)
glRotatef(self.pitch, 1.0, 0.0, 0.0) 

is not the same as

glRotatef(self.pitch, 1.0, 0.0, 0.0)
gluLookAt(5.0, 5.0, 5.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0)

You have to care about the order.