完成所有绘图代码后，我是否需要重复glOrtho调用？

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

but glLoadIdentity also resets glOrtho call, so basically I ending with something like this

glOrtho belongs into the projection matrix, not the modelview matrix. The answer you accepted is not conceptually wrong, but it did miss that part. That part of your code should look like

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

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
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();

glRotatef( 25,0,0,1);

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

glFlush();

glfwSwapBuffers();

Answer 2

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

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