CODEWITHSUNDEEP
c++copengl
Neomex
Neomex

Reputation: 1660

Do I need to repeat glOrtho call after all my drawing code?

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?

Upvotes: 1

Views: 1447

Answers (2)

datenwolf
datenwolf

Reputation: 162164

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();

Upvotes: 6

ahoffer
ahoffer

Reputation: 6526

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

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

Upvotes: 4

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