Unable to rotate 2D object properly with opengl - c++

Question

I'm using opengl, glfw and glew. I want to rotate the tip of the spaceship without moving the whole spaceship when users press on the left and right arrow respectively. I've tried using gltransform glrotate, it does rotate but it also move my spaceship along. The output of the movement of spaces
objects objects::rotateSpaceship(GLFWwindow * window, objects & sp)
{
GLfloat spaceship_angle = sp.getAngle();
GLfloat newAngle = sp.getAngle() + 0.5;
sp.setAngle(newAngle);
while (!glfwWindowShouldClose(window))
{
glClear(GL_COLOR_BUFFER_BIT);
GLfloat spaceShipRightLine[] =
{
x, y + 80, 0,
x - 50, y - 50, 0
};
GLfloat spaceShipLeftLine[] = {
x, y + 80, 0,
x + 50, y - 50, 0
};
GLfloat spaceShipMiddleLine[] = {
sp.x - 30 ,sp.y,0,
sp.x + 35,sp.y,0
};
glPushMatrix();
glTranslatef(0.0, 0.01, 0.0); // 3. Translate to the object's position.
glRotatef(sp.getAngle(), 0.0, 0.0, 1.0); // 2. Rotate the object.
glTranslatef(0.0, 0.01, 0.0); // 1. Translate to the origin.
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, spaceShipRightLine);
glDrawArrays(GL_LINES, 0, 2);
glDisableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, spaceShipLeftLine);
glDrawArrays(GL_LINES, 0, 2);
glDisableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, spaceShipMiddleLine);
glDrawArrays(GL_LINES, 0, 2);
glDisableClientState(GL_VERTEX_ARRAY);
glPopMatrix();
// Swap front and back buffers
glfwSwapBuffers(window);
// Poll for and process events
glfwPollEvents();
}
glfwTerminate();
return sp;
}

Answer 1

Note, that drawing by glBegin/glEnd sequences and the fixed function matrix stack is deprecated since decades.
Read about Fixed Function Pipeline and see Vertex Specification and Shader for a state of the art way of rendering.
I want to rotate the tip of the spaceship without moving the whole spaceship [...]
First you have to define the pivot, which is the "tip" of the spaceship:
float pivot_x = sp.x;
float pivot_y = sp.y + 80.0f;
To rotate around a pivot you have to define a model matrix, which displaces by the inverted pivot, then rotate (glRotatef) and final transforms back by to the pivot (glTranslate):
glTranslatef(pivot_x, pivot_y, 0);
glRotatef(angle, 0, 0, 1);
glTranslatef(-pivot_x, -pivot_y, 0);
The final code may look like that:
float pivot_x = x;
float pivot_y = y + 80.0f;
glPushMatrix();
glTranslatef(pivot_x, pivot_y, 0 ); // 3. Translate to the object's position.
glRotatef(sp.getAngle(), 0.0, 0.0, 1.0); // 2. Rotate the object.
glTranslatef(-pivot_x, -pivot_y, 0.0); // 1. Translate to the origin.
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, spaceShipRightLine);
glDrawArrays(GL_LINES, 0, 2);
glDisableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, spaceShipLeftLine);
glDrawArrays(GL_LINES, 0, 2);
glDisableClientState(GL_VERTEX_ARRAY);
glEnableClientState(GL_VERTEX_ARRAY);
glVertexPointer(3, GL_FLOAT, 0, spaceShipMiddleLine);
glDrawArrays(GL_LINES, 0, 2);
glDisableClientState(GL_VERTEX_ARRAY);
glPopMatrix();