Question

I am creating 2D game with OpenGL. Everytime I move my object, I have to call glLoadIdentity, because otherwise all objects will be moved, but glLoadIdentity also resets glOrtho call, so basically I ending with something like this:

#include <GL/glfw.h>

int main()
{
glfwInit();

glfwOpenWindowHint( GLFW_WINDOW_NO_RESIZE, GL_TRUE );
glfwOpenWindowHint( GLFW_FSAA_SAMPLES, 8 );
glfwOpenWindow( 800, 600, 0, 0, 255, 0, 32, 0, GLFW_WINDOW );

glfwSetWindowTitle( "title" );

glfwSwapInterval( 1 ); // also known as 'vsync'

glfwEnable( GLFW_KEY_REPEAT );
//glfwDisable( GLFW_MOUSE_CURSOR );

glOrtho(0.0, 1024, 768, 0, -1.0, 1.0); 
glMatrixMode( GL_MODELVIEW );

while( !glfwGetKey( GLFW_KEY_ESC ) )
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glLoadIdentity();
    glOrtho(0.0, 1024, 768, 0, -1.0, 1.0); 

    glTranslatef( 100, 0, 0 );

    glColor3f(0.5f,0.5f,1.0f);
    glBegin(GL_POLYGON);
      glVertex2f(100, 100);
      glVertex2f(100, 250);
      glVertex2f(250, 250);
      glVertex2f(250, 100);
    glEnd();

    glLoadIdentity();
    glOrtho(0.0, 1024, 768, 0, -1.0, 1.0); 

    glRotatef( 25,0,0,1);

    glBegin(GL_POLYGON);
      glVertex2f(300, 300);
      glVertex2f(300, 450);
      glVertex2f(450, 450);
      glVertex2f(450, 300);
    glEnd();

    glFlush();

    glfwSwapBuffers();
}

glfwTerminate();
}

How should it be done properly, to display, move and rotate objects with glOrtho enabled?

Answer 1

You need glPushMatrix() and glPopMatrix(). Checkout this tutorial:

http://www.swiftless.com/tutorials/opengl/pop_and_push_matrices.html